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Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/15428 RocksDB counts per-operation statistics (blocks read, bytes read, mutex waits, timings) in `perf_context`. It is a `thread_local` variable: each thread has its own copy, so threads never contend on it. This per-thread copy is "thread-local storage" (TLS). **Problem**: `PerfContext` has a non-trivial destructor. For such a variable, C++ must make sure the calling thread's copy is constructed before each use from another source file, so the compiler emits a call to a small "TLS init" function. That call goes through the PLT (procedure linkage table, the jump table used to call functions that may live in another shared library). Every `PERF_COUNTER_ADD` therefore paid a real function call before incrementing a counter, and these macros run in hot loops such as key comparisons during block iteration. `perf_level`, the on/off switch, has a smaller cost: it is `extern thread_local` without `constinit`, so each read also tests whether it has been initialized. **Fix**: - The perf macros read a `constinit thread_local PerfContext*` that each thread sets on first use. After that, a counter update is a pointer load, a null check and the increment. - `perf_level` is declared and defined `constinit`, so reading it is a plain thread-local load. - No public API change. `perf_context`, `get_perf_context()` and macro behavior are unchanged, and so is thread-exit behavior. **Results**: - Microbenchmark (`PERF_COUNTER_ADD` in a loop): 2.10 -> 0.84 ns per update with counting enabled; unchanged at 0.42 ns with counting disabled. - Calls to the TLS init functions drop from 40 and 96 to 0 in block.cc.o and memtable.cc.o. - In CPU profiles of large production RocksDB deployments, these accessors cost roughly 0.3-0.6% of total CPU. This change removes most of the `perf_context` part. Reviewed By: xingbowang Differential Revision: D125982209 fbshipit-source-id: f38b4fffda09611d28322e7534b135fb35631389
15 lines
452 B
C++
15 lines
452 B
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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#pragma once
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#include "port/port.h"
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#include "rocksdb/perf_level.h"
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namespace ROCKSDB_NAMESPACE {
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extern thread_local constinit PerfLevel perf_level;
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} // namespace ROCKSDB_NAMESPACE
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