Horton

Sub-millisecond [autovacuum_]vacuum_cost_delay broken

Melanie Plageman <melanieplageman@gmail.com>

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From: Melanie Plageman <melanieplageman@gmail.com>
To: Pg Hackers <pgsql-hackers@postgresql.org>
Cc: Stephen Frost <sfrost@snowman.net>, Thomas Munro <thomas.munro@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>
Date: 2023-03-09T21:26:02Z
Lists: pgsql-hackers 
Hi,

I think that 4753ef37e0ed undid the work caf626b2c did to support
sub-millisecond delays for vacuum and autovacuum.

After 4753ef37e0ed, vacuum_delay_point()'s local variable msec is a
double which, after being passed to WaitLatch() as timeout, which is a
long, ends up being 0, so we don't end up waiting AFAICT.

When I set [autovacuum_]vacuum_delay_point to 0.5, SHOW will report that
it is 500us, but WaitLatch() is still getting 0 as timeout.

- Melanie

Commits

  1. Use nanosleep() to implement pg_usleep().

  2. Update obsolete comment about pg_usleep() accuracy.

  3. Fix fractional vacuum_cost_delay.