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

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Date:	Tue, 29 Jan 2013 09:12:03 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Valdis.Kletnieks@...edu
Cc:	ling.ma.program@...il.com, 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


* Valdis.Kletnieks@...edu <Valdis.Kletnieks@...edu> wrote:

> 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.

cycles and elapsed time is down in both tests - the speedup 
seems statistically a wash in the first test and significant for 
the second workload.

the instruction count might be an artifact of byte wise versus 
word wise REP; MOV.

> 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?

Look at the numbers, it counts a whole lot of other things as 
well beyond instructions - elapsed time being the most important 
one.

But more numbers never hurt.

Thanks,

	Ingo
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