From owner-freebsd-performance@FreeBSD.ORG  Tue Apr 10 22:26:38 2007
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To: Kris Kennaway <kris@obsecurity.org>
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Comments: In-reply-to Kris Kennaway <kris@obsecurity.org>
	message dated "Tue, 10 Apr 2007 18:09:24 -0400"
Date: Tue, 10 Apr 2007 18:26:37 -0400
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Cc: performance@FreeBSD.org, current@FreeBSD.org,
	Mark Kirkwood <markir@paradise.net.nz>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Anyone interested in improving postgresql scaling? 
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Kris Kennaway <kris@obsecurity.org> writes:
> On Tue, Apr 10, 2007 at 05:36:17PM -0400, Tom Lane wrote:
>> Anyway I'd be interested to know what the test case is, and which PG
>> version you were testing.

> I used 8.2 (and some older version when I first noticed it a year ago)
> and either sysbench or supersmack will show it - presumably anything
> that makes simultaneous queries.  Just instrument sleepq_broadcast()
> to e.g. log a KTR event when it wakes more than 1 process and you'll
> see it happening.

Sorry, I'm not much of a BSD kernel hacker ... but sleepq_broadcast
seems a rather generic name.  Is that called *only* from semop?
I'm wondering if you are seeing simultaneous wakeup from some other
cause --- sleep timeout being the obvious possibility.  We are aware
of behaviors (search the PG lists for "context swap storm") where a
number of backends will all fail to get a spinlock and do short usleep
or select-timeout waits.  In this situation they'd all wake up at the
next scheduler clock tick ...

			regards, tom lane