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

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Message-ID: <4B869682.9010709@linux.vnet.ibm.com>
Date:	Thu, 25 Feb 2010 16:25:54 +0100
From:	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.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>, 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



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>

What I state here is for read ahead in a "multi iozone sequential" 
setup, I can't speak for real "read around" workloads.
So probably your table is fine to cover read-around+read-ahead in one 
number.

I have tested 256MB mem systems with 512kb readahead quite a lot.
On those 512kb is still by far superior to smaller readaheads and I 
didn't see major trashing or memory pressure impact.

Therefore I would recommend a table like:
                >=256MB mem => 512KB readahead size
                  128MB mem => 128KB readahead size
                   32MB mem =>  32KB readahead size (minimal)

-- 

Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, System z Linux Performance
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