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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
77 lines
2.4 KiB
C++
77 lines
2.4 KiB
C++
#pragma once
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#include <string>
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#include <unordered_set>
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#include <vector>
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#include "db/column_family.h"
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#include "db/external_sst_file_ingestion_job.h"
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#include "db/snapshot_impl.h"
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#include "options/db_options.h"
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#include "rocksdb/db.h"
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#include "rocksdb/metadata.h"
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#include "rocksdb/sst_file_writer.h"
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#include "util/autovector.h"
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namespace ROCKSDB_NAMESPACE {
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struct EnvOptions;
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class SystemClock;
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// Imports a set of sst files as is into a new column family. Logic is similar
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// to ExternalSstFileIngestionJob.
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class ImportColumnFamilyJob {
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public:
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ImportColumnFamilyJob(VersionSet* versions, ColumnFamilyData* cfd,
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const ImmutableDBOptions& db_options,
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const EnvOptions& env_options,
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const ImportColumnFamilyOptions& import_options,
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const std::vector<LiveFileMetaData>& metadata,
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const std::shared_ptr<IOTracer>& io_tracer)
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: clock_(db_options.clock),
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versions_(versions),
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cfd_(cfd),
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db_options_(db_options),
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fs_(db_options_.fs, io_tracer),
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env_options_(env_options),
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import_options_(import_options),
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metadata_(metadata),
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io_tracer_(io_tracer) {}
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// Prepare the job by copying external files into the DB.
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Status Prepare(uint64_t next_file_number, SuperVersion* sv);
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// Will execute the import job and prepare edit() to be applied.
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// REQUIRES: Mutex held
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Status Run();
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// Cleanup after successful/failed job
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void Cleanup(const Status& status);
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VersionEdit* edit() { return &edit_; }
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const autovector<IngestedFileInfo>& files_to_import() const {
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return files_to_import_;
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}
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private:
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// Open the external file and populate `file_to_import` with all the
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// external information we need to import this file.
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Status GetIngestedFileInfo(const std::string& external_file,
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uint64_t new_file_number,
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IngestedFileInfo* file_to_import,
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SuperVersion* sv);
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SystemClock* clock_;
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VersionSet* versions_;
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ColumnFamilyData* cfd_;
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const ImmutableDBOptions& db_options_;
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const FileSystemPtr fs_;
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const EnvOptions& env_options_;
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autovector<IngestedFileInfo> files_to_import_;
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VersionEdit edit_;
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const ImportColumnFamilyOptions& import_options_;
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std::vector<LiveFileMetaData> metadata_;
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const std::shared_ptr<IOTracer> io_tracer_;
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};
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} // namespace ROCKSDB_NAMESPACE
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