Subject: Re: tweaking MemSet() performance - 7.4.5
From: Karel Zak <zakkr@zf.jcu.cz>
To: Manfred Spraul <manfred@colorfullife.com>
Cc: mcolosimo@smtp-bedford.mitre.org, Marc Colosimo <mcolosimo@mitre.org>,
	List pgsql-hackers <pgsql-hackers@postgreSQL.org>
In-Reply-To: <4155E1C1.4030503@colorfullife.com>
References: <22741566.1095718812574.JavaMail.ipsadmin@emiisrv>
	<4155E1C1.4030503@colorfullife.com>
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Date: Wed, 29 Sep 2004 10:46:18 +0200
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On Sat, 2004-09-25 at 23:23 +0200, Manfred Spraul wrote:
> mcolosimo@mitre.org wrote:
> 
> >>If the memset 
> >>bypasses the cache then the following access will cause a cache line 
> >>miss, which can be so slow that using the faster memset can result in a 
> >>net performance loss.
> >>
> >>    
> >>
> >
> >Could you suggest some structs to test? If I get your meaning, I would make a loop that sets then reads from the structure. 
> >
> >  
> >
> Read the sources and the cpu specs. Benchmarking such problems is 
> virtually impossible.
> I don't have OS-X, thus I checked the Linux-kernel sources: It seems 
> that the power architecture doesn't have the same problem as x86.
> There is a special clear cacheline instruction for large memsets and the 
> rest is done through carefully optimized store byte/halfword/word/double 
> word sequences.
> 
> Thus I'd check what happens if you memset not perfectly aligned buffers. 
> That's another point where over-optimized functions sometimes break 
> down. If there is no slowdown, then I'd replace the postgres function 
> with the OS provided function.
> 
> I'd add some __builtin_constant_p() optimizations, but I guess Tom won't 
> like gcc hacks ;-)

I think it cannot be problem if you write it to some .h file (in port
directory?) as macro with "#ifdef GCC". The other thing is real
advantage of hacks like this in practical PG usage :-)

	Karel

-- 
Karel Zak
http://home.zf.jcu.cz/~zakkr