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0050a73a4f
Summary: This change standardizes on a new 16-byte cache key format for block cache (incl compressed and secondary) and persistent cache (but not table cache and row cache). The goal is a really fast cache key with practically ideal stability and uniqueness properties without external dependencies (e.g. from FileSystem). A fixed key size of 16 bytes should enable future optimizations to the concurrent hash table for block cache, which is a heavy CPU user / bottleneck, but there appears to be measurable performance improvement even with no changes to LRUCache. This change replaces a lot of disjointed and ugly code handling cache keys with calls to a simple, clean new internal API (cache_key.h). (Preserving the old cache key logic under an option would be very ugly and likely negate the performance gain of the new approach. Complete replacement carries some inherent risk, but I think that's acceptable with sufficient analysis and testing.) The scheme for encoding new cache keys is complicated but explained in cache_key.cc. Also: EndianSwapValue is moved to math.h to be next to other bit operations. (Explains some new include "math.h".) ReverseBits operation added and unit tests added to hash_test for both. Fixes https://github.com/facebook/rocksdb/issues/7405 (presuming a root cause) Pull Request resolved: https://github.com/facebook/rocksdb/pull/9126 Test Plan: ### Basic correctness Several tests needed updates to work with the new functionality, mostly because we are no longer relying on filesystem for stable cache keys so table builders & readers need more context info to agree on cache keys. This functionality is so core, a huge number of existing tests exercise the cache key functionality. ### Performance Create db with `TEST_TMPDIR=/dev/shm ./db_bench -bloom_bits=10 -benchmarks=fillrandom -num=3000000 -partition_index_and_filters` And test performance with `TEST_TMPDIR=/dev/shm ./db_bench -readonly -use_existing_db -bloom_bits=10 -benchmarks=readrandom -num=3000000 -duration=30 -cache_index_and_filter_blocks -cache_size=250000 -threads=4` using DEBUG_LEVEL=0 and simultaneous before & after runs. Before ops/sec, avg over 100 runs: 121924 After ops/sec, avg over 100 runs: 125385 (+2.8%) ### Collision probability I have built a tool, ./cache_bench -stress_cache_key to broadly simulate host-wide cache activity over many months, by making some pessimistic simplifying assumptions: * Every generated file has a cache entry for every byte offset in the file (contiguous range of cache keys) * All of every file is cached for its entire lifetime We use a simple table with skewed address assignment and replacement on address collision to simulate files coming & going, with quite a variance (super-Poisson) in ages. Some output with `./cache_bench -stress_cache_key -sck_keep_bits=40`: ``` Total cache or DBs size: 32TiB Writing 925.926 MiB/s or 76.2939TiB/day Multiply by 9.22337e+18 to correct for simulation losses (but still assume whole file cached) ``` These come from default settings of 2.5M files per day of 32 MB each, and `-sck_keep_bits=40` means that to represent a single file, we are only keeping 40 bits of the 128-bit cache key. With file size of 2\*\*25 contiguous keys (pessimistic), our simulation is about 2\*\*(128-40-25) or about 9 billion billion times more prone to collision than reality. More default assumptions, relatively pessimistic: * 100 DBs in same process (doesn't matter much) * Re-open DB in same process (new session ID related to old session ID) on average every 100 files generated * Restart process (all new session IDs unrelated to old) 24 times per day After enough data, we get a result at the end: ``` (keep 40 bits) 17 collisions after 2 x 90 days, est 10.5882 days between (9.76592e+19 corrected) ``` If we believe the (pessimistic) simulation and the mathematical generalization, we would need to run a billion machines all for 97 billion days to expect a cache key collision. To help verify that our generalization ("corrected") is robust, we can make our simulation more precise with `-sck_keep_bits=41` and `42`, which takes more running time to get enough data: ``` (keep 41 bits) 16 collisions after 4 x 90 days, est 22.5 days between (1.03763e+20 corrected) (keep 42 bits) 19 collisions after 10 x 90 days, est 47.3684 days between (1.09224e+20 corrected) ``` The generalized prediction still holds. With the `-sck_randomize` option, we can see that we are beating "random" cache keys (except offsets still non-randomized) by a modest amount (roughly 20x less collision prone than random), which should make us reasonably comfortable even in "degenerate" cases: ``` 197 collisions after 1 x 90 days, est 0.456853 days between (4.21372e+18 corrected) ``` I've run other tests to validate other conditions behave as expected, never behaving "worse than random" unless we start chopping off structured data. Reviewed By: zhichao-cao Differential Revision: D33171746 Pulled By: pdillinger fbshipit-source-id: f16a57e369ed37be5e7e33525ace848d0537c88f
72 lines
2.8 KiB
C++
72 lines
2.8 KiB
C++
// Copyright (c) Facebook, Inc. and its affiliates. 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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// This file is for functions that generate unique identifiers by
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// (at least in part) by extracting novel entropy or sources of uniqueness
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// from the execution environment. (By contrast, random.h is for algorithmic
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// pseudorandomness.)
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//
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// These functions could eventually migrate to public APIs, such as in Env.
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#pragma once
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#include <atomic>
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#include <cstdint>
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#include "rocksdb/rocksdb_namespace.h"
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namespace ROCKSDB_NAMESPACE {
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// Generates a new 128-bit identifier that is universally unique
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// (with high probability) for each call. The result is split into
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// two 64-bit pieces. This function has NOT been validated for use in
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// cryptography.
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//
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// This is used in generating DB session IDs and by Env::GenerateUniqueId
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// (used for DB IDENTITY) if the platform does not provide a generator of
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// RFC 4122 UUIDs or fails somehow. (Set exclude_port_uuid=true if this
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// function is used as a fallback for GenerateRfcUuid, because no need
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// trying it again.)
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void GenerateRawUniqueId(uint64_t* a, uint64_t* b,
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bool exclude_port_uuid = false);
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#ifndef NDEBUG
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// A version of above with options for challenge testing
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void TEST_GenerateRawUniqueId(uint64_t* a, uint64_t* b, bool exclude_port_uuid,
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bool exclude_env_details,
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bool exclude_random_device);
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#endif
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// Generates globally unique ids with lower probability of any collisions
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// vs. each unique id being independently random (GenerateRawUniqueId).
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// We call this "semi-structured" because between different
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// SemiStructuredUniqueIdGen objects, the IDs are separated by random
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// intervals (unstructured), but within a single SemiStructuredUniqueIdGen
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// object, the generated IDs are trivially related (structured). See
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// https://github.com/pdillinger/unique_id for how this improves probability
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// of no collision. In short, if we have n SemiStructuredUniqueIdGen
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// objects each generating m IDs, the first collision is expected at
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// around n = sqrt(2^128 / m), equivalently n * sqrt(m) = 2^64,
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// rather than n * m = 2^64 for fully random IDs.
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class SemiStructuredUniqueIdGen {
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public:
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// Initializes with random starting state (from GenerateRawUniqueId)
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SemiStructuredUniqueIdGen() { Reset(); }
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// Re-initializes, but not thread safe
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void Reset();
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// Assuming no fork(), `lower` is guaranteed unique from one call
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// to the next (thread safe).
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void GenerateNext(uint64_t* upper, uint64_t* lower);
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private:
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uint64_t base_upper_;
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uint64_t base_lower_;
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std::atomic<uint64_t> counter_;
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int64_t saved_process_id_;
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};
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} // namespace ROCKSDB_NAMESPACE
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