From: Manfred Spraul <manfred@colorfullife.com>
To: Marc Colosimo <mcolosimo@mitre.org>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@postgresql.org
Subject: Re: tweaking MemSet() performance - 7.4.5
Date: Sat, 18 Sep 2004 17:38:36 +0200
Message-ID: <414C567C.3060503@colorfullife.com> (raw)
In-Reply-To: <E60A72E0-08E7-11D9-B617-000A95A5D8B2@mitre.org>
References: <8A6E7DA4-0654-11D9-800C-000A95A5D8B2@mitre.org>
<24165.1095450900@sss.pgh.pa.us>
<E60A72E0-08E7-11D9-B617-000A95A5D8B2@mitre.org>
Marc Colosimo wrote:
> Oops, I used the same setting as in the old hacking message (-O2, gcc
> 3.3). If I understand what you are saying, then it turns out yes, PG's
> MemSet is faster for smaller blocksizes (see below, between 32 and
> 64). I just replaced the whole MemSet with memset and it is not very
> low when I profile.
Could you check what the OS-X memset function does internally?
One trick to speed up memset it to bypass the cache and bulk-write
directly from write buffers to main memory. i386 cpus support that and
in microbenchmarks it's 3 times faster (or something like that).
Unfortunately it's a loss in real-world tests: Typically a structure is
initialized with memset and then immediately accessed. 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.
> I could squeeze more out of it if I spent more time trying to
> understand it (change MEMSET_LOOP_LIMIT to 32 and then add memset
> after that?). I'm now working one understanding Spin Locks and
> friends. Putting in a sync call (in s_lock.h) is really a time killer
> and bad for performance (it takes up 35 cycles).
>
That's the price you pay for weakly ordered memory access.
Linux on ppc uses eieio, on ppc64 lwsync is used. Could you check if
they are faster?
--
Manfred
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To: pgsql-hackers@postgresql.org
Cc: manfred@colorfullife.com, mcolosimo@mitre.org, tgl@sss.pgh.pa.us
Subject: Re: tweaking MemSet() performance - 7.4.5
In-Reply-To: <414C567C.3060503@colorfullife.com>
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