On Tue, Apr 12, 2016 at 3:48 PM, Alexander Korotkov <
a.korot...@postgrespro.ru> wrote:

> 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.

Yes, it seems generally it is a good idea, but not sure if it is a complete
fix for variation in performance we are seeing when we change shared memory
structures.  Andres suggested me on IM to take performance data on x86 m/c
by padding PGXACT and the data for the same is as below:

median of 3, 5-min runs

Client_Count/Patch_ver 8 64 128
HEAD 59708 329560 173655
PATCH 61480 379798 157580

Here, at 128 client-count the performance with patch still seems to have
variation.  The highest tps with patch (170363) is close to HEAD (175718).
This could be run-to-run variation, but I think it indicates that there are
more places where we might need such padding or may be optimize them, so
that they are aligned.

I can do some more experiments on similar lines, but I am out on vacation
and might not be able to access the m/c for 3-4 days.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com

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