Block cache tracing: Fix minor bugs with downsampling and some benchmark results. (#5473)
Summary: As the code changes for block cache tracing are almost complete, I did a benchmark to compare the performance when block cache tracing is enabled/disabled. With 1% downsampling ratio, the performance overhead of block cache tracing is negligible. When we trace all block accesses, the throughput drops by 6 folds with 16 threads issuing random reads and all reads are served in block cache. Setup: RocksDB: version 6.2 Date: Mon Jun 17 17:11:13 2019 CPU: 24 * Intel Core Processor (Skylake) CPUCache: 16384 KB Keys: 20 bytes each Values: 100 bytes each (100 bytes after compression) Entries: 10000000 Prefix: 20 bytes Keys per prefix: 0 RawSize: 1144.4 MB (estimated) FileSize: 1144.4 MB (estimated) Write rate: 0 bytes/second Read rate: 0 ops/second Compression: NoCompression Compression sampling rate: 0 Memtablerep: skip_list Perf Level: 1 I ran the readrandom workload for 1 minute. Detailed throughput results: (ops/second) Sample rate 0: no block cache tracing. Sample rate 1: trace all block accesses. Sample rate 100: trace accesses 1% blocks. 1 thread | | | -- | -- | -- | -- Sample rate | 0 | 1 | 100 1 MB block cache size | 13,094 | 13,166 | 13,341 10 GB block cache size | 202,243 | 188,677 | 229,182 16 threads | | | -- | -- | -- | -- Sample rate | 0 | 1 | 100 1 MB block cache size | 208,761 | 178,700 | 201,872 10 GB block cache size | 2,645,996 | 426,295 | 2,587,605 Pull Request resolved: https://github.com/facebook/rocksdb/pull/5473 Differential Revision: D15869479 Pulled By: HaoyuHuang fbshipit-source-id: 7ae802abe84811281a6af8649f489887cd7c4618
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