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Re: Wierd context-switching issue on Xeon

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: josh(at)agliodbs(dot)com
Cc: Joe Conway <mail(at)joeconway(dot)com>,"scott(dot)marlowe" <scott(dot)marlowe(at)ihs(dot)com>,Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>, lutzeb(at)aeccom(dot)com,pgsql-performance(at)postgresql(dot)org, Neil Conway <neilc(at)samurai(dot)com>
Subject: Re: Wierd context-switching issue on Xeon
Date: 2004-04-20 00:01:56
Message-ID: 1407.1082419316@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Here is a test case.  To set up, run the "test_setup.sql" script once;
then launch two copies of the "test_run.sql" script.  (For those of
you with more than two CPUs, see whether you need one per CPU to make
trouble, or whether two test_runs are enough.)  Check that you get a
nestloops-with-index-scans plan shown by the EXPLAIN in test_run.

In isolation, test_run.sql should do essentially no syscalls at all once
it's past the initial ramp-up.  On a machine that's functioning per
expectations, multiple copies of test_run show a relatively low rate of
semop() calls --- a few per second, at most --- and maybe a delaying
select() here and there.

What I actually see on Josh's client's machine is a context swap storm:
"vmstat 1" shows CS rates around 170K/sec.  strace'ing the backends
shows a corresponding rate of semop() syscalls, with a few delaying
select()s sprinkled in.  top(1) shows system CPU percent of 25-30
and idle CPU percent of 16-20.

I haven't bothered to check how long the test_run query takes, but if it
ends while you're still examining the behavior, just start it again.

Note the test case assumes you've got shared_buffers set to at least
1000; with smaller values, you may get some I/O syscalls, which will
probably skew the results.

			regards, tom lane

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Subject: Re: Wierd context-switching issue on Xeon
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