linux-kernel - Re: [PATCH 05/15] readahead: limit readahead size for small memory systems

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Date:	Thu, 25 Feb 2010 10:00:46 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Matt Mackall <mpm@...enic.com>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Clemens Ladisch <clemens@...isch.de>,
	Olivier Galibert <galibert@...ox.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>,
	Nick Piggin <npiggin@...e.de>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/15] readahead: limit readahead size for small memory
 systems

On 02/23/2010 10:10 PM, Wu Fengguang wrote:
> When lifting the default readahead size from 128KB to 512KB,
> make sure it won't add memory pressure to small memory systems.
>
> For read-ahead, the memory pressure is mainly readahead buffers consumed
> by too many concurrent streams. The context readahead can adapt
> readahead size to thrashing threshold well.  So in principle we don't
> need to adapt the default _max_ read-ahead size to memory pressure.
>
> For read-around, the memory pressure is mainly read-around misses on
> executables/libraries. Which could be reduced by scaling down
> read-around size on fast "reclaim passes".
>
> This patch presents a straightforward solution: to limit default
> readahead size proportional to available system memory, ie.
>                  512MB mem =>  512KB readahead size
>                  128MB mem =>  128KB readahead size
>                   32MB mem =>   32KB readahead size (minimal)
>
> Strictly speaking, only read-around size has to be limited.  However we
> don't bother to seperate read-around size from read-ahead size for now.
>
> CC: Matt Mackall<mpm@...enic.com>
> Signed-off-by: Wu Fengguang<fengguang.wu@...el.com>

Acked-by: Rik van Riel <riel@...hat.com>

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