linux-kernel - Re: [PATCH 0/3] readahead drop behind and size adjustment

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Date:	Sun, 22 Jul 2007 16:29:23 +0800
From:	Fengguang Wu <fengguang.wu@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Dave Jones <davej@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Tim Pepper <lnxninja@...ibm.com>,
	Chris Snook <csnook@...hat.com>
Subject: Re: [PATCH 0/3] readahead drop behind and size adjustment

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.

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