Hui Xiao 719f5511f6 No file system prefetching when Options::compaction_readahead_size is 0 (#11887) ... Summary: **Context/Summary:** https://github.com/facebook/rocksdb/pull/11631 introduced `readahead()` system call for compaction read under non direct IO. When `Options::compaction_readahead_size` is 0, the `readahead()` will issued with a small size (i.e, the block size, by default 4KB) Benchmarks shows that such readahead() call regresses the compaction read compared with "no readahead()" case (see Test Plan for more). Therefore we decided to not issue such `readhead() ` when `Options::compaction_readahead_size` is 0. Pull Request resolved: https://github.com/facebook/rocksdb/pull/11887 Test Plan: Settings: `compaction_readahead_size = 0, use_direct_reads=false` Setup: ``` TEST_TMPDIR=../ ./db_bench -benchmarks=filluniquerandom -disable_auto_compactions=true -write_buffer_size=1048576 -compression_type=none -value_size=10240 && tar -cf ../dbbench.tar -C ../dbbench/ . ``` Run: ``` for i in $(seq 3); do rm -rf ../dbbench/ && mkdir -p ../dbbench/ && tar -xf ../dbbench.tar -C ../dbbench/ . && sudo bash -c 'sync && echo 3 > /proc/sys/vm/drop_caches' && TEST_TMPDIR=../ /usr/bin/time ./db_bench_{pre_PR11631|PR11631|PR11631_with_improvementPR11887} -benchmarks=compact -use_existing_db=true -db=../dbbench/ -disable_auto_compactions=true -compression_type=none ; done |& grep elapsed ``` pre-PR11631("no readahead()" case): PR11631: PR11631+this improvement: Reviewed By: ajkr Differential Revision: D49607266 Pulled By: hx235 fbshipit-source-id: 2efa0dc91bac3c11cc2be057c53d894645f683ef		2023-09-26 10:08:43 -07:00
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no_fs_prefetch_on_zero_compaction_readahead.md	No file system prefetching when Options::compaction_readahead_size is 0 (#11887)	2023-09-26 10:08:43 -07:00