linux-kernel - Re: [PATCH] [x86]: Compiler Option Os is better on latest x86

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Message-ID: <26437.1359393357@turing-police.cc.vt.edu>
Date:	Mon, 28 Jan 2013 12:15:57 -0500
From:	Valdis.Kletnieks@...edu
To:	ling.ma.program@...il.com
Cc:	mingo@...hat.com, tglx@...utronix.de, hpa@...or.com,
	linux-kernel@...r.kernel.org, Ma Ling <ling.ml@...pay.com>
Subject: Re: [PATCH] [x86]: Compiler Option Os is better on latest x86

On Fri, 25 Jan 2013 09:11:01 -0500, ling.ma.program@...il.com said:

> Based on above reasons, we compiled linux kernel 3.6.9 with O2 and Os
> respectively. The results show Os improve performance netperf 4.8%,
> 2.7% for volano as below

Am I allowed to NAK this?  What the numbers given so far *actually*
show is 4.8% more instructions executed, *not* 4.8% better performance.

I'm having a *very* hard time convincing myself that what we're seeing isn't
simply the expected behavior of loops *not* being unrolled and similar
non-optimizations done by -Os, so more instructions get executed to do the same
amount of work.

Rather than "run for 10 seconds and count instructions", can we
"run for 50,000 syscalls and count clock time" or similar that shows
an *actual* improvement?

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