linux-kernel - Re: [REPORT] cfs-v5 vs sd-0.46

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Message-ID: <20070424085105.GA12329@elte.hu>
Date:	Tue, 24 Apr 2007 10:51:05 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Michael Gerdau <mgd@...hnosis.de>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Gene Heskett <gene.heskett@...il.com>,
	Juliusz Chroboczek <jch@....jussieu.fr>,
	Mike Galbraith <efault@....de>,
	Peter Williams <pwil3058@...pond.net.au>,
	ck list <ck@....kolivas.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	William Lee Irwin III <wli@...omorphy.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bill Davidsen <davidsen@....com>, Willy Tarreau <w@....eu>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [REPORT] cfs-v5 vs sd-0.46


* Michael Gerdau <mgd@...hnosis.de> wrote:

> > Here i'm assuming that the vmstats are directly comparable: that 
> > your number-crunchers behave the same during the full runtime - is 
> > that correct?
> 
> Yes, basically it does (disregarding small fluctuations)

ok, good.

> I'll see whether I can produce some type of absolute performance 
> measure as well. Thinking about it I guess this should be fairly 
> simple to implement.

oh, you are writing the number-cruncher? In general the 'best' 
performance metrics for scheduler validation are the ones where you have 
immediate feedback: i.e. some ops/sec (or ops per minute) value in some 
readily accessible place, or some "milliseconds-per-100,000 ops" type of 
metric - whichever lends itself better to the workload at hand. If you 
measure time then the best is to use long long and nanoseconds and the 
monotonic clocksource:

 unsigned long long rdclock(void)
 {
	struct timespec ts;

	clock_gettime(CLOCK_MONOTONIC, &ts);

        return ts.tv_sec * 1000000000ULL + ts.tv_nsec;
 }

(link to librt via -lrt to pick up clock_gettime())

The cost of a clock_gettime() (or of a gettimeofday()) can be a couple 
of microseconds on some systems, so it shouldnt be done too frequently.

Plus an absolute metric of "the whole workload took X.Y seconds" is 
useful too.

	Ingo
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