Re: shared-memory based stats collector

Hello.

At Mon, 21 Jan 2019 21:19:07 +0900 (Tokyo Standard Time), Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote in <20190121(dot)211907(dot)59625409(dot)horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
> I'll reconsider the referer side of the stats.

The most significant cause of the slowdown is repeated search for
non-existent entries both on local and shared hash each time.

Negative cache in addition to cache expiration interval
eliminates the slowdown.

1000 times repetition with -O2 binary:

master : 124.99 tps
patched: 125.48 tps (+0.4%)

> I didn't merge the suggested two pairs of commits. I'll do that
> after adressing the slowdown issue.

I agree to commiting 0001-0003 separately. In the attached patch
set, old 0004+0006 are merged as 0004 and old 0005+0007 are
merged as new 0005.

Changed cacheing policy.
Expired at every xact end
-> Keeps at least for PGSTAT_STAT_MIN_INTERVAL (500ms).

Added negative cache feature (snapshot_statentry).

Improved separation between pgstat and bestat (separated AtEOXact_* functions).

Fixed doubious memory context usage.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center