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rocksdb/cache/clock_cache.cc

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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.
#include "cache/clock_cache.h"
#include <algorithm>
#include <atomic>
#include <bitset>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <exception>
#include <functional>
#include <numeric>
#include <string>
#include <thread>
#include <type_traits>
#include "cache/cache_key.h"
#include "cache/secondary_cache_adapter.h"
#include "logging/logging.h"
#include "monitoring/perf_context_imp.h"
#include "monitoring/statistics_impl.h"
#include "port/lang.h"
#include "rocksdb/env.h"
#include "util/hash.h"
#include "util/math.h"
#include "util/random.h"
namespace ROCKSDB_NAMESPACE {
namespace clock_cache {
namespace {
inline uint64_t GetRefcount(uint64_t meta) {
return ((meta >> ClockHandle::kAcquireCounterShift) -
(meta >> ClockHandle::kReleaseCounterShift)) &
ClockHandle::kCounterMask;
}
inline uint64_t GetInitialCountdown(Cache::Priority priority) {
// Set initial clock data from priority
// TODO: configuration parameters for priority handling and clock cycle
// count?
switch (priority) {
case Cache::Priority::HIGH:
return ClockHandle::kHighCountdown;
default:
assert(false);
FALLTHROUGH_INTENDED;
case Cache::Priority::LOW:
return ClockHandle::kLowCountdown;
case Cache::Priority::BOTTOM:
return ClockHandle::kBottomCountdown;
}
}
inline void MarkEmpty(ClockHandle& h) {
#ifndef NDEBUG
// Mark slot as empty, with assertion
uint64_t meta = h.meta.exchange(0, std::memory_order_release);
assert(meta >> ClockHandle::kStateShift == ClockHandle::kStateConstruction);
#else
// Mark slot as empty
h.meta.store(0, std::memory_order_release);
#endif
}
inline void FreeDataMarkEmpty(ClockHandle& h, MemoryAllocator* allocator) {
// NOTE: in theory there's more room for parallelism if we copy the handle
// data and delay actions like this until after marking the entry as empty,
// but performance tests only show a regression by copying the few words
// of data.
h.FreeData(allocator);
MarkEmpty(h);
}
// Called to undo the effect of referencing an entry for internal purposes,
// so it should not be marked as having been used.
inline void Unref(const ClockHandle& h, uint64_t count = 1) {
// Pretend we never took the reference
// WART: there's a tiny chance we release last ref to invisible
// entry here. If that happens, we let eviction take care of it.
uint64_t old_meta = h.meta.fetch_sub(ClockHandle::kAcquireIncrement * count,
std::memory_order_release);
assert(GetRefcount(old_meta) != 0);
(void)old_meta;
}
inline bool ClockUpdate(ClockHandle& h) {
uint64_t meta = h.meta.load(std::memory_order_relaxed);
uint64_t acquire_count =
(meta >> ClockHandle::kAcquireCounterShift) & ClockHandle::kCounterMask;
uint64_t release_count =
(meta >> ClockHandle::kReleaseCounterShift) & ClockHandle::kCounterMask;
if (acquire_count != release_count) {
// Only clock update entries with no outstanding refs
return false;
}
if (!((meta >> ClockHandle::kStateShift) & ClockHandle::kStateShareableBit)) {
// Only clock update Shareable entries
return false;
}
if ((meta >> ClockHandle::kStateShift == ClockHandle::kStateVisible) &&
acquire_count > 0) {
// Decrement clock
uint64_t new_count =
std::min(acquire_count - 1, uint64_t{ClockHandle::kMaxCountdown} - 1);
// Compare-exchange in the decremented clock info, but
// not aggressively
uint64_t new_meta =
(uint64_t{ClockHandle::kStateVisible} << ClockHandle::kStateShift) |
(new_count << ClockHandle::kReleaseCounterShift) |
(new_count << ClockHandle::kAcquireCounterShift);
h.meta.compare_exchange_strong(meta, new_meta, std::memory_order_relaxed);
return false;
}
// Otherwise, remove entry (either unreferenced invisible or
// unreferenced and expired visible).
if (h.meta.compare_exchange_strong(
meta,
uint64_t{ClockHandle::kStateConstruction} << ClockHandle::kStateShift,
std::memory_order_acquire)) {
// Took ownership.
return true;
} else {
// Compare-exchange failing probably
// indicates the entry was used, so skip it in that case.
return false;
}
}
// If an entry doesn't receive clock updates but is repeatedly referenced &
// released, the acquire and release counters could overflow without some
// intervention. This is that intervention, which should be inexpensive
// because it only incurs a simple, very predictable check. (Applying a bit
// mask in addition to an increment to every Release likely would be
// relatively expensive, because it's an extra atomic update.)
//
// We do have to assume that we never have many millions of simultaneous
// references to a cache handle, because we cannot represent so many
// references with the difference in counters, masked to the number of
// counter bits. Similarly, we assume there aren't millions of threads
// holding transient references (which might be "undone" rather than
// released by the way).
//
// Consider these possible states for each counter:
// low: less than kMaxCountdown
// medium: kMaxCountdown to half way to overflow + kMaxCountdown
// high: half way to overflow + kMaxCountdown, or greater
//
// And these possible states for the combination of counters:
// acquire / release
// ------- -------
// low low - Normal / common, with caveats (see below)
// medium low - Can happen while holding some refs
// high low - Violates assumptions (too many refs)
// low medium - Violates assumptions (refs underflow, etc.)
// medium medium - Normal (very read heavy cache)
// high medium - Can happen while holding some refs
// low high - This function is supposed to prevent
// medium high - Violates assumptions (refs underflow, etc.)
// high high - Needs CorrectNearOverflow
//
// Basically, this function detects (high, high) state (inferred from
// release alone being high) and bumps it back down to (medium, medium)
// state with the same refcount and the same logical countdown counter
// (everything > kMaxCountdown is logically the same). Note that bumping
// down to (low, low) would modify the countdown counter, so is "reserved"
// in a sense.
//
// If near-overflow correction is triggered here, there's no guarantee
// that another thread hasn't freed the entry and replaced it with another.
// Therefore, it must be the case that the correction does not affect
// entries unless they are very old (many millions of acquire-release cycles).
// (Our bit manipulation is indeed idempotent and only affects entries in
// exceptional cases.) We assume a pre-empted thread will not stall that long.
// If it did, the state could be corrupted in the (unlikely) case that the top
// bit of the acquire counter is set but not the release counter, and thus
// we only clear the top bit of the acquire counter on resumption. It would
// then appear that there are too many refs and the entry would be permanently
// pinned (which is not terrible for an exceptionally rare occurrence), unless
// it is referenced enough (at least kMaxCountdown more times) for the release
// counter to reach "high" state again and bumped back to "medium." (This
// motivates only checking for release counter in high state, not both in high
// state.)
inline void CorrectNearOverflow(uint64_t old_meta,
std::atomic<uint64_t>& meta) {
// We clear both top-most counter bits at the same time.
constexpr uint64_t kCounterTopBit = uint64_t{1}
<< (ClockHandle::kCounterNumBits - 1);
constexpr uint64_t kClearBits =
(kCounterTopBit << ClockHandle::kAcquireCounterShift) |
(kCounterTopBit << ClockHandle::kReleaseCounterShift);
// A simple check that allows us to initiate clearing the top bits for
// a large portion of the "high" state space on release counter.
constexpr uint64_t kCheckBits =
(kCounterTopBit | (ClockHandle::kMaxCountdown + 1))
<< ClockHandle::kReleaseCounterShift;
if (UNLIKELY(old_meta & kCheckBits)) {
meta.fetch_and(~kClearBits, std::memory_order_relaxed);
}
}
inline bool BeginSlotInsert(const ClockHandleBasicData& proto, ClockHandle& h,
uint64_t initial_countdown, bool* already_matches) {
assert(*already_matches == false);
// Optimistically transition the slot from "empty" to
// "under construction" (no effect on other states)
uint64_t old_meta = h.meta.fetch_or(
uint64_t{ClockHandle::kStateOccupiedBit} << ClockHandle::kStateShift,
std::memory_order_acq_rel);
uint64_t old_state = old_meta >> ClockHandle::kStateShift;
if (old_state == ClockHandle::kStateEmpty) {
// We've started inserting into an available slot, and taken
// ownership.
return true;
} else if (old_state != ClockHandle::kStateVisible) {
// Slot not usable / touchable now
return false;
}
// Existing, visible entry, which might be a match.
// But first, we need to acquire a ref to read it. In fact, number of
// refs for initial countdown, so that we boost the clock state if
// this is a match.
old_meta =
h.meta.fetch_add(ClockHandle::kAcquireIncrement * initial_countdown,
std::memory_order_acq_rel);
// Like Lookup
if ((old_meta >> ClockHandle::kStateShift) == ClockHandle::kStateVisible) {
// Acquired a read reference
if (h.hashed_key == proto.hashed_key) {
// Match. Release in a way that boosts the clock state
old_meta =
h.meta.fetch_add(ClockHandle::kReleaseIncrement * initial_countdown,
std::memory_order_acq_rel);
// Correct for possible (but rare) overflow
CorrectNearOverflow(old_meta, h.meta);
// Insert detached instead (only if return handle needed)
*already_matches = true;
return false;
} else {
// Mismatch.
Unref(h, initial_countdown);
}
} else if (UNLIKELY((old_meta >> ClockHandle::kStateShift) ==
ClockHandle::kStateInvisible)) {
// Pretend we never took the reference
Unref(h, initial_countdown);
} else {
// For other states, incrementing the acquire counter has no effect
// so we don't need to undo it.
// Slot not usable / touchable now.
}
return false;
}
inline void FinishSlotInsert(const ClockHandleBasicData& proto, ClockHandle& h,
uint64_t initial_countdown, bool keep_ref) {
// Save data fields
ClockHandleBasicData* h_alias = &h;
*h_alias = proto;
// Transition from "under construction" state to "visible" state
uint64_t new_meta = uint64_t{ClockHandle::kStateVisible}
<< ClockHandle::kStateShift;
// Maybe with an outstanding reference
new_meta |= initial_countdown << ClockHandle::kAcquireCounterShift;
new_meta |= (initial_countdown - keep_ref)
<< ClockHandle::kReleaseCounterShift;
#ifndef NDEBUG
// Save the state transition, with assertion
uint64_t old_meta = h.meta.exchange(new_meta, std::memory_order_release);
assert(old_meta >> ClockHandle::kStateShift ==
ClockHandle::kStateConstruction);
#else
// Save the state transition
h.meta.store(new_meta, std::memory_order_release);
#endif
}
bool TryInsert(const ClockHandleBasicData& proto, ClockHandle& h,
uint64_t initial_countdown, bool keep_ref,
bool* already_matches) {
bool b = BeginSlotInsert(proto, h, initial_countdown, already_matches);
if (b) {
FinishSlotInsert(proto, h, initial_countdown, keep_ref);
}
return b;
}
// Func must be const HandleImpl& -> void callable
template <class HandleImpl, class Func>
void ConstApplyToEntriesRange(const Func& func, const HandleImpl* begin,
const HandleImpl* end,
bool apply_if_will_be_deleted) {
uint64_t check_state_mask = ClockHandle::kStateShareableBit;
if (!apply_if_will_be_deleted) {
check_state_mask |= ClockHandle::kStateVisibleBit;
}
for (const HandleImpl* h = begin; h < end; ++h) {
// Note: to avoid using compare_exchange, we have to be extra careful.
uint64_t old_meta = h->meta.load(std::memory_order_relaxed);
// Check if it's an entry visible to lookups
if ((old_meta >> ClockHandle::kStateShift) & check_state_mask) {
// Increment acquire counter. Note: it's possible that the entry has
// completely changed since we loaded old_meta, but incrementing acquire
// count is always safe. (Similar to optimistic Lookup here.)
old_meta = h->meta.fetch_add(ClockHandle::kAcquireIncrement,
std::memory_order_acquire);
// Check whether we actually acquired a reference.
if ((old_meta >> ClockHandle::kStateShift) &
ClockHandle::kStateShareableBit) {
// Apply func if appropriate
if ((old_meta >> ClockHandle::kStateShift) & check_state_mask) {
func(*h);
}
// Pretend we never took the reference
Unref(*h);
// No net change, so don't need to check for overflow
} 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.
}
}
}
}
} // namespace
void ClockHandleBasicData::FreeData(MemoryAllocator* allocator) const {
if (helper->del_cb) {
helper->del_cb(value, allocator);
}
}
template <class HandleImpl>
HandleImpl* BaseClockTable::StandaloneInsert(
const ClockHandleBasicData& proto) {
// Heap allocated separate from table
HandleImpl* h = new HandleImpl();
ClockHandleBasicData* h_alias = h;
*h_alias = proto;
h->SetStandalone();
// Single reference (standalone entries only created if returning a refed
// Handle back to user)
uint64_t meta = uint64_t{ClockHandle::kStateInvisible}
<< ClockHandle::kStateShift;
meta |= uint64_t{1} << ClockHandle::kAcquireCounterShift;
h->meta.store(meta, std::memory_order_release);
// Keep track of how much of usage is standalone
standalone_usage_.fetch_add(proto.GetTotalCharge(),
std::memory_order_relaxed);
return h;
}
template <class Table>
typename Table::HandleImpl* BaseClockTable::CreateStandalone(
ClockHandleBasicData& proto, size_t capacity, bool strict_capacity_limit,
bool allow_uncharged) {
Table& derived = static_cast<Table&>(*this);
typename Table::InsertState state;
derived.StartInsert(state);
const size_t total_charge = proto.GetTotalCharge();
if (strict_capacity_limit) {
Status s = ChargeUsageMaybeEvictStrict<Table>(
total_charge, capacity,
/*need_evict_for_occupancy=*/false, state);
if (!s.ok()) {
if (allow_uncharged) {
proto.total_charge = 0;
} else {
return nullptr;
}
}
} else {
// Case strict_capacity_limit == false
bool success = ChargeUsageMaybeEvictNonStrict<Table>(
total_charge, capacity,
/*need_evict_for_occupancy=*/false, state);
if (!success) {
// Force the issue
usage_.fetch_add(total_charge, std::memory_order_relaxed);
}
}
return StandaloneInsert<typename Table::HandleImpl>(proto);
}
template <class Table>
Status BaseClockTable::ChargeUsageMaybeEvictStrict(
size_t total_charge, size_t capacity, bool need_evict_for_occupancy,
typename Table::InsertState& state) {
if (total_charge > capacity) {
return Status::MemoryLimit(
"Cache entry too large for a single cache shard: " +
std::to_string(total_charge) + " > " + std::to_string(capacity));
}
// Grab any available capacity, and free up any more required.
size_t old_usage = usage_.load(std::memory_order_relaxed);
size_t new_usage;
do {
new_usage = std::min(capacity, old_usage + total_charge);
if (new_usage == old_usage) {
// No change needed
break;
}
} while (!usage_.compare_exchange_weak(old_usage, new_usage,
std::memory_order_relaxed));
// How much do we need to evict then?
size_t need_evict_charge = old_usage + total_charge - new_usage;
size_t request_evict_charge = need_evict_charge;
if (UNLIKELY(need_evict_for_occupancy) && request_evict_charge == 0) {
// Require at least 1 eviction.
request_evict_charge = 1;
}
if (request_evict_charge > 0) {
EvictionData data;
static_cast<Table*>(this)->Evict(request_evict_charge, state, &data);
occupancy_.fetch_sub(data.freed_count, std::memory_order_release);
if (LIKELY(data.freed_charge > need_evict_charge)) {
assert(data.freed_count > 0);
// Evicted more than enough
usage_.fetch_sub(data.freed_charge - need_evict_charge,
std::memory_order_relaxed);
} else if (data.freed_charge < need_evict_charge ||
(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));
}
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
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
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