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a1743e85be
Summary: This PR implements a new admission policy for the compressed secondary cache, which includes the functionality of the existing policy, and also admits items evicted from the primary block cache with the hit bit set. Effectively, the new policy works as follows - 1. When an item is demoted from the primary cache without a hit, a placeholder is inserted in the compressed cache. A second demotion will insert the full entry. 2. When an item is promoted from the compressed cache to the primary cache for the first time, a placeholder is inserted in the primary. The second promotion inserts the full entry, while erasing it form the compressed cache. 3. If an item is demoted from the primary cache with the hit bit set, it is immediately inserted in the compressed secondary cache. The ```TieredVolatileCacheOptions``` has been updated with a new option, ```adm_policy```, which allows the policy to be selected. Pull Request resolved: https://github.com/facebook/rocksdb/pull/11713 Reviewed By: pdillinger Differential Revision: D48444512 Pulled By: anand1976 fbshipit-source-id: b4cbf8c169a88097dff08e36e8bc4b3088de1492
1660 lines
62 KiB
C++
1660 lines
62 KiB
C++
// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
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// This source code is licensed under both the GPLv2 (found in the
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// COPYING file in the root directory) and Apache 2.0 License
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// (found in the LICENSE.Apache file in the root directory).
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//
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// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#include "cache/clock_cache.h"
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#include <algorithm>
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#include <atomic>
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#include <bitset>
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#include <cassert>
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#include <cstddef>
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#include <cstdint>
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#include <exception>
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#include <functional>
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#include <numeric>
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#include <string>
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#include <thread>
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#include <type_traits>
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#include "cache/cache_key.h"
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#include "cache/secondary_cache_adapter.h"
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#include "logging/logging.h"
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#include "monitoring/perf_context_imp.h"
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#include "monitoring/statistics_impl.h"
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#include "port/lang.h"
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#include "rocksdb/env.h"
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#include "util/hash.h"
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#include "util/math.h"
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#include "util/random.h"
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namespace ROCKSDB_NAMESPACE {
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namespace clock_cache {
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namespace {
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inline uint64_t GetRefcount(uint64_t meta) {
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return ((meta >> ClockHandle::kAcquireCounterShift) -
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(meta >> ClockHandle::kReleaseCounterShift)) &
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ClockHandle::kCounterMask;
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}
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inline uint64_t GetInitialCountdown(Cache::Priority priority) {
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// Set initial clock data from priority
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// TODO: configuration parameters for priority handling and clock cycle
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// count?
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switch (priority) {
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case Cache::Priority::HIGH:
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return ClockHandle::kHighCountdown;
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default:
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assert(false);
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FALLTHROUGH_INTENDED;
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case Cache::Priority::LOW:
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return ClockHandle::kLowCountdown;
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case Cache::Priority::BOTTOM:
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return ClockHandle::kBottomCountdown;
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}
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}
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inline void MarkEmpty(ClockHandle& h) {
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#ifndef NDEBUG
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// Mark slot as empty, with assertion
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uint64_t meta = h.meta.exchange(0, std::memory_order_release);
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assert(meta >> ClockHandle::kStateShift == ClockHandle::kStateConstruction);
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#else
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// Mark slot as empty
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h.meta.store(0, std::memory_order_release);
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#endif
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}
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inline void FreeDataMarkEmpty(ClockHandle& h, MemoryAllocator* allocator) {
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// NOTE: in theory there's more room for parallelism if we copy the handle
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// data and delay actions like this until after marking the entry as empty,
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// but performance tests only show a regression by copying the few words
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// of data.
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h.FreeData(allocator);
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MarkEmpty(h);
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}
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// Called to undo the effect of referencing an entry for internal purposes,
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// so it should not be marked as having been used.
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inline void Unref(const ClockHandle& h, uint64_t count = 1) {
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// Pretend we never took the reference
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// WART: there's a tiny chance we release last ref to invisible
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// entry here. If that happens, we let eviction take care of it.
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uint64_t old_meta = h.meta.fetch_sub(ClockHandle::kAcquireIncrement * count,
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std::memory_order_release);
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assert(GetRefcount(old_meta) != 0);
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(void)old_meta;
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}
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inline bool ClockUpdate(ClockHandle& h) {
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uint64_t meta = h.meta.load(std::memory_order_relaxed);
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uint64_t acquire_count =
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(meta >> ClockHandle::kAcquireCounterShift) & ClockHandle::kCounterMask;
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uint64_t release_count =
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(meta >> ClockHandle::kReleaseCounterShift) & ClockHandle::kCounterMask;
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if (acquire_count != release_count) {
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// Only clock update entries with no outstanding refs
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return false;
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}
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if (!((meta >> ClockHandle::kStateShift) & ClockHandle::kStateShareableBit)) {
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// Only clock update Shareable entries
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return false;
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}
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if ((meta >> ClockHandle::kStateShift == ClockHandle::kStateVisible) &&
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acquire_count > 0) {
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// Decrement clock
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uint64_t new_count =
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std::min(acquire_count - 1, uint64_t{ClockHandle::kMaxCountdown} - 1);
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// Compare-exchange in the decremented clock info, but
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// not aggressively
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uint64_t new_meta =
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(uint64_t{ClockHandle::kStateVisible} << ClockHandle::kStateShift) |
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(meta & ClockHandle::kHitBitMask) |
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(new_count << ClockHandle::kReleaseCounterShift) |
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(new_count << ClockHandle::kAcquireCounterShift);
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h.meta.compare_exchange_strong(meta, new_meta, std::memory_order_relaxed);
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return false;
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}
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// Otherwise, remove entry (either unreferenced invisible or
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// unreferenced and expired visible).
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if (h.meta.compare_exchange_strong(meta,
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(uint64_t{ClockHandle::kStateConstruction}
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<< ClockHandle::kStateShift) |
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(meta & ClockHandle::kHitBitMask),
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std::memory_order_acquire)) {
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// Took ownership.
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return true;
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} else {
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// Compare-exchange failing probably
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// indicates the entry was used, so skip it in that case.
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return false;
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}
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}
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// If an entry doesn't receive clock updates but is repeatedly referenced &
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// released, the acquire and release counters could overflow without some
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// intervention. This is that intervention, which should be inexpensive
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// because it only incurs a simple, very predictable check. (Applying a bit
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// mask in addition to an increment to every Release likely would be
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// relatively expensive, because it's an extra atomic update.)
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//
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// We do have to assume that we never have many millions of simultaneous
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// references to a cache handle, because we cannot represent so many
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// references with the difference in counters, masked to the number of
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// counter bits. Similarly, we assume there aren't millions of threads
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// holding transient references (which might be "undone" rather than
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// released by the way).
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//
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// Consider these possible states for each counter:
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// low: less than kMaxCountdown
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// medium: kMaxCountdown to half way to overflow + kMaxCountdown
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// high: half way to overflow + kMaxCountdown, or greater
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//
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// And these possible states for the combination of counters:
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// acquire / release
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// ------- -------
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// low low - Normal / common, with caveats (see below)
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// medium low - Can happen while holding some refs
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// high low - Violates assumptions (too many refs)
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// low medium - Violates assumptions (refs underflow, etc.)
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// medium medium - Normal (very read heavy cache)
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// high medium - Can happen while holding some refs
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// low high - This function is supposed to prevent
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// medium high - Violates assumptions (refs underflow, etc.)
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// high high - Needs CorrectNearOverflow
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//
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// Basically, this function detects (high, high) state (inferred from
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// release alone being high) and bumps it back down to (medium, medium)
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// state with the same refcount and the same logical countdown counter
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// (everything > kMaxCountdown is logically the same). Note that bumping
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// down to (low, low) would modify the countdown counter, so is "reserved"
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// in a sense.
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//
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// If near-overflow correction is triggered here, there's no guarantee
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// that another thread hasn't freed the entry and replaced it with another.
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// Therefore, it must be the case that the correction does not affect
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// entries unless they are very old (many millions of acquire-release cycles).
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// (Our bit manipulation is indeed idempotent and only affects entries in
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// exceptional cases.) We assume a pre-empted thread will not stall that long.
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// If it did, the state could be corrupted in the (unlikely) case that the top
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// bit of the acquire counter is set but not the release counter, and thus
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// we only clear the top bit of the acquire counter on resumption. It would
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// then appear that there are too many refs and the entry would be permanently
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// pinned (which is not terrible for an exceptionally rare occurrence), unless
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// it is referenced enough (at least kMaxCountdown more times) for the release
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// counter to reach "high" state again and bumped back to "medium." (This
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// motivates only checking for release counter in high state, not both in high
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// state.)
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inline void CorrectNearOverflow(uint64_t old_meta,
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std::atomic<uint64_t>& meta) {
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// We clear both top-most counter bits at the same time.
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constexpr uint64_t kCounterTopBit = uint64_t{1}
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<< (ClockHandle::kCounterNumBits - 1);
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constexpr uint64_t kClearBits =
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(kCounterTopBit << ClockHandle::kAcquireCounterShift) |
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(kCounterTopBit << ClockHandle::kReleaseCounterShift);
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// A simple check that allows us to initiate clearing the top bits for
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// a large portion of the "high" state space on release counter.
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constexpr uint64_t kCheckBits =
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(kCounterTopBit | (ClockHandle::kMaxCountdown + 1))
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<< ClockHandle::kReleaseCounterShift;
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if (UNLIKELY(old_meta & kCheckBits)) {
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meta.fetch_and(~kClearBits, std::memory_order_relaxed);
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}
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}
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inline bool BeginSlotInsert(const ClockHandleBasicData& proto, ClockHandle& h,
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uint64_t initial_countdown, bool* already_matches) {
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assert(*already_matches == false);
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// Optimistically transition the slot from "empty" to
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// "under construction" (no effect on other states)
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uint64_t old_meta = h.meta.fetch_or(
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uint64_t{ClockHandle::kStateOccupiedBit} << ClockHandle::kStateShift,
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std::memory_order_acq_rel);
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uint64_t old_state = old_meta >> ClockHandle::kStateShift;
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if (old_state == ClockHandle::kStateEmpty) {
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// We've started inserting into an available slot, and taken
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// ownership.
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return true;
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} else if (old_state != ClockHandle::kStateVisible) {
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// Slot not usable / touchable now
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return false;
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}
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// Existing, visible entry, which might be a match.
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// But first, we need to acquire a ref to read it. In fact, number of
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// refs for initial countdown, so that we boost the clock state if
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// this is a match.
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old_meta =
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h.meta.fetch_add(ClockHandle::kAcquireIncrement * initial_countdown,
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std::memory_order_acq_rel);
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// Like Lookup
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if ((old_meta >> ClockHandle::kStateShift) == ClockHandle::kStateVisible) {
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// Acquired a read reference
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if (h.hashed_key == proto.hashed_key) {
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// Match. Release in a way that boosts the clock state
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old_meta =
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h.meta.fetch_add(ClockHandle::kReleaseIncrement * initial_countdown,
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std::memory_order_acq_rel);
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// Correct for possible (but rare) overflow
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CorrectNearOverflow(old_meta, h.meta);
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// Insert detached instead (only if return handle needed)
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*already_matches = true;
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return false;
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} else {
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// Mismatch.
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Unref(h, initial_countdown);
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}
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} else if (UNLIKELY((old_meta >> ClockHandle::kStateShift) ==
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ClockHandle::kStateInvisible)) {
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// Pretend we never took the reference
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Unref(h, initial_countdown);
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} else {
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// For other states, incrementing the acquire counter has no effect
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// so we don't need to undo it.
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// Slot not usable / touchable now.
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}
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return false;
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}
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inline void FinishSlotInsert(const ClockHandleBasicData& proto, ClockHandle& h,
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uint64_t initial_countdown, bool keep_ref) {
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// Save data fields
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ClockHandleBasicData* h_alias = &h;
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*h_alias = proto;
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// Transition from "under construction" state to "visible" state
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uint64_t new_meta = uint64_t{ClockHandle::kStateVisible}
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<< ClockHandle::kStateShift;
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// Maybe with an outstanding reference
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new_meta |= initial_countdown << ClockHandle::kAcquireCounterShift;
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new_meta |= (initial_countdown - keep_ref)
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<< ClockHandle::kReleaseCounterShift;
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#ifndef NDEBUG
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// Save the state transition, with assertion
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uint64_t old_meta = h.meta.exchange(new_meta, std::memory_order_release);
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assert(old_meta >> ClockHandle::kStateShift ==
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ClockHandle::kStateConstruction);
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#else
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// Save the state transition
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h.meta.store(new_meta, std::memory_order_release);
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#endif
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}
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bool TryInsert(const ClockHandleBasicData& proto, ClockHandle& h,
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uint64_t initial_countdown, bool keep_ref,
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bool* already_matches) {
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bool b = BeginSlotInsert(proto, h, initial_countdown, already_matches);
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if (b) {
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FinishSlotInsert(proto, h, initial_countdown, keep_ref);
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}
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return b;
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}
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// Func must be const HandleImpl& -> void callable
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template <class HandleImpl, class Func>
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void ConstApplyToEntriesRange(const Func& func, const HandleImpl* begin,
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const HandleImpl* end,
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bool apply_if_will_be_deleted) {
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uint64_t check_state_mask = ClockHandle::kStateShareableBit;
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if (!apply_if_will_be_deleted) {
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check_state_mask |= ClockHandle::kStateVisibleBit;
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}
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for (const HandleImpl* h = begin; h < end; ++h) {
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// Note: to avoid using compare_exchange, we have to be extra careful.
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uint64_t old_meta = h->meta.load(std::memory_order_relaxed);
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// Check if it's an entry visible to lookups
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if ((old_meta >> ClockHandle::kStateShift) & check_state_mask) {
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// Increment acquire counter. Note: it's possible that the entry has
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// completely changed since we loaded old_meta, but incrementing acquire
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// count is always safe. (Similar to optimistic Lookup here.)
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old_meta = h->meta.fetch_add(ClockHandle::kAcquireIncrement,
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std::memory_order_acquire);
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// Check whether we actually acquired a reference.
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if ((old_meta >> ClockHandle::kStateShift) &
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ClockHandle::kStateShareableBit) {
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// Apply func if appropriate
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if ((old_meta >> ClockHandle::kStateShift) & check_state_mask) {
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func(*h);
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}
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// Pretend we never took the reference
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Unref(*h);
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// No net change, so don't need to check for overflow
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} else {
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// For other states, incrementing the acquire counter has no effect
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// so we don't need to undo it. Furthermore, we cannot safely undo
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// it because we did not acquire a read reference to lock the
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// entry in a Shareable state.
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}
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}
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}
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}
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} // namespace
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void ClockHandleBasicData::FreeData(MemoryAllocator* allocator) const {
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if (helper->del_cb) {
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helper->del_cb(value, allocator);
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}
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}
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template <class HandleImpl>
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HandleImpl* BaseClockTable::StandaloneInsert(
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const ClockHandleBasicData& proto) {
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// Heap allocated separate from table
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HandleImpl* h = new HandleImpl();
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ClockHandleBasicData* h_alias = h;
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*h_alias = proto;
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h->SetStandalone();
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// Single reference (standalone entries only created if returning a refed
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// Handle back to user)
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uint64_t meta = uint64_t{ClockHandle::kStateInvisible}
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<< ClockHandle::kStateShift;
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meta |= uint64_t{1} << ClockHandle::kAcquireCounterShift;
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h->meta.store(meta, std::memory_order_release);
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// Keep track of how much of usage is standalone
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standalone_usage_.fetch_add(proto.GetTotalCharge(),
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std::memory_order_relaxed);
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return h;
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}
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template <class Table>
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typename Table::HandleImpl* BaseClockTable::CreateStandalone(
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ClockHandleBasicData& proto, size_t capacity, bool strict_capacity_limit,
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bool allow_uncharged) {
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Table& derived = static_cast<Table&>(*this);
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typename Table::InsertState state;
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derived.StartInsert(state);
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const size_t total_charge = proto.GetTotalCharge();
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if (strict_capacity_limit) {
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Status s = ChargeUsageMaybeEvictStrict<Table>(
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total_charge, capacity,
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/*need_evict_for_occupancy=*/false, state);
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if (!s.ok()) {
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if (allow_uncharged) {
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proto.total_charge = 0;
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} else {
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return nullptr;
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}
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}
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} else {
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// Case strict_capacity_limit == false
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bool success = ChargeUsageMaybeEvictNonStrict<Table>(
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total_charge, capacity,
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/*need_evict_for_occupancy=*/false, state);
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if (!success) {
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// Force the issue
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usage_.fetch_add(total_charge, std::memory_order_relaxed);
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}
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}
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return StandaloneInsert<typename Table::HandleImpl>(proto);
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}
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template <class Table>
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Status BaseClockTable::ChargeUsageMaybeEvictStrict(
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size_t total_charge, size_t capacity, bool need_evict_for_occupancy,
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typename Table::InsertState& state) {
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if (total_charge > capacity) {
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return Status::MemoryLimit(
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"Cache entry too large for a single cache shard: " +
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std::to_string(total_charge) + " > " + std::to_string(capacity));
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}
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// Grab any available capacity, and free up any more required.
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size_t old_usage = usage_.load(std::memory_order_relaxed);
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size_t new_usage;
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do {
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new_usage = std::min(capacity, old_usage + total_charge);
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if (new_usage == old_usage) {
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// No change needed
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break;
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}
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} while (!usage_.compare_exchange_weak(old_usage, new_usage,
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std::memory_order_relaxed));
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// How much do we need to evict then?
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size_t need_evict_charge = old_usage + total_charge - new_usage;
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size_t request_evict_charge = need_evict_charge;
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if (UNLIKELY(need_evict_for_occupancy) && request_evict_charge == 0) {
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// Require at least 1 eviction.
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request_evict_charge = 1;
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}
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if (request_evict_charge > 0) {
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EvictionData data;
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static_cast<Table*>(this)->Evict(request_evict_charge, state, &data);
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occupancy_.fetch_sub(data.freed_count, std::memory_order_release);
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if (LIKELY(data.freed_charge > need_evict_charge)) {
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assert(data.freed_count > 0);
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// Evicted more than enough
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usage_.fetch_sub(data.freed_charge - need_evict_charge,
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std::memory_order_relaxed);
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} else if (data.freed_charge < need_evict_charge ||
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(UNLIKELY(need_evict_for_occupancy) && data.freed_count == 0)) {
|
|
// Roll back to old usage minus evicted
|
|
usage_.fetch_sub(data.freed_charge + (new_usage - old_usage),
|
|
std::memory_order_relaxed);
|
|
if (data.freed_charge < need_evict_charge) {
|
|
return Status::MemoryLimit(
|
|
"Insert failed because unable to evict entries to stay within "
|
|
"capacity limit.");
|
|
} else {
|
|
return Status::MemoryLimit(
|
|
"Insert failed because unable to evict entries to stay within "
|
|
"table occupancy limit.");
|
|
}
|
|
}
|
|
// If we needed to evict something and we are proceeding, we must have
|
|
// evicted something.
|
|
assert(data.freed_count > 0);
|
|
}
|
|
return Status::OK();
|
|
}
|
|
|
|
template <class Table>
|
|
inline bool BaseClockTable::ChargeUsageMaybeEvictNonStrict(
|
|
size_t total_charge, size_t capacity, bool need_evict_for_occupancy,
|
|
typename Table::InsertState& state) {
|
|
// For simplicity, we consider that either the cache can accept the insert
|
|
// with no evictions, or we must evict enough to make (at least) enough
|
|
// space. It could lead to unnecessary failures or excessive evictions in
|
|
// some extreme cases, but allows a fast, simple protocol. If we allow a
|
|
// race to get us over capacity, then we might never get back to capacity
|
|
// limit if the sizes of entries allow each insertion to evict the minimum
|
|
// charge. Thus, we should evict some extra if it's not a signifcant
|
|
// portion of the shard capacity. This can have the side benefit of
|
|
// involving fewer threads in eviction.
|
|
size_t old_usage = usage_.load(std::memory_order_relaxed);
|
|
size_t need_evict_charge;
|
|
// NOTE: if total_charge > old_usage, there isn't yet enough to evict
|
|
// `total_charge` amount. Even if we only try to evict `old_usage` amount,
|
|
// there's likely something referenced and we would eat CPU looking for
|
|
// enough to evict.
|
|
if (old_usage + total_charge <= capacity || total_charge > old_usage) {
|
|
// Good enough for me (might run over with a race)
|
|
need_evict_charge = 0;
|
|
} else {
|
|
// Try to evict enough space, and maybe some extra
|
|
need_evict_charge = total_charge;
|
|
if (old_usage > capacity) {
|
|
// Not too much to avoid thundering herd while avoiding strict
|
|
// synchronization, such as the compare_exchange used with strict
|
|
// capacity limit.
|
|
need_evict_charge += std::min(capacity / 1024, total_charge) + 1;
|
|
}
|
|
}
|
|
if (UNLIKELY(need_evict_for_occupancy) && need_evict_charge == 0) {
|
|
// Special case: require at least 1 eviction if we only have to
|
|
// deal with occupancy
|
|
need_evict_charge = 1;
|
|
}
|
|
EvictionData data;
|
|
if (need_evict_charge > 0) {
|
|
static_cast<Table*>(this)->Evict(need_evict_charge, state, &data);
|
|
// Deal with potential occupancy deficit
|
|
if (UNLIKELY(need_evict_for_occupancy) && data.freed_count == 0) {
|
|
assert(data.freed_charge == 0);
|
|
// Can't meet occupancy requirement
|
|
return false;
|
|
} else {
|
|
// Update occupancy for evictions
|
|
occupancy_.fetch_sub(data.freed_count, std::memory_order_release);
|
|
}
|
|
}
|
|
// Track new usage even if we weren't able to evict enough
|
|
usage_.fetch_add(total_charge - data.freed_charge, std::memory_order_relaxed);
|
|
// No underflow
|
|
assert(usage_.load(std::memory_order_relaxed) < SIZE_MAX / 2);
|
|
// Success
|
|
return true;
|
|
}
|
|
|
|
void BaseClockTable::TrackAndReleaseEvictedEntry(
|
|
ClockHandle* h, BaseClockTable::EvictionData* data) {
|
|
data->freed_charge += h->GetTotalCharge();
|
|
data->freed_count += 1;
|
|
|
|
bool took_value_ownership = false;
|
|
if (eviction_callback_) {
|
|
// For key reconstructed from hash
|
|
UniqueId64x2 unhashed;
|
|
took_value_ownership = eviction_callback_(
|
|
ClockCacheShard<FixedHyperClockTable>::ReverseHash(
|
|
h->GetHash(), &unhashed, hash_seed_),
|
|
reinterpret_cast<Cache::Handle*>(h),
|
|
h->meta.load(std::memory_order_relaxed) & ClockHandle::kHitBitMask);
|
|
}
|
|
if (!took_value_ownership) {
|
|
h->FreeData(allocator_);
|
|
}
|
|
MarkEmpty(*h);
|
|
}
|
|
|
|
template <class Table>
|
|
Status BaseClockTable::Insert(const ClockHandleBasicData& proto,
|
|
typename Table::HandleImpl** handle,
|
|
Cache::Priority priority, size_t capacity,
|
|
bool strict_capacity_limit) {
|
|
using HandleImpl = typename Table::HandleImpl;
|
|
Table& derived = static_cast<Table&>(*this);
|
|
|
|
typename Table::InsertState state;
|
|
derived.StartInsert(state);
|
|
|
|
// Do we have the available occupancy? Optimistically assume we do
|
|
// and deal with it if we don't.
|
|
size_t old_occupancy = occupancy_.fetch_add(1, std::memory_order_acquire);
|
|
// Whether we over-committed and need an eviction to make up for it
|
|
bool need_evict_for_occupancy =
|
|
!derived.GrowIfNeeded(old_occupancy + 1, state);
|
|
|
|
// Usage/capacity handling is somewhat different depending on
|
|
// strict_capacity_limit, but mostly pessimistic.
|
|
bool use_standalone_insert = false;
|
|
const size_t total_charge = proto.GetTotalCharge();
|
|
if (strict_capacity_limit) {
|
|
Status s = ChargeUsageMaybeEvictStrict<Table>(
|
|
total_charge, capacity, need_evict_for_occupancy, state);
|
|
if (!s.ok()) {
|
|
// Revert occupancy
|
|
occupancy_.fetch_sub(1, std::memory_order_relaxed);
|
|
return s;
|
|
}
|
|
} else {
|
|
// Case strict_capacity_limit == false
|
|
bool success = ChargeUsageMaybeEvictNonStrict<Table>(
|
|
total_charge, capacity, need_evict_for_occupancy, state);
|
|
if (!success) {
|
|
// Revert occupancy
|
|
occupancy_.fetch_sub(1, std::memory_order_relaxed);
|
|
if (handle == nullptr) {
|
|
// Don't insert the entry but still return ok, as if the entry
|
|
// inserted into cache and evicted immediately.
|
|
proto.FreeData(allocator_);
|
|
return Status::OK();
|
|
} else {
|
|
// Need to track usage of fallback standalone insert
|
|
usage_.fetch_add(total_charge, std::memory_order_relaxed);
|
|
use_standalone_insert = true;
|
|
}
|
|
}
|
|
}
|
|
|
|
if (!use_standalone_insert) {
|
|
// Attempt a table insert, but abort if we find an existing entry for the
|
|
// key. If we were to overwrite old entries, we would either
|
|
// * Have to gain ownership over an existing entry to overwrite it, which
|
|
// would only work if there are no outstanding (read) references and would
|
|
// create a small gap in availability of the entry (old or new) to lookups.
|
|
// * Have to insert into a suboptimal location (more probes) so that the
|
|
// old entry can be kept around as well.
|
|
|
|
uint64_t initial_countdown = GetInitialCountdown(priority);
|
|
assert(initial_countdown > 0);
|
|
|
|
HandleImpl* e =
|
|
derived.DoInsert(proto, initial_countdown, handle != nullptr, state);
|
|
|
|
if (e) {
|
|
// Successfully inserted
|
|
if (handle) {
|
|
*handle = e;
|
|
}
|
|
return Status::OK();
|
|
}
|
|
// Not inserted
|
|
// Revert occupancy
|
|
occupancy_.fetch_sub(1, std::memory_order_relaxed);
|
|
// Maybe fall back on standalone insert
|
|
if (handle == nullptr) {
|
|
// Revert usage
|
|
usage_.fetch_sub(total_charge, std::memory_order_relaxed);
|
|
// No underflow
|
|
assert(usage_.load(std::memory_order_relaxed) < SIZE_MAX / 2);
|
|
// As if unrefed entry immdiately evicted
|
|
proto.FreeData(allocator_);
|
|
return Status::OK();
|
|
}
|
|
|
|
use_standalone_insert = true;
|
|
}
|
|
|
|
// Run standalone insert
|
|
assert(use_standalone_insert);
|
|
|
|
*handle = StandaloneInsert<HandleImpl>(proto);
|
|
|
|
// The OkOverwritten status is used to count "redundant" insertions into
|
|
// block cache. This implementation doesn't strictly check for redundant
|
|
// insertions, but we instead are probably interested in how many insertions
|
|
// didn't go into the table (instead "standalone"), which could be redundant
|
|
// Insert or some other reason (use_standalone_insert reasons above).
|
|
return Status::OkOverwritten();
|
|
}
|
|
|
|
void BaseClockTable::Ref(ClockHandle& h) {
|
|
// Increment acquire counter
|
|
uint64_t old_meta = h.meta.fetch_add(ClockHandle::kAcquireIncrement,
|
|
std::memory_order_acquire);
|
|
|
|
assert((old_meta >> ClockHandle::kStateShift) &
|
|
ClockHandle::kStateShareableBit);
|
|
// Must have already had a reference
|
|
assert(GetRefcount(old_meta) > 0);
|
|
(void)old_meta;
|
|
}
|
|
|
|
#ifndef NDEBUG
|
|
void BaseClockTable::TEST_RefN(ClockHandle& h, size_t n) {
|
|
// Increment acquire counter
|
|
uint64_t old_meta = h.meta.fetch_add(n * ClockHandle::kAcquireIncrement,
|
|
std::memory_order_acquire);
|
|
|
|
assert((old_meta >> ClockHandle::kStateShift) &
|
|
ClockHandle::kStateShareableBit);
|
|
(void)old_meta;
|
|
}
|
|
|
|
void BaseClockTable::TEST_ReleaseNMinus1(ClockHandle* h, size_t n) {
|
|
assert(n > 0);
|
|
|
|
// Like n-1 Releases, but assumes one more will happen in the caller to take
|
|
// care of anything like erasing an unreferenced, invisible entry.
|
|
uint64_t old_meta = h->meta.fetch_add(
|
|
(n - 1) * ClockHandle::kReleaseIncrement, std::memory_order_acquire);
|
|
assert((old_meta >> ClockHandle::kStateShift) &
|
|
ClockHandle::kStateShareableBit);
|
|
(void)old_meta;
|
|
}
|
|
#endif
|
|
|
|
FixedHyperClockTable::FixedHyperClockTable(
|
|
size_t capacity, bool /*strict_capacity_limit*/,
|
|
CacheMetadataChargePolicy metadata_charge_policy,
|
|
MemoryAllocator* allocator,
|
|
const Cache::EvictionCallback* eviction_callback, const uint32_t* hash_seed,
|
|
const Opts& opts)
|
|
: BaseClockTable(metadata_charge_policy, allocator, eviction_callback,
|
|
hash_seed),
|
|
length_bits_(CalcHashBits(capacity, opts.estimated_value_size,
|
|
metadata_charge_policy)),
|
|
length_bits_mask_((size_t{1} << length_bits_) - 1),
|
|
occupancy_limit_(static_cast<size_t>((uint64_t{1} << length_bits_) *
|
|
kStrictLoadFactor)),
|
|
array_(new HandleImpl[size_t{1} << length_bits_]) {
|
|
if (metadata_charge_policy ==
|
|
CacheMetadataChargePolicy::kFullChargeCacheMetadata) {
|
|
usage_ += size_t{GetTableSize()} * sizeof(HandleImpl);
|
|
}
|
|
|
|
static_assert(sizeof(HandleImpl) == 64U,
|
|
"Expecting size / alignment with common cache line size");
|
|
}
|
|
|
|
FixedHyperClockTable::~FixedHyperClockTable() {
|
|
// Assumes there are no references or active operations on any slot/element
|
|
// in the table.
|
|
for (size_t i = 0; i < GetTableSize(); i++) {
|
|
HandleImpl& h = array_[i];
|
|
switch (h.meta >> ClockHandle::kStateShift) {
|
|
case ClockHandle::kStateEmpty:
|
|
// noop
|
|
break;
|
|
case ClockHandle::kStateInvisible: // rare but possible
|
|
case ClockHandle::kStateVisible:
|
|
assert(GetRefcount(h.meta) == 0);
|
|
h.FreeData(allocator_);
|
|
#ifndef NDEBUG
|
|
Rollback(h.hashed_key, &h);
|
|
ReclaimEntryUsage(h.GetTotalCharge());
|
|
#endif
|
|
break;
|
|
// otherwise
|
|
default:
|
|
assert(false);
|
|
break;
|
|
}
|
|
}
|
|
|
|
#ifndef NDEBUG
|
|
for (size_t i = 0; i < GetTableSize(); i++) {
|
|
assert(array_[i].displacements.load() == 0);
|
|
}
|
|
#endif
|
|
|
|
assert(usage_.load() == 0 ||
|
|
usage_.load() == size_t{GetTableSize()} * sizeof(HandleImpl));
|
|
assert(occupancy_ == 0);
|
|
}
|
|
|
|
void FixedHyperClockTable::StartInsert(InsertState&) {}
|
|
|
|
bool FixedHyperClockTable::GrowIfNeeded(size_t new_occupancy, InsertState&) {
|
|
return new_occupancy <= occupancy_limit_;
|
|
}
|
|
|
|
FixedHyperClockTable::HandleImpl* FixedHyperClockTable::DoInsert(
|
|
const ClockHandleBasicData& proto, uint64_t initial_countdown,
|
|
bool keep_ref, InsertState&) {
|
|
bool already_matches = false;
|
|
HandleImpl* e = FindSlot(
|
|
proto.hashed_key,
|
|
[&](HandleImpl* h) {
|
|
return TryInsert(proto, *h, initial_countdown, keep_ref,
|
|
&already_matches);
|
|
},
|
|
[&](HandleImpl* h) {
|
|
if (already_matches) {
|
|
// Stop searching & roll back displacements
|
|
Rollback(proto.hashed_key, h);
|
|
return true;
|
|
} else {
|
|
// Keep going
|
|
return false;
|
|
}
|
|
},
|
|
[&](HandleImpl* h, bool is_last) {
|
|
if (is_last) {
|
|
// Search is ending. Roll back displacements
|
|
Rollback(proto.hashed_key, h);
|
|
} else {
|
|
h->displacements.fetch_add(1, std::memory_order_relaxed);
|
|
}
|
|
});
|
|
if (already_matches) {
|
|
// Insertion skipped
|
|
return nullptr;
|
|
}
|
|
if (e != nullptr) {
|
|
// Successfully inserted
|
|
return e;
|
|
}
|
|
// Else, no available slot found. Occupancy check should generally prevent
|
|
// this, except it's theoretically possible for other threads to evict and
|
|
// replace entries in the right order to hit every slot when it is populated.
|
|
// Assuming random hashing, the chance of that should be no higher than
|
|
// pow(kStrictLoadFactor, n) for n slots. That should be infeasible for
|
|
// roughly n >= 256, so if this assertion fails, that suggests something is
|
|
// going wrong.
|
|
assert(GetTableSize() < 256);
|
|
return nullptr;
|
|
}
|
|
|
|
FixedHyperClockTable::HandleImpl* FixedHyperClockTable::Lookup(
|
|
const UniqueId64x2& hashed_key) {
|
|
HandleImpl* e = FindSlot(
|
|
hashed_key,
|
|
[&](HandleImpl* h) {
|
|
// Mostly branch-free version (similar performance)
|
|
/*
|
|
uint64_t old_meta = h->meta.fetch_add(ClockHandle::kAcquireIncrement,
|
|
std::memory_order_acquire);
|
|
bool Shareable = (old_meta >> (ClockHandle::kStateShift + 1)) & 1U;
|
|
bool visible = (old_meta >> ClockHandle::kStateShift) & 1U;
|
|
bool match = (h->key == key) & visible;
|
|
h->meta.fetch_sub(static_cast<uint64_t>(Shareable & !match) <<
|
|
ClockHandle::kAcquireCounterShift, std::memory_order_release); return
|
|
match;
|
|
*/
|
|
// Optimistic lookup should pay off when the table is relatively
|
|
// sparse.
|
|
constexpr bool kOptimisticLookup = true;
|
|
uint64_t old_meta;
|
|
if (!kOptimisticLookup) {
|
|
old_meta = h->meta.load(std::memory_order_acquire);
|
|
if ((old_meta >> ClockHandle::kStateShift) !=
|
|
ClockHandle::kStateVisible) {
|
|
return false;
|
|
}
|
|
}
|
|
// (Optimistically) increment acquire counter
|
|
old_meta = h->meta.fetch_add(ClockHandle::kAcquireIncrement,
|
|
std::memory_order_acquire);
|
|
// Check if it's an entry visible to lookups
|
|
if ((old_meta >> ClockHandle::kStateShift) ==
|
|
ClockHandle::kStateVisible) {
|
|
// Acquired a read reference
|
|
if (h->hashed_key == hashed_key) {
|
|
// Match
|
|
// Update the hit bit
|
|
if (eviction_callback_) {
|
|
h->meta.fetch_or(uint64_t{1} << ClockHandle::kHitBitShift,
|
|
std::memory_order_relaxed);
|
|
}
|
|
return true;
|
|
} else {
|
|
// Mismatch. Pretend we never took the reference
|
|
Unref(*h);
|
|
}
|
|
} else if (UNLIKELY((old_meta >> ClockHandle::kStateShift) ==
|
|
ClockHandle::kStateInvisible)) {
|
|
// Pretend we never took the reference
|
|
Unref(*h);
|
|
} else {
|
|
// For other states, incrementing the acquire counter has no effect
|
|
// so we don't need to undo it. Furthermore, we cannot safely undo
|
|
// it because we did not acquire a read reference to lock the
|
|
// entry in a Shareable state.
|
|
}
|
|
return false;
|
|
},
|
|
[&](HandleImpl* h) {
|
|
return h->displacements.load(std::memory_order_relaxed) == 0;
|
|
},
|
|
[&](HandleImpl* /*h*/, bool /*is_last*/) {});
|
|
|
|
return e;
|
|
}
|
|
|
|
bool FixedHyperClockTable::Release(HandleImpl* h, bool useful,
|
|
bool erase_if_last_ref) {
|
|
// In contrast with LRUCache's Release, this function won't delete the handle
|
|
// when the cache is above capacity and the reference is the last one. Space
|
|
// is only freed up by EvictFromClock (called by Insert when space is needed)
|
|
// and Erase. We do this to avoid an extra atomic read of the variable usage_.
|
|
|
|
uint64_t old_meta;
|
|
if (useful) {
|
|
// Increment release counter to indicate was used
|
|
old_meta = h->meta.fetch_add(ClockHandle::kReleaseIncrement,
|
|
std::memory_order_release);
|
|
} else {
|
|
// Decrement acquire counter to pretend it never happened
|
|
old_meta = h->meta.fetch_sub(ClockHandle::kAcquireIncrement,
|
|
std::memory_order_release);
|
|
}
|
|
|
|
assert((old_meta >> ClockHandle::kStateShift) &
|
|
ClockHandle::kStateShareableBit);
|
|
// No underflow
|
|
assert(((old_meta >> ClockHandle::kAcquireCounterShift) &
|
|
ClockHandle::kCounterMask) !=
|
|
((old_meta >> ClockHandle::kReleaseCounterShift) &
|
|
ClockHandle::kCounterMask));
|
|
|
|
if (erase_if_last_ref || UNLIKELY(old_meta >> ClockHandle::kStateShift ==
|
|
ClockHandle::kStateInvisible)) {
|
|
// Update for last fetch_add op
|
|
if (useful) {
|
|
old_meta += ClockHandle::kReleaseIncrement;
|
|
} else {
|
|
old_meta -= ClockHandle::kAcquireIncrement;
|
|
}
|
|
// Take ownership if no refs
|
|
do {
|
|
if (GetRefcount(old_meta) != 0) {
|
|
// Not last ref at some point in time during this Release call
|
|
// Correct for possible (but rare) overflow
|
|
CorrectNearOverflow(old_meta, h->meta);
|
|
return false;
|
|
}
|
|
if ((old_meta & (uint64_t{ClockHandle::kStateShareableBit}
|
|
<< ClockHandle::kStateShift)) == 0) {
|
|
// Someone else took ownership
|
|
return false;
|
|
}
|
|
// Note that there's a small chance that we release, another thread
|
|
// replaces this entry with another, reaches zero refs, and then we end
|
|
// up erasing that other entry. That's an acceptable risk / imprecision.
|
|
} while (!h->meta.compare_exchange_weak(
|
|
old_meta,
|
|
uint64_t{ClockHandle::kStateConstruction} << ClockHandle::kStateShift,
|
|
std::memory_order_acquire));
|
|
// Took ownership
|
|
size_t total_charge = h->GetTotalCharge();
|
|
if (UNLIKELY(h->IsStandalone())) {
|
|
h->FreeData(allocator_);
|
|
// Delete standalone handle
|
|
delete h;
|
|
standalone_usage_.fetch_sub(total_charge, std::memory_order_relaxed);
|
|
usage_.fetch_sub(total_charge, std::memory_order_relaxed);
|
|
} else {
|
|
Rollback(h->hashed_key, h);
|
|
FreeDataMarkEmpty(*h, allocator_);
|
|
ReclaimEntryUsage(total_charge);
|
|
}
|
|
return true;
|
|
} else {
|
|
// Correct for possible (but rare) overflow
|
|
CorrectNearOverflow(old_meta, h->meta);
|
|
return false;
|
|
}
|
|
}
|
|
|
|
#ifndef NDEBUG
|
|
void FixedHyperClockTable::TEST_ReleaseN(HandleImpl* h, size_t n) {
|
|
if (n > 0) {
|
|
// Do n-1 simple releases first
|
|
TEST_ReleaseNMinus1(h, n);
|
|
|
|
// Then the last release might be more involved
|
|
Release(h, /*useful*/ true, /*erase_if_last_ref*/ false);
|
|
}
|
|
}
|
|
#endif
|
|
|
|
void FixedHyperClockTable::Erase(const UniqueId64x2& hashed_key) {
|
|
(void)FindSlot(
|
|
hashed_key,
|
|
[&](HandleImpl* h) {
|
|
// Could be multiple entries in rare cases. Erase them all.
|
|
// Optimistically increment acquire counter
|
|
uint64_t old_meta = h->meta.fetch_add(ClockHandle::kAcquireIncrement,
|
|
std::memory_order_acquire);
|
|
// Check if it's an entry visible to lookups
|
|
if ((old_meta >> ClockHandle::kStateShift) ==
|
|
ClockHandle::kStateVisible) {
|
|
// Acquired a read reference
|
|
if (h->hashed_key == hashed_key) {
|
|
// Match. Set invisible.
|
|
old_meta =
|
|
h->meta.fetch_and(~(uint64_t{ClockHandle::kStateVisibleBit}
|
|
<< ClockHandle::kStateShift),
|
|
std::memory_order_acq_rel);
|
|
// Apply update to local copy
|
|
old_meta &= ~(uint64_t{ClockHandle::kStateVisibleBit}
|
|
<< ClockHandle::kStateShift);
|
|
for (;;) {
|
|
uint64_t refcount = GetRefcount(old_meta);
|
|
assert(refcount > 0);
|
|
if (refcount > 1) {
|
|
// Not last ref at some point in time during this Erase call
|
|
// Pretend we never took the reference
|
|
Unref(*h);
|
|
break;
|
|
} else if (h->meta.compare_exchange_weak(
|
|
old_meta,
|
|
uint64_t{ClockHandle::kStateConstruction}
|
|
<< ClockHandle::kStateShift,
|
|
std::memory_order_acq_rel)) {
|
|
// Took ownership
|
|
assert(hashed_key == h->hashed_key);
|
|
size_t total_charge = h->GetTotalCharge();
|
|
FreeDataMarkEmpty(*h, allocator_);
|
|
ReclaimEntryUsage(total_charge);
|
|
// We already have a copy of hashed_key in this case, so OK to
|
|
// delay Rollback until after releasing the entry
|
|
Rollback(hashed_key, h);
|
|
break;
|
|
}
|
|
}
|
|
} else {
|
|
// Mismatch. Pretend we never took the reference
|
|
Unref(*h);
|
|
}
|
|
} else if (UNLIKELY((old_meta >> ClockHandle::kStateShift) ==
|
|
ClockHandle::kStateInvisible)) {
|
|
// Pretend we never took the reference
|
|
Unref(*h);
|
|
} else {
|
|
// For other states, incrementing the acquire counter has no effect
|
|
// so we don't need to undo it.
|
|
}
|
|
return false;
|
|
},
|
|
[&](HandleImpl* h) {
|
|
return h->displacements.load(std::memory_order_relaxed) == 0;
|
|
},
|
|
[&](HandleImpl* /*h*/, bool /*is_last*/) {});
|
|
}
|
|
|
|
void FixedHyperClockTable::EraseUnRefEntries() {
|
|
for (size_t i = 0; i <= this->length_bits_mask_; i++) {
|
|
HandleImpl& h = array_[i];
|
|
|
|
uint64_t old_meta = h.meta.load(std::memory_order_relaxed);
|
|
if (old_meta & (uint64_t{ClockHandle::kStateShareableBit}
|
|
<< ClockHandle::kStateShift) &&
|
|
GetRefcount(old_meta) == 0 &&
|
|
h.meta.compare_exchange_strong(old_meta,
|
|
uint64_t{ClockHandle::kStateConstruction}
|
|
<< ClockHandle::kStateShift,
|
|
std::memory_order_acquire)) {
|
|
// Took ownership
|
|
size_t total_charge = h.GetTotalCharge();
|
|
Rollback(h.hashed_key, &h);
|
|
FreeDataMarkEmpty(h, allocator_);
|
|
ReclaimEntryUsage(total_charge);
|
|
}
|
|
}
|
|
}
|
|
|
|
template <typename MatchFn, typename AbortFn, typename UpdateFn>
|
|
inline FixedHyperClockTable::HandleImpl* FixedHyperClockTable::FindSlot(
|
|
const UniqueId64x2& hashed_key, const MatchFn& match_fn,
|
|
const AbortFn& abort_fn, const UpdateFn& update_fn) {
|
|
// NOTE: upper 32 bits of hashed_key[0] is used for sharding
|
|
//
|
|
// We use double-hashing probing. Every probe in the sequence is a
|
|
// pseudorandom integer, computed as a linear function of two random hashes,
|
|
// which we call base and increment. Specifically, the i-th probe is base + i
|
|
// * increment modulo the table size.
|
|
size_t base = static_cast<size_t>(hashed_key[1]);
|
|
// We use an odd increment, which is relatively prime with the power-of-two
|
|
// table size. This implies that we cycle back to the first probe only
|
|
// after probing every slot exactly once.
|
|
// TODO: we could also reconsider linear probing, though locality benefits
|
|
// are limited because each slot is a full cache line
|
|
size_t increment = static_cast<size_t>(hashed_key[0]) | 1U;
|
|
size_t first = ModTableSize(base);
|
|
size_t current = first;
|
|
bool is_last;
|
|
do {
|
|
HandleImpl* h = &array_[current];
|
|
if (match_fn(h)) {
|
|
return h;
|
|
}
|
|
if (abort_fn(h)) {
|
|
return nullptr;
|
|
}
|
|
current = ModTableSize(current + increment);
|
|
is_last = current == first;
|
|
update_fn(h, is_last);
|
|
} while (!is_last);
|
|
// We looped back.
|
|
return nullptr;
|
|
}
|
|
|
|
inline void FixedHyperClockTable::Rollback(const UniqueId64x2& hashed_key,
|
|
const HandleImpl* h) {
|
|
size_t current = ModTableSize(hashed_key[1]);
|
|
size_t increment = static_cast<size_t>(hashed_key[0]) | 1U;
|
|
while (&array_[current] != h) {
|
|
array_[current].displacements.fetch_sub(1, std::memory_order_relaxed);
|
|
current = ModTableSize(current + increment);
|
|
}
|
|
}
|
|
|
|
inline void FixedHyperClockTable::ReclaimEntryUsage(size_t total_charge) {
|
|
auto old_occupancy = occupancy_.fetch_sub(1U, std::memory_order_release);
|
|
(void)old_occupancy;
|
|
// No underflow
|
|
assert(old_occupancy > 0);
|
|
auto old_usage = usage_.fetch_sub(total_charge, std::memory_order_relaxed);
|
|
(void)old_usage;
|
|
// No underflow
|
|
assert(old_usage >= total_charge);
|
|
}
|
|
|
|
inline void FixedHyperClockTable::Evict(size_t requested_charge, InsertState&,
|
|
EvictionData* data) {
|
|
// precondition
|
|
assert(requested_charge > 0);
|
|
|
|
// TODO: make a tuning parameter?
|
|
constexpr size_t step_size = 4;
|
|
|
|
// First (concurrent) increment clock pointer
|
|
uint64_t old_clock_pointer =
|
|
clock_pointer_.fetch_add(step_size, std::memory_order_relaxed);
|
|
|
|
// Cap the eviction effort at this thread (along with those operating in
|
|
// parallel) circling through the whole structure kMaxCountdown times.
|
|
// In other words, this eviction run must find something/anything that is
|
|
// unreferenced at start of and during the eviction run that isn't reclaimed
|
|
// by a concurrent eviction run.
|
|
uint64_t max_clock_pointer =
|
|
old_clock_pointer + (ClockHandle::kMaxCountdown << length_bits_);
|
|
|
|
for (;;) {
|
|
for (size_t i = 0; i < step_size; i++) {
|
|
HandleImpl& h = array_[ModTableSize(Lower32of64(old_clock_pointer + i))];
|
|
bool evicting = ClockUpdate(h);
|
|
if (evicting) {
|
|
Rollback(h.hashed_key, &h);
|
|
TrackAndReleaseEvictedEntry(&h, data);
|
|
}
|
|
}
|
|
|
|
// Loop exit condition
|
|
if (data->freed_charge >= requested_charge) {
|
|
return;
|
|
}
|
|
if (old_clock_pointer >= max_clock_pointer) {
|
|
return;
|
|
}
|
|
|
|
// Advance clock pointer (concurrently)
|
|
old_clock_pointer =
|
|
clock_pointer_.fetch_add(step_size, std::memory_order_relaxed);
|
|
}
|
|
}
|
|
|
|
template <class Table>
|
|
ClockCacheShard<Table>::ClockCacheShard(
|
|
size_t capacity, bool strict_capacity_limit,
|
|
CacheMetadataChargePolicy metadata_charge_policy,
|
|
MemoryAllocator* allocator,
|
|
const Cache::EvictionCallback* eviction_callback, const uint32_t* hash_seed,
|
|
const typename Table::Opts& opts)
|
|
: CacheShardBase(metadata_charge_policy),
|
|
table_(capacity, strict_capacity_limit, metadata_charge_policy, allocator,
|
|
eviction_callback, hash_seed, opts),
|
|
capacity_(capacity),
|
|
strict_capacity_limit_(strict_capacity_limit) {
|
|
// Initial charge metadata should not exceed capacity
|
|
assert(table_.GetUsage() <= capacity_ || capacity_ < sizeof(HandleImpl));
|
|
}
|
|
|
|
template <class Table>
|
|
void ClockCacheShard<Table>::EraseUnRefEntries() {
|
|
table_.EraseUnRefEntries();
|
|
}
|
|
|
|
template <class Table>
|
|
void ClockCacheShard<Table>::ApplyToSomeEntries(
|
|
const std::function<void(const Slice& key, Cache::ObjectPtr value,
|
|
size_t charge,
|
|
const Cache::CacheItemHelper* helper)>& callback,
|
|
size_t average_entries_per_lock, size_t* state) {
|
|
// The state will be a simple index into the table. Even with a dynamic
|
|
// hyper clock cache, entries will generally stay in their existing
|
|
// slots, so we don't need to be aware of the high-level organization
|
|
// that makes lookup efficient.
|
|
size_t length = table_.GetTableSize();
|
|
|
|
assert(average_entries_per_lock > 0);
|
|
|
|
size_t index_begin = *state;
|
|
size_t index_end = index_begin + average_entries_per_lock;
|
|
if (index_end >= length) {
|
|
// Going to end.
|
|
index_end = length;
|
|
*state = SIZE_MAX;
|
|
} else {
|
|
*state = index_end;
|
|
}
|
|
|
|
auto hash_seed = table_.GetHashSeed();
|
|
ConstApplyToEntriesRange(
|
|
[callback, hash_seed](const HandleImpl& h) {
|
|
UniqueId64x2 unhashed;
|
|
callback(ReverseHash(h.hashed_key, &unhashed, hash_seed), h.value,
|
|
h.GetTotalCharge(), h.helper);
|
|
},
|
|
table_.HandlePtr(index_begin), table_.HandlePtr(index_end), false);
|
|
}
|
|
|
|
int FixedHyperClockTable::CalcHashBits(
|
|
size_t capacity, size_t estimated_value_size,
|
|
CacheMetadataChargePolicy metadata_charge_policy) {
|
|
double average_slot_charge = estimated_value_size * kLoadFactor;
|
|
if (metadata_charge_policy == kFullChargeCacheMetadata) {
|
|
average_slot_charge += sizeof(HandleImpl);
|
|
}
|
|
assert(average_slot_charge > 0.0);
|
|
uint64_t num_slots =
|
|
static_cast<uint64_t>(capacity / average_slot_charge + 0.999999);
|
|
|
|
int hash_bits = FloorLog2((num_slots << 1) - 1);
|
|
if (metadata_charge_policy == kFullChargeCacheMetadata) {
|
|
// For very small estimated value sizes, it's possible to overshoot
|
|
while (hash_bits > 0 &&
|
|
uint64_t{sizeof(HandleImpl)} << hash_bits > capacity) {
|
|
hash_bits--;
|
|
}
|
|
}
|
|
return hash_bits;
|
|
}
|
|
|
|
template <class Table>
|
|
void ClockCacheShard<Table>::SetCapacity(size_t capacity) {
|
|
capacity_.store(capacity, std::memory_order_relaxed);
|
|
// next Insert will take care of any necessary evictions
|
|
}
|
|
|
|
template <class Table>
|
|
void ClockCacheShard<Table>::SetStrictCapacityLimit(
|
|
bool strict_capacity_limit) {
|
|
strict_capacity_limit_.store(strict_capacity_limit,
|
|
std::memory_order_relaxed);
|
|
// next Insert will take care of any necessary evictions
|
|
}
|
|
|
|
template <class Table>
|
|
Status ClockCacheShard<Table>::Insert(const Slice& key,
|
|
const UniqueId64x2& hashed_key,
|
|
Cache::ObjectPtr value,
|
|
const Cache::CacheItemHelper* helper,
|
|
size_t charge, HandleImpl** handle,
|
|
Cache::Priority priority) {
|
|
if (UNLIKELY(key.size() != kCacheKeySize)) {
|
|
return Status::NotSupported("ClockCache only supports key size " +
|
|
std::to_string(kCacheKeySize) + "B");
|
|
}
|
|
ClockHandleBasicData proto;
|
|
proto.hashed_key = hashed_key;
|
|
proto.value = value;
|
|
proto.helper = helper;
|
|
proto.total_charge = charge;
|
|
return table_.template Insert<Table>(
|
|
proto, handle, priority, capacity_.load(std::memory_order_relaxed),
|
|
strict_capacity_limit_.load(std::memory_order_relaxed));
|
|
}
|
|
|
|
template <class Table>
|
|
typename Table::HandleImpl* ClockCacheShard<Table>::CreateStandalone(
|
|
const Slice& key, const UniqueId64x2& hashed_key, Cache::ObjectPtr obj,
|
|
const Cache::CacheItemHelper* helper, size_t charge, bool allow_uncharged) {
|
|
if (UNLIKELY(key.size() != kCacheKeySize)) {
|
|
return nullptr;
|
|
}
|
|
ClockHandleBasicData proto;
|
|
proto.hashed_key = hashed_key;
|
|
proto.value = obj;
|
|
proto.helper = helper;
|
|
proto.total_charge = charge;
|
|
return table_.template CreateStandalone<Table>(
|
|
proto, capacity_.load(std::memory_order_relaxed),
|
|
strict_capacity_limit_.load(std::memory_order_relaxed), allow_uncharged);
|
|
}
|
|
|
|
template <class Table>
|
|
typename ClockCacheShard<Table>::HandleImpl* ClockCacheShard<Table>::Lookup(
|
|
const Slice& key, const UniqueId64x2& hashed_key) {
|
|
if (UNLIKELY(key.size() != kCacheKeySize)) {
|
|
return nullptr;
|
|
}
|
|
return table_.Lookup(hashed_key);
|
|
}
|
|
|
|
template <class Table>
|
|
bool ClockCacheShard<Table>::Ref(HandleImpl* h) {
|
|
if (h == nullptr) {
|
|
return false;
|
|
}
|
|
table_.Ref(*h);
|
|
return true;
|
|
}
|
|
|
|
template <class Table>
|
|
bool ClockCacheShard<Table>::Release(HandleImpl* handle, bool useful,
|
|
bool erase_if_last_ref) {
|
|
if (handle == nullptr) {
|
|
return false;
|
|
}
|
|
return table_.Release(handle, useful, erase_if_last_ref);
|
|
}
|
|
|
|
#ifndef NDEBUG
|
|
template <class Table>
|
|
void ClockCacheShard<Table>::TEST_RefN(HandleImpl* h, size_t n) {
|
|
table_.TEST_RefN(*h, n);
|
|
}
|
|
|
|
template <class Table>
|
|
void ClockCacheShard<Table>::TEST_ReleaseN(HandleImpl* h, size_t n) {
|
|
table_.TEST_ReleaseN(h, n);
|
|
}
|
|
#endif
|
|
|
|
template <class Table>
|
|
bool ClockCacheShard<Table>::Release(HandleImpl* handle,
|
|
bool erase_if_last_ref) {
|
|
return Release(handle, /*useful=*/true, erase_if_last_ref);
|
|
}
|
|
|
|
template <class Table>
|
|
void ClockCacheShard<Table>::Erase(const Slice& key,
|
|
const UniqueId64x2& hashed_key) {
|
|
if (UNLIKELY(key.size() != kCacheKeySize)) {
|
|
return;
|
|
}
|
|
table_.Erase(hashed_key);
|
|
}
|
|
|
|
template <class Table>
|
|
size_t ClockCacheShard<Table>::GetUsage() const {
|
|
return table_.GetUsage();
|
|
}
|
|
|
|
template <class Table>
|
|
size_t ClockCacheShard<Table>::GetStandaloneUsage() const {
|
|
return table_.GetStandaloneUsage();
|
|
}
|
|
|
|
template <class Table>
|
|
size_t ClockCacheShard<Table>::GetCapacity() const {
|
|
return capacity_;
|
|
}
|
|
|
|
template <class Table>
|
|
size_t ClockCacheShard<Table>::GetPinnedUsage() const {
|
|
// Computes the pinned usage by scanning the whole hash table. This
|
|
// is slow, but avoids keeping an exact counter on the clock usage,
|
|
// i.e., the number of not externally referenced elements.
|
|
// Why avoid this counter? Because Lookup removes elements from the clock
|
|
// list, so it would need to update the pinned usage every time,
|
|
// which creates additional synchronization costs.
|
|
size_t table_pinned_usage = 0;
|
|
const bool charge_metadata =
|
|
metadata_charge_policy_ == kFullChargeCacheMetadata;
|
|
ConstApplyToEntriesRange(
|
|
[&table_pinned_usage, charge_metadata](const HandleImpl& h) {
|
|
uint64_t meta = h.meta.load(std::memory_order_relaxed);
|
|
uint64_t refcount = GetRefcount(meta);
|
|
// Holding one ref for ConstApplyToEntriesRange
|
|
assert(refcount > 0);
|
|
if (refcount > 1) {
|
|
table_pinned_usage += h.GetTotalCharge();
|
|
if (charge_metadata) {
|
|
table_pinned_usage += sizeof(HandleImpl);
|
|
}
|
|
}
|
|
},
|
|
table_.HandlePtr(0), table_.HandlePtr(table_.GetTableSize()), true);
|
|
|
|
return table_pinned_usage + table_.GetStandaloneUsage();
|
|
}
|
|
|
|
template <class Table>
|
|
size_t ClockCacheShard<Table>::GetOccupancyCount() const {
|
|
return table_.GetOccupancy();
|
|
}
|
|
|
|
template <class Table>
|
|
size_t ClockCacheShard<Table>::GetOccupancyLimit() const {
|
|
return table_.GetOccupancyLimit();
|
|
}
|
|
|
|
template <class Table>
|
|
size_t ClockCacheShard<Table>::GetTableAddressCount() const {
|
|
return table_.GetTableSize();
|
|
}
|
|
|
|
// Explicit instantiation
|
|
template class ClockCacheShard<FixedHyperClockTable>;
|
|
template class ClockCacheShard<AutoHyperClockTable>;
|
|
|
|
template <class Table>
|
|
BaseHyperClockCache<Table>::BaseHyperClockCache(
|
|
const HyperClockCacheOptions& opts)
|
|
: ShardedCache<ClockCacheShard<Table>>(opts) {
|
|
// TODO: should not need to go through two levels of pointer indirection to
|
|
// get to table entries
|
|
size_t per_shard = this->GetPerShardCapacity();
|
|
MemoryAllocator* alloc = this->memory_allocator();
|
|
this->InitShards([&](Shard* cs) {
|
|
typename Table::Opts table_opts{opts};
|
|
new (cs) Shard(per_shard, opts.strict_capacity_limit,
|
|
opts.metadata_charge_policy, alloc,
|
|
&this->eviction_callback_, &this->hash_seed_, table_opts);
|
|
});
|
|
}
|
|
|
|
template <class Table>
|
|
Cache::ObjectPtr BaseHyperClockCache<Table>::Value(Handle* handle) {
|
|
return reinterpret_cast<const typename Table::HandleImpl*>(handle)->value;
|
|
}
|
|
|
|
template <class Table>
|
|
size_t BaseHyperClockCache<Table>::GetCharge(Handle* handle) const {
|
|
return reinterpret_cast<const typename Table::HandleImpl*>(handle)
|
|
->GetTotalCharge();
|
|
}
|
|
|
|
template <class Table>
|
|
const Cache::CacheItemHelper* BaseHyperClockCache<Table>::GetCacheItemHelper(
|
|
Handle* handle) const {
|
|
auto h = reinterpret_cast<const typename Table::HandleImpl*>(handle);
|
|
return h->helper;
|
|
}
|
|
|
|
namespace {
|
|
|
|
// For each cache shard, estimate what the table load factor would be if
|
|
// cache filled to capacity with average entries. This is considered
|
|
// indicative of a potential problem if the shard is essentially operating
|
|
// "at limit", which we define as high actual usage (>80% of capacity)
|
|
// or actual occupancy very close to limit (>95% of limit).
|
|
// Also, for each shard compute the recommended estimated_entry_charge,
|
|
// and keep the minimum one for use as overall recommendation.
|
|
void AddShardEvaluation(const FixedHyperClockCache::Shard& shard,
|
|
std::vector<double>& predicted_load_factors,
|
|
size_t& min_recommendation) {
|
|
size_t usage = shard.GetUsage() - shard.GetStandaloneUsage();
|
|
size_t capacity = shard.GetCapacity();
|
|
double usage_ratio = 1.0 * usage / capacity;
|
|
|
|
size_t occupancy = shard.GetOccupancyCount();
|
|
size_t occ_limit = shard.GetOccupancyLimit();
|
|
double occ_ratio = 1.0 * occupancy / occ_limit;
|
|
if (usage == 0 || occupancy == 0 || (usage_ratio < 0.8 && occ_ratio < 0.95)) {
|
|
// Skip as described above
|
|
return;
|
|
}
|
|
|
|
// If filled to capacity, what would the occupancy ratio be?
|
|
double ratio = occ_ratio / usage_ratio;
|
|
// Given max load factor, what that load factor be?
|
|
double lf = ratio * FixedHyperClockTable::kStrictLoadFactor;
|
|
predicted_load_factors.push_back(lf);
|
|
|
|
// Update min_recommendation also
|
|
size_t recommendation = usage / occupancy;
|
|
min_recommendation = std::min(min_recommendation, recommendation);
|
|
}
|
|
|
|
bool IsSlotOccupied(const ClockHandle& h) {
|
|
return (h.meta.load(std::memory_order_relaxed) >> ClockHandle::kStateShift) !=
|
|
0;
|
|
}
|
|
} // namespace
|
|
|
|
// NOTE: GCC might warn about subobject linkage if this is in anon namespace
|
|
template <size_t N = 500>
|
|
class LoadVarianceStats {
|
|
public:
|
|
std::string Report() const {
|
|
return "Overall " + PercentStr(positive_count_, samples_) + " (" +
|
|
std::to_string(positive_count_) + "/" + std::to_string(samples_) +
|
|
"), Min/Max/Window = " + PercentStr(min_, N) + "/" +
|
|
PercentStr(max_, N) + "/" + std::to_string(N) +
|
|
", MaxRun{Pos/Neg} = " + std::to_string(max_pos_run_) + "/" +
|
|
std::to_string(max_neg_run_) + "\n";
|
|
}
|
|
|
|
void Add(bool positive) {
|
|
recent_[samples_ % N] = positive;
|
|
if (positive) {
|
|
++positive_count_;
|
|
++cur_pos_run_;
|
|
max_pos_run_ = std::max(max_pos_run_, cur_pos_run_);
|
|
cur_neg_run_ = 0;
|
|
} else {
|
|
++cur_neg_run_;
|
|
max_neg_run_ = std::max(max_neg_run_, cur_neg_run_);
|
|
cur_pos_run_ = 0;
|
|
}
|
|
++samples_;
|
|
if (samples_ >= N) {
|
|
size_t count_set = recent_.count();
|
|
max_ = std::max(max_, count_set);
|
|
min_ = std::min(min_, count_set);
|
|
}
|
|
}
|
|
|
|
private:
|
|
size_t max_ = 0;
|
|
size_t min_ = N;
|
|
size_t positive_count_ = 0;
|
|
size_t samples_ = 0;
|
|
size_t max_pos_run_ = 0;
|
|
size_t cur_pos_run_ = 0;
|
|
size_t max_neg_run_ = 0;
|
|
size_t cur_neg_run_ = 0;
|
|
std::bitset<N> recent_;
|
|
|
|
static std::string PercentStr(size_t a, size_t b) {
|
|
return std::to_string(uint64_t{100} * a / b) + "%";
|
|
}
|
|
};
|
|
|
|
template <class Table>
|
|
void BaseHyperClockCache<Table>::ReportProblems(
|
|
const std::shared_ptr<Logger>& info_log) const {
|
|
if (info_log->GetInfoLogLevel() <= InfoLogLevel::DEBUG_LEVEL) {
|
|
LoadVarianceStats slot_stats;
|
|
this->ForEachShard([&](const BaseHyperClockCache<Table>::Shard* shard) {
|
|
size_t count = shard->GetTableAddressCount();
|
|
for (size_t i = 0; i < count; ++i) {
|
|
slot_stats.Add(IsSlotOccupied(*shard->GetTable().HandlePtr(i)));
|
|
}
|
|
});
|
|
ROCKS_LOG_AT_LEVEL(info_log, InfoLogLevel::DEBUG_LEVEL,
|
|
"Slot occupancy stats: %s", slot_stats.Report().c_str());
|
|
}
|
|
}
|
|
|
|
void FixedHyperClockCache::ReportProblems(
|
|
const std::shared_ptr<Logger>& info_log) const {
|
|
BaseHyperClockCache::ReportProblems(info_log);
|
|
|
|
uint32_t shard_count = GetNumShards();
|
|
std::vector<double> predicted_load_factors;
|
|
size_t min_recommendation = SIZE_MAX;
|
|
ForEachShard([&](const FixedHyperClockCache::Shard* shard) {
|
|
AddShardEvaluation(*shard, predicted_load_factors, min_recommendation);
|
|
});
|
|
|
|
if (predicted_load_factors.empty()) {
|
|
// None operating "at limit" -> nothing to report
|
|
return;
|
|
}
|
|
std::sort(predicted_load_factors.begin(), predicted_load_factors.end());
|
|
|
|
// First, if the average load factor is within spec, we aren't going to
|
|
// complain about a few shards being out of spec.
|
|
// NOTE: this is only the average among cache shards operating "at limit,"
|
|
// which should be representative of what we care about. It it normal, even
|
|
// desirable, for a cache to operate "at limit" so this should not create
|
|
// selection bias. See AddShardEvaluation().
|
|
// TODO: Consider detecting cases where decreasing the number of shards
|
|
// would be good, e.g. serious imbalance among shards.
|
|
double average_load_factor =
|
|
std::accumulate(predicted_load_factors.begin(),
|
|
predicted_load_factors.end(), 0.0) /
|
|
shard_count;
|
|
|
|
constexpr double kLowSpecLoadFactor = FixedHyperClockTable::kLoadFactor / 2;
|
|
constexpr double kMidSpecLoadFactor =
|
|
FixedHyperClockTable::kLoadFactor / 1.414;
|
|
if (average_load_factor > FixedHyperClockTable::kLoadFactor) {
|
|
// Out of spec => Consider reporting load factor too high
|
|
// Estimate effective overall capacity loss due to enforcing occupancy limit
|
|
double lost_portion = 0.0;
|
|
int over_count = 0;
|
|
for (double lf : predicted_load_factors) {
|
|
if (lf > FixedHyperClockTable::kStrictLoadFactor) {
|
|
++over_count;
|
|
lost_portion +=
|
|
(lf - FixedHyperClockTable::kStrictLoadFactor) / lf / shard_count;
|
|
}
|
|
}
|
|
// >= 20% loss -> error
|
|
// >= 10% loss -> consistent warning
|
|
// >= 1% loss -> intermittent warning
|
|
InfoLogLevel level = InfoLogLevel::INFO_LEVEL;
|
|
bool report = true;
|
|
if (lost_portion > 0.2) {
|
|
level = InfoLogLevel::ERROR_LEVEL;
|
|
} else if (lost_portion > 0.1) {
|
|
level = InfoLogLevel::WARN_LEVEL;
|
|
} else if (lost_portion > 0.01) {
|
|
int report_percent = static_cast<int>(lost_portion * 100.0);
|
|
if (Random::GetTLSInstance()->PercentTrue(report_percent)) {
|
|
level = InfoLogLevel::WARN_LEVEL;
|
|
}
|
|
} else {
|
|
// don't report
|
|
report = false;
|
|
}
|
|
if (report) {
|
|
ROCKS_LOG_AT_LEVEL(
|
|
info_log, level,
|
|
"FixedHyperClockCache@%p unable to use estimated %.1f%% capacity "
|
|
"because of full occupancy in %d/%u cache shards "
|
|
"(estimated_entry_charge too high). "
|
|
"Recommend estimated_entry_charge=%zu",
|
|
this, lost_portion * 100.0, over_count, (unsigned)shard_count,
|
|
min_recommendation);
|
|
}
|
|
} else if (average_load_factor < kLowSpecLoadFactor) {
|
|
// Out of spec => Consider reporting load factor too low
|
|
// But cautiously because low is not as big of a problem.
|
|
|
|
// Only report if highest occupancy shard is also below
|
|
// spec and only if average is substantially out of spec
|
|
if (predicted_load_factors.back() < kLowSpecLoadFactor &&
|
|
average_load_factor < kLowSpecLoadFactor / 1.414) {
|
|
InfoLogLevel level = InfoLogLevel::INFO_LEVEL;
|
|
if (average_load_factor < kLowSpecLoadFactor / 2) {
|
|
level = InfoLogLevel::WARN_LEVEL;
|
|
}
|
|
ROCKS_LOG_AT_LEVEL(
|
|
info_log, level,
|
|
"FixedHyperClockCache@%p table has low occupancy at full capacity. "
|
|
"Higher estimated_entry_charge (about %.1fx) would likely improve "
|
|
"performance. Recommend estimated_entry_charge=%zu",
|
|
this, kMidSpecLoadFactor / average_load_factor, min_recommendation);
|
|
}
|
|
}
|
|
}
|
|
|
|
} // namespace clock_cache
|
|
|
|
// DEPRECATED (see public API)
|
|
std::shared_ptr<Cache> NewClockCache(
|
|
size_t capacity, int num_shard_bits, bool strict_capacity_limit,
|
|
CacheMetadataChargePolicy metadata_charge_policy) {
|
|
return NewLRUCache(capacity, num_shard_bits, strict_capacity_limit,
|
|
/* high_pri_pool_ratio */ 0.5, nullptr,
|
|
kDefaultToAdaptiveMutex, metadata_charge_policy,
|
|
/* low_pri_pool_ratio */ 0.0);
|
|
}
|
|
|
|
std::shared_ptr<Cache> HyperClockCacheOptions::MakeSharedCache() const {
|
|
// For sanitized options
|
|
HyperClockCacheOptions opts = *this;
|
|
if (opts.num_shard_bits >= 20) {
|
|
return nullptr; // The cache cannot be sharded into too many fine pieces.
|
|
}
|
|
if (opts.num_shard_bits < 0) {
|
|
// Use larger shard size to reduce risk of large entries clustering
|
|
// or skewing individual shards.
|
|
constexpr size_t min_shard_size = 32U * 1024U * 1024U;
|
|
opts.num_shard_bits =
|
|
GetDefaultCacheShardBits(opts.capacity, min_shard_size);
|
|
}
|
|
std::shared_ptr<Cache> cache;
|
|
if (opts.estimated_entry_charge == 0) {
|
|
// BEGIN placeholder logic to be removed
|
|
// This is sufficient to get the placeholder Auto working in unit tests
|
|
// much like the Fixed version.
|
|
opts.estimated_entry_charge = opts.min_avg_entry_charge;
|
|
// END placeholder logic to be removed
|
|
cache = std::make_shared<clock_cache::AutoHyperClockCache>(opts);
|
|
} else {
|
|
cache = std::make_shared<clock_cache::FixedHyperClockCache>(opts);
|
|
}
|
|
if (opts.secondary_cache) {
|
|
cache = std::make_shared<CacheWithSecondaryAdapter>(cache,
|
|
opts.secondary_cache);
|
|
}
|
|
return cache;
|
|
}
|
|
|
|
} // namespace ROCKSDB_NAMESPACE
|