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

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Date:	Fri, 9 Oct 2009 19:53:45 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Thu, Oct 08, 2009 at 09:39:30AM +0200, Nick Piggin wrote:
> > My point (probably not very well written expressed)
> > was that in a typical VFS operation there are hundreds
> > of cache lines touched for various things (code, global, various
> > objects, random stuff) and one more or less in the final dentry 
> > is not that big a difference in the global picture.
> > (ok I realize final in this case means the elements in the path)
> 
> Yes I do know what you were getting at... Although I would
> say dcache is still fairly significant. It's 3 cachelines
> for every single path element we look up. Now I suspect it

That's true, for deeply nested paths where the elements add up 
it's a concern. I was assuming that paths are not that deep, but that might 
be too optimistic.

> Actually no, it does look relatively sane (I guess it doesn't
> change that much), except it might actualy be a nice idea
> to move d_iname up to about above d_lru, so we could actually
> do path lookup in 2 cachelines per entry (or even 1 if we
> they have very short names).
> 
> I will do a bit of profiling on the lookup code and see...

Yes numbers would be great.

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