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pg_stat_wal: Accumulate time as instr_time instead of microseconds

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In instr_time.h it is stated that:

* When summing multiple measurements, it's recommended to leave the
* running sum in instr_time form (ie, use INSTR_TIME_ADD or
* INSTR_TIME_ACCUM_DIFF) and convert to a result format only at the end.

The reason for that is that converting to microseconds is not cheap, and can
loose precision.  Therefore this commit changes 'PendingWalStats' to use
'instr_time' instead of 'PgStat_Counter' while accumulating 'wal_write_time'
and 'wal_sync_time'.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/1feedb83-7aa9-cb4b-5086-598349d3f555@gmail.com
This commit is contained in:
Andres Freund
2023-03-30 14:23:14 -07:00
parent 122376f028
commit ca7b3c4c00
5 changed files with 37 additions and 24 deletions
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6
src/backend/access/transam/xlog.c
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@@ -2206,8 +2206,7 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
instr_time duration;
INSTR_TIME_SET_CURRENT(duration);
INSTR_TIME_SUBTRACT(duration, start);
PendingWalStats.wal_write_time += INSTR_TIME_GET_MICROSEC(duration);
INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_write_time, duration, start);
}
PendingWalStats.wal_write++;
@@ -8204,8 +8203,7 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
instr_time duration;
INSTR_TIME_SET_CURRENT(duration);
INSTR_TIME_SUBTRACT(duration, start);
PendingWalStats.wal_sync_time += INSTR_TIME_GET_MICROSEC(duration);
INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_sync_time, duration, start);
}
PendingWalStats.wal_sync++;