Re: Move PinBuffer and UnpinBuffer to atomics

On Tue, Apr 12, 2016 at 5:39 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:

> On 2016-04-11 14:40:29 -0700, Andres Freund wrote:
> > On 2016-04-11 12:17:20 -0700, Andres Freund 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.
> >
> > But I'm unclear why the magnitude of the effect depends on other
> > allocations. With the previously posted patches allPgXact is always
> > cacheline-aligned.
>
> I've spent considerable amount experimenting around this. The alignment
> of allPgXact does *not* apear to play a significant role; rather it
> apears to be the the "distance" between the allPgXact and pgprocno
> arrays.
>
> Alexander, could you post dmidecode output, and install numactl &
> numastat on the machine? I wonder if the box has cluster-on-die
> activated or not.

Dmidecode output is in the attachment. Numactl & numastat are installed.

> Do I see correctly that this is a system that could
> potentially have 8 sockets, but actually has only four? Because I see
> physical id : 3 in /proc/cpuinfo only going up to three (from zero),
> not 7? And there's only 144 processorcs, while each E7-8890 v3 should
> have 36 threads.
>

There are definitely 4 of used sockets. I'm not sure about potential count
though.

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