linux-kernel - Re: [patch, v2.6.22-rc6] sys

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Message-ID: <20070626152629.GA3342@elte.hu>
Date:	Tue, 26 Jun 2007 17:26:29 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, John Stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch, v2.6.22-rc6] sys_time() speedup

* Andrew Morton <akpm@...ux-foundation.org> wrote:

> > > the patch improves the sysbench OLTP macrobenchmark significantly:
> > 
> > Has that any real practical relevance?
> 
> Interesting question. [...]

i'm missing the <sarcastic> tag i guess ;-)

<sarcastic> Oh my, does database macro-performance have any relevance to 
Linux bread and butter markets in general. Boggle, it is a really 
difficult question i suspect. </sarcastic>

If we ignore those few million database and web server Linux boxes on
the market and concentrate purely on the few m68k boxes that are still
in existance, _then_ we might be doubtful about this question ;-)

> [...] The patch adds a new test-n-branch to gettimeofday() so if 
> gettimeofday() is used much more frequently than time(), we lose.

given that the cost to sys_gettimeofday() is less than a cycle (we test
a value already in a register, with an unlikely hint), and the benefit
to sys_time() is around 6000 cycles (or more), sys_gettimeofday() would
have to be used thousands of times more frequently than sys_time() -
which it clearly isnt. As a test i just triggered a really X-intense
workload and for that gettimeofday-dominated landscape there was still 1
sys_time() call for every 50 gettimeofday calls - so it's a small win
even for this X workload.

	Ingo
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