Re: Possible performance regression in version 10.1 with pgbench read-write tests.

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: Mithun Cy <mithun(dot)cy(at)enterprisedb(dot)com>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Possible performance regression in version 10.1 with pgbench read-write tests.
Date: 2018-07-21 20:19:51
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Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2018-07-20 16:43:33 -0400, Tom Lane wrote:
>> On my RHEL6 machine, with unmodified HEAD and 8 sessions (since I've
>> only got 8 cores) but other parameters matching Mithun's example,
>> I just got

> It's *really* common to have more actual clients than cpus for oltp
> workloads, so I don't think it's insane to test with more clients.

I finished a set of runs using similar parameters to Mithun's test except
for using 8 clients, and another set using 72 clients (but, being
impatient, 5-minute runtime) just to verify that the results wouldn't
be markedly different. I got TPS numbers like this:

8 clients 72 clients

unmodified HEAD 16112 16284
with padding patch 16096 16283
with SysV semas 15926 16064
with padding+SysV 15949 16085

This is on RHEL6 (kernel 2.6.32-754.2.1.el6.x86_64), hardware is dual
4-core Intel E5-2609 (Sandy Bridge era). This hardware does show NUMA
effects, although no doubt less strongly than Mithun's machine.

I would like to see some other results with a newer kernel. I tried to
repeat this test on a laptop running Fedora 28, but soon concluded that
anything beyond very short runs was mainly going to tell me about thermal
throttling :-(. I could possibly get repeatable numbers from, say,
1-minute SELECT-only runs, but that would be a different test scenario,
likely one with a lot less lock contention.

Anyway, for me, the padding change is a don't-care. Given that both
Mithun and Thomas showed some positive effect from it, I'm not averse
to applying it. I'm still -1 on going back to SysV semas.

regards, tom lane

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