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pg_stat_statements: Avoid some locking during PGSS entry scans

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A single PGSS entry's spinlock is used to be able to modify "counters"
without holding pgss->lock exclusively, as mentioned at the top of
pg_stat_statements.c and within pgssEntry.

Within a single pgssEntry, stats_since and minmax_stats_since are never
modified without holding pgss->lock exclusively, so there is no need to
hold an entry's spinlock when reading stats_since and
minmax_stats_since, as done when scanning all the PGSS entries for
function calls of pg_stat_statements().

This also restores the consistency between the code and the comments
about the entry's spinlock usage.  This change is a performance
improvement (it can be argued that this is a logic bug), so there is no
need for a backpatch.  This saves two instructions from being read while
holding an entry's spinlock.

Author: Karina Litskevich
Reviewed-by: Michael Paquier, wenhui qiu
Discussion: https://postgr.es/m/CACiT8ibhCmzbcOxM0v4pRLH3abk-95LPkt7_uC2JMP+miPjxsg@mail.gmail.com
This commit is contained in:
Michael Paquier
2024-11-11 09:02:30 +09:00
parent 29d66b2d2f
commit 5d4298e75f
1 changed files with 6 additions and 1 deletions
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7
contrib/pg_stat_statements/pg_stat_statements.c
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@ -1869,9 +1869,14 @@ pg_stat_statements_internal(FunctionCallInfo fcinfo,
/* copy counters to a local variable to keep locking time short */ /* copy counters to a local variable to keep locking time short */
SpinLockAcquire(&entry->mutex); SpinLockAcquire(&entry->mutex);
tmp = entry->counters; tmp = entry->counters;
SpinLockRelease(&entry->mutex);
/*
* The spinlock is not required when reading these two as they are
* always updated when holding pgss->lock exclusively.
*/
stats_since = entry->stats_since; stats_since = entry->stats_since;
minmax_stats_since = entry->minmax_stats_since; minmax_stats_since = entry->minmax_stats_since;
SpinLockRelease(&entry->mutex);
/* Skip entry if unexecuted (ie, it's a pending "sticky" entry) */ /* Skip entry if unexecuted (ie, it's a pending "sticky" entry) */
if (IS_STICKY(tmp)) if (IS_STICKY(tmp))