On Tue, Apr 12, 2016 at 12:40 AM, Andres Freund <and...@anarazel.de> wrote:

> I did get access to the machine (thanks!). My testing shows that
> performance is sensitive to various parameters influencing memory
> allocation. E.g. twiddling with max_connections changes
> performance. With max_connections=400 and the previous patches applied I
> get ~1220000 tps, with 402 ~1620000 tps.  This sorta confirms that we're
> dealing with an alignment/sharing related issue.
> Padding PGXACT to a full cache-line seems to take care of the largest
> part of the performance irregularity. I looked at perf profiles and saw
> that most cache misses stem from there, and that the percentage (not
> absolute amount!) changes between fast/slow settings.
> To me it makes intuitive sense why you'd want PGXACTs to be on separate
> cachelines - they're constantly dirtied via SnapshotResetXmin(). Indeed
> making it immediately return propels performance up to 1720000, without
> other changes. Additionally cacheline-padding PGXACT speeds things up to
> 1750000 tps.

It seems like padding PGXACT to a full cache-line is a great improvement.
We have not so many PGXACTs to care about bytes wasted to padding.  But
could it have another negative side-effect?

Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company

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