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Date: Sun, 22 Jul 2007 16:29:23 +0800
From: Fengguang Wu <fengguang.wu@gmail.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Dave Jones <davej@redhat.com>, linux-kernel <linux-kernel@vger.kernel.org>,
       riel <riel@redhat.com>, Andrew Morton <akpm@linux-foundation.org>,
       Rusty Russell <rusty@rustcorp.com.au>, Tim Pepper <lnxninja@us.ibm.com>,
       Chris Snook <csnook@redhat.com>
Subject: Re: [PATCH 0/3] readahead drop behind and size adjustment
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On Sun, Jul 22, 2007 at 10:24:37AM +0200, Peter Zijlstra wrote:
> On Sun, 2007-07-22 at 16:10 +0800, Fengguang Wu wrote:
> 
> > - It will avoid large-file-reads-thrashing-my-desktop problem,
> >   so most desktop users should like it. But sure there will be counter
> >   cases when a user want to keep the data cached.
> > - File servers may hurt from it. Imagine a mp3/png file server. The
> >   files are large enough to trigger drop-behind, but small (and hot)
> >   enough to be cached. Also when a new fedora DVD iso is released, it
> >   may be cached for some days. These are only the obvious cases.
> > 
> > So I opt for it being made tunable, safe, and turned off by default.
> 
> I'm still not convinced (Rik wasn't either last time around). When these
> files really are hot, they will be kept in memory due to them becoming
> Active.
> 
> Also, by scaling up the max readahead size it takes a larger file before
> it starts dropping. If say this server has 4G of memory (not much at all
> for a server) resulting in a 1M readahead window, the file needs to be >
> ~2M before it starts drop behind.

[snip]

> But I guess it takes someone to try this IRL before we can settle this
> debate :-/

Yeah, some real workload numbers would help.