linux-kernel - Re: [PATCH RFC 2/2] [x86] Optimize copy_page by re-arranging instruction sequence and saving register

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Message-ID: <20121012133554.GA16230@one.firstfloor.org>
Date:	Fri, 12 Oct 2012 15:35:54 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"Ma, Ling" <ling.ma@...el.com>
Cc:	Andi Kleen <andi@...stfloor.org>, "mingo@...e.hu" <mingo@...e.hu>,
	"hpa@...or.com" <hpa@...or.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 2/2] [x86] Optimize copy_page by re-arranging instruction sequence and saving register

> I tested new and original version on core2, the patch improved performance about 9%,

That's not useful because core2 doesn't use this variant, it uses the
rep string variant. Primary user is P4.

> Although core2 is out-of-order pipeline and weaken instruction sequence requirement, 
> because of ROB size limitation, new patch issues write operation earlier and
> get more parallelism possibility for the pair of write and load ops and better result.
> Attached core2-cpu-info (I have no older machine)

If you can't test the CPUs who run this code I think it's safer if you
add a new variant for Atom, not change the existing well tested code.
Otherwise you risk performance regressions on these older CPUs.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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