linux-kernel - Re: [PATCH 5/8] readahead: add /debug/readahead/stats

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Message-ID: <20111121233255.GF24062@one.firstfloor.org>
Date:	Tue, 22 Nov 2011 00:32:55 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux-fsdevel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Jens Axboe <jens.axboe@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH 5/8] readahead: add /debug/readahead/stats

> I may be wrong, but I don't think the CPU cost of this code matters a
> lot.  People will rarely turn it on and disk IO is a lot slower than
> CPU actions and it's waaaaaaay more important to get high-quality info
> about readahead than it is to squeeze out a few CPU cycles.

In its current form it would cache line bounce, which tends to be 
extremly slow. But the solution is probably to make it per CPU.

-Andi

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