linux-kernel - Re: [rfc][patch] store-free path walking

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Message-ID: <1158166a0910080612h29d93d50y875d5305cd4d985f@mail.gmail.com>
Date:	Thu, 8 Oct 2009 15:12:08 +0200
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andi Kleen <andi@...stfloor.org>, Nick Piggin <npiggin@...e.de>,
	Jens Axboe <jens.axboe@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org,
	Ravikiran G Thirumalai <kiran@...lex86.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [rfc][patch] store-free path walking

On Wed, Oct 7, 2009 at 11:57 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> This, btw, is exactly the kind of thing we saw with some of the
> non-temporal work, when we used nontemporal stores to copy pages on COW
> faults, or when doing pre-zeroing of pages. You get rid of some of the
> hot-spots in the kernel, and you then replace them with user space taking
> the cache misses in random spots instead. The kernel profile looks better,
> and system time may go down, but actual performace never went down - you
> just moved your cache miss cost from one place to another.

A few years ago when K7s were not ancient yet, after hearing
argument for and against non-temporal stores,
I decided to finally figure it for myself.

I tested kernel build workload on two kernels with the only
one difference - clear_page with and without non-temporal stores.

"Non-temporal stores" kernel was faster, not slower. Just a little bit,
but reproducibly.
--
vda
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