linux-kernel - Re: [PATCH] i386-pda UP optimization

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Message-ID: <456387F9.3010000@goop.org>
Date:	Tue, 21 Nov 2006 15:12:57 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	Andi Kleen <ak@...e.de>, Ingo Molnar <mingo@...e.hu>,
	akpm@...l.org, Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i386-pda UP optimization

Eric Dumazet wrote:
> for umask/getppid(), its a basic loop with 100.000.000 iterations

Ah, OK, so there's about 2.5-3.5ns difference due to the instructions
you removed.  That's very much in line with that I saw in my measurements.

> for read/write(), loop with 10.000.000 iterations

2 syscalls/iteration?  It's interesting you measured about the same
absolute time difference (.42s) even though you're doing 1/5th the
number of syscalls.

> elapsed time (/usr/bin/time ./prog)
> 10 runs, and the minimum time is taken.

Hm, but "time" measures user, system and real time.  You used real time?

> Hum... Do you mean a cache miss every time we do a syscall ? What
> could invalidate this cache exactly ?

Well, there might be a miss simply because the line got evicted.  But as
Andi pointed out, a hot benchmark like this is very unlikely to get any
cache misses unless there's something very unfortunate happening.

    J
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