No file system prefetching when Options::compaction_readahead_size is 0

[hx235]

Context/Summary:

#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 when Options::compaction_readahead_size is 0 under non direct IO. (see Test Plan for more).

Options::compaction_readahead_size is 0 by default in 8.5. So such regression is undesirable for upgrading from 8.4 to 8.5. Therefore we decided to not issue such readhead() when Options::compaction_readahead_size is 0.

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 -compaction_readahead_size=0 -use_direct_reads=0 ; done |& grep elapsed

pre-PR11631("no readahead()" case):

0:34.39elapsed
0:34.16elapsed
0:33.60elapsed

PR11631 (call readahead(), even when compaction_readahead_size is 0):

2:08.09elapsed
2:10.51elapsed
2:06.93elapsed

PR11631+this improvement (no readahead() call when compaction_readahead_size is 0):

0:35.73elapsed 
0:33.46elapsed
0:35.58elapsed

[facebook-github-bot]

@hx235 has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[ajkr]

LGTM

[facebook-github-bot]

@hx235 merged this pull request in 719f551.