[Fwd: performance with gcc -O0/-O2]

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[Fwd: performance with gcc -O0/-O2]
\ Howard Chu (27 Nov 2007)
. \ Richard Guenther (27 Nov 2007)
. . \ Tim Prince (27 Nov 2007)
. . \ Howard Chu (27 Nov 2007)
. \ Andrew Haley (27 Nov 2007)
. \ J.C. Pizarro (27 Nov 2007)
. . \ Howard Chu (27 Nov 2007)
. \ Andi Kleen (27 Nov 2007)
. . \ Andrew Haley (27 Nov 2007)

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Subject:	[Fwd: performance with gcc -O0/-O2]
Group:	Gcc
From:	Howard Chu
Date:	27 Nov 2007

A bit of a minor mystery. Not a problem, just a curiosity. If someone knew off
the top of their head a reason for it, that'd be cool, but otherwise no sweat.

-------- Original Message --------
Subject: Re: commit: ldap/servers/slapd connection.c daemon.c proto-slap.h
syncrepl.c
Date: Tue, 27 Nov 2007 05:17:04 -0800
From: Howard Chu <hyc>
To: OpenLDAP-devel
References: <200711261603.lAQG3R7e010741>
<474AFA54.6080805> <474B0620.8030706>
<474B92F5.50306>

Howard Chu wrote:
> Howard Chu wrote:
>> Howard Chu wrote:
>>> For reference, the peak throughput with back-null on the previous code was
>>> only 7,800 auths/sec (with 8 client threads). With this patch it's 11,140
>>> auths/sec.

Those numbers are for Windows Server 2003 x86_64 on a Celestica A8440 with 4
Opteron 875s, using OpenLDAP compiled with gcc 4.3.0. The following numbers
are for Linux 2.6.23.1 x86_64, on the same machine, compiled first with gcc
4.1.2 and then later with gcc 4.2.2. There's no disk I/O in these tests.

>>> In both cases the throughput declines as more client threads are
>>> used. (Compare to 35,553 auths/sec for the same machine running Linux, and no
>>> drop in throughput all the way up to hundreds/thousands of connections.)

> Re-running on Linux with a non-optimized build, peaked at 40,101 auths/sec. (I
> guess HEAD has sped up a bit more in the past week or so...)

OK, this is odd. The code compiled without optimization peaks at 40K auths/sec
at around 124-132 client threads. The code compiled with -O2 peaks at 37K sec
at around 128 client threads.

The -O2 build is faster from about 4 to 24 client threads. From 28 on up, the
nonoptimized code is faster at every load level. I was originally using gcc
4.1.2 but I'm seeing the same result now using gcc 4.2.2. Also, slapd is only
configured with 8 worker threads in all of these tests. Strange that whatever
optimizations the compiler has generated speeds things up for lighter load,
but works against it under heavier load.
--
-- Howard Chu
Chief Architect, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/