Josh Kang
a7ae6cd0a8
Summary: Pull Request resolved: https://github.com/meta/rocksdb/pull/15407 X-link: https://github.com/facebook/rocksdb/pull/15407 ## Summary A full clang-tidy scan of RocksDB reports 65 dead-code findings in existing code: - `clang-analyzer-deadcode.DeadStores` (20) - `-Wunused-member-function` (24) - `-Wunreachable-code-return` / `-Wunreachable-code-loop-increment` (6) - `-Wmissing-noreturn` (9) - `-Wextra-semi` (6) Every PR that touches nearby lines gets flagged for them, and some hide real mistakes (an unchecked result, a loop that can only run once). This diff fixes 57 of them. What changed: - Unused private and file-local member functions are removed, along with members that only fed them and local classes that are never instantiated. Examples: `EmptyIterator` in the hash memtable reps, `PersistentCacheFromCache` in `db_block_cache_test.cc`, and `LegacyFileSystemWrapper::target()`. Each one was confirmed unused in every build configuration and is file-local or private. - Unreachable code after `return`, after a `throw`, or after an infinite loop is removed, but only where every compiler still sees every path returning. A loop that always returned on its first pass is now an `if`. A trailing return after `#ifdef` branches is now an `#else`/`#endif`. - Dead stores are removed. Google Benchmark loop variables are marked `[[maybe_unused]]` (`for ([[maybe_unused]] auto _ : state)`). - `[[noreturn]]` is added to functions that never return, such as `ErrorExit` in `db_bench`. - Stray `;` after member function bodies are removed. Not changed: `[[noreturn]]` was not added to `NoopDelete` helpers whose body is `assert(false)`, because they return in release builds. It was also not added to `MemTableRep::InsertConcurrently` (public virtual) or to `LDBTool::Run` (public, deprecated), and lambdas have no portable C++20 syntax for it. Reviewed By: xingbowang Differential Revision: D123186311 fbshipit-source-id: 378348c02a494aef6786827e3d18b44cbd280a04
RocksDB Micro-Benchmark
Overview
RocksDB micro-benchmark is a set of tests for benchmarking a single component or simple DB operations. The test artificially generates input data and executes the same operation with it to collect and report performance metrics. As it's focusing on testing a single, well-defined operation, the result is more precise and reproducible, which also has its limitation of not representing a real production use case. The test author needs to carefully design the microbench to represent its true purpose.
The tests are based on Google Benchmark library, which provides a standard framework for writing benchmarks.
How to Run
Prerequisite
Install the Google Benchmark version 1.6.0 or above.
Note: Google Benchmark 1.6.x is incompatible with previous versions like 1.5.x, please make sure you're using the newer version.
Build and Run
With Makefile:
$ DEBUG_LEVEL=0 make run_microbench
Or with cmake:
$ mkdir build && cd build && cmake .. -DCMAKE_BUILD_TYPE=Release -DWITH_BENCHMARK
$ make run_microbench
Note: Please run the benchmark code in release build.
Run Single Test
Example:
$ make db_basic_bench
$ ./db_basic_bench --benchmark_filter=<TEST_NAME>
Best Practices
* Use the Same Test Directory Setting as Unittest
Most of the Micro-benchmark tests use the same test directory setup as unittest, so it could be overridden by:
$ TEST_TMPDIR=/mydata/tmp/ ./db_basic_bench --benchmark_filter=<TEST_NAME>
Please also follow that when designing new tests.
* Avoid Using Debug API
Even though micro-benchmark is a test, avoid using internal Debug API like TEST_WaitForRun() which is designed for unittest. As benchmark tests are designed for release build, don't use any of that.
* Pay Attention to Local Optimization
As a micro-benchmark is focusing on a single component or area, make sure it is a key part for impacting the overall application performance.
The compiler might be able to optimize the code that not the same way as the whole application, and if the test data input is simple and small, it may be able to all cached in CPU memory, which is leading to a wrong metric. Take these into consideration when designing the tests.
* Names of user-defined counters/metrics has to be [A-Za-z0-9_]
It's a restriction of the metrics collecting and reporting system RocksDB is using internally. It will also help integrate with more systems.
* Minimize the Metrics Variation
Try reducing the test result variation, one way to check that is running the test multiple times and check the CV (Coefficient of Variation) reported by gbenchmark.
$ ./db_basic_bench --benchmark_filter=<TEST_NAME> --benchmark_repetitions=10
...
<TEST_NAME>_cv 3.2%
RocksDB has background compaction jobs which may cause the test result to vary a lot. If the micro-benchmark is not purposely testing the operation while compaction is in progress, it should wait for the compaction to finish (db_impl->WaitForCompact()) or disable auto-compaction.