linux-kernel - Re: [PATCH RFC] [X86] Compile Option Os versus O2 on latest x86 platform

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Message-ID: <4B154018.4060605@zytor.com>
Date:	Tue, 01 Dec 2009 08:11:04 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	"Ma, Ling" <ling.ma@...el.com>, Ingo Molnar <mingo@...e.hu>,
	Dave Jones <davej@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] [X86] Compile Option Os versus O2 on latest x86 platform

On 12/01/2009 02:14 AM, Arjan van de Ven wrote:
> On Tue, 1 Dec 2009 16:54:04 +0800
> "Ma, Ling" <ling.ma@...el.com> wrote:
> 
>> Hi Ingo
>>
>> Thanks for your correction, so we use perf stat --repeat 3 to test
>> volano, tbench, and kbuild, Because netperf has multiple items we may
>> send out later.
> 
> a key question is.. how much more memory do you have free due to -Os?
> (because memory is cache is performance on a system level as well)
> and how much less icache pressure is there?
> 

>From the re-run, it sounds like the only test that actually shows a
significant difference is volano.  From reading the numbers, it looks
like the improvements are almost exclusively in IPC i.e. better
scheduling -- all the other metrics are substantially worse; including a
10% increase in cache misses.

It would be interesting to see what functions are hot in volano.  It
might very well be that we could get a boost without significantly bloat
the kernel as a whole by picking out a couple of hot object files and
compiling those with -O2 or -O3.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.
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