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Date: Tue, 22 Nov 2011 00:32:55 +0100
From: Andi Kleen <andi@firstfloor.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Wu Fengguang <fengguang.wu@intel.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
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        Rik van Riel <riel@redhat.com>, LKML <linux-kernel@vger.kernel.org>,
        Andi Kleen <andi@firstfloor.org>
Subject: Re: [PATCH 5/8] readahead: add /debug/readahead/stats
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References: <20111121091819.394895091@intel.com> <20111121093846.636765408@intel.com> <20111121152958.e4fd76d4.akpm@linux-foundation.org>
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> 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