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

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Message-ID: <alpine.LFD.2.01.0910071417300.3432@localhost.localdomain>
Date:	Wed, 7 Oct 2009 14:20:50 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
cc:	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, 7 Oct 2009, Andi Kleen wrote:
> 
> It won't double it, because there are many more cache lines from
> all kinds of other things in these paths.

Not all that many. The big part of the D$ is the hash table lookup in this 
path.

> The point was just to hide the latency of fetching two at different 
> times. The actual cache foot print is not that important, as long as 
> it's not excessive

You're full of sh*t.

Every extra line you fetch pushes out another line. No amount of 
prefetching will hide that. So stop spreading idiotic fairytales.

The best optimization is to not do the work at all. It really is that 
simple.  Prefetching as an optimization technique absolutely sucks.

		Linus
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