To: Marc Colosimo <mcolosimo@mitre.org>
Cc: pgsql-hackers@postgresql.org
Subject: Re: tweaking MemSet() performance - 7.4.5 
In-reply-to: <8A6E7DA4-0654-11D9-800C-000A95A5D8B2@mitre.org> 
References: <8A6E7DA4-0654-11D9-800C-000A95A5D8B2@mitre.org>
Comments: In-reply-to Marc Colosimo <mcolosimo@mitre.org>
	message dated "Tue, 14 Sep 2004 09:46:55 -0400"
Date: Fri, 17 Sep 2004 15:55:00 -0400
Message-ID: <24165.1095450900@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Marc Colosimo <mcolosimo@mitre.org> writes:
> I'm using 7.4.5 on Mac OS X (G5) and was profiling it to see why it is 
> SO SLOW at committing  inserts and deletes into a large database. One 
> of the many slowdowns was from MemSet. I found an old (2002) thread 
> about this and retried the tests  (see below). The main point is that 
> the system memset crushes pg's!!

Hmm.  I tried to duplicate this on my G4 laptop, and found that they
were more or less on a par for small-to-middling block sizes (using
"gcc -O2").  Darwin's memset code must have some additional tweaks for
use on G5 hardware.  Good for Apple --- this is the sort of thing that
OS vendors *ought* to be doing.  The fact that we can beat the system
memset on so many platforms is an indictment of those platforms.

> Is it possible to add a define to call 
> the system memset at build time! This probably isn't the case on other 
> systems.

Feel free to hack the definition of MemSet in src/include/c.h.  See the
comments for it for more context.

Note that for small compile-time-constant block sizes (a case your test
program doesn't test, but it's common in pgsql), gcc with a sufficiently
high optimization setting can unroll the loop into a linear sequence of
words zeroings.  I would expect that to beat the system memset up to a
few dozen words, no matter how tense the memset coding is.  So you
probably want to think in terms of reducing MEMSET_LOOP_LIMIT rather
than diking out the macro code altogether.  Or maybe reduce MemSet to
"memset(...)" but leave MemSetAligned and/or MemSetTest/MemSetLoop
as-is.  In any case, reporting results without mentioning the compiler
and optimization level in use isn't going to convince anybody ...

			regards, tom lane