Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964837Ab0BZOSs (ORCPT ); Fri, 26 Feb 2010 09:18:48 -0500 Received: from mx1.redhat.com ([209.132.183.28]:63998 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964795Ab0BZOSr (ORCPT ); Fri, 26 Feb 2010 09:18:47 -0500 Date: Fri, 26 Feb 2010 09:17:40 -0500 From: Vivek Goyal To: Wu Fengguang Cc: Christian Ehrhardt , Andrew Morton , Jens Axboe , Matt Mackall , Chris Mason , Peter Zijlstra , Clemens Ladisch , Olivier Galibert , Nick Piggin , Linux Memory Management List , "linux-fsdevel@vger.kernel.org" , LKML , Rik van Riel Subject: Re: [PATCH] readahead: add notes on readahead size Message-ID: <20100226141740.GB6023@redhat.com> References: <20100224031001.026464755@intel.com> <20100224031054.307027163@intel.com> <4B869682.9010709@linux.vnet.ibm.com> <20100226022907.GA22226@localhost> <20100226024837.GA22859@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100226024837.GA22859@localhost> User-Agent: Mutt/1.5.19 (2009-01-05) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2555 Lines: 68 On Fri, Feb 26, 2010 at 10:48:37AM +0800, Wu Fengguang wrote: > > readahead: limit read-ahead size for small memory systems > > > > When lifting the default readahead size from 128KB to 512KB, > > make sure it won't add memory pressure to small memory systems. > > btw, I wrote some comments to summarize the now complex readahead size > rules.. > > == > readahead: add notes on readahead size > > Basically, currently the default max readahead size > - is 512k > - is boot time configurable with "readahead=" > and is auto scaled down: > - for small devices > - for small memory systems (read-around size alone) > > CC: Matt Mackall > CC: Christian Ehrhardt > Signed-off-by: Wu Fengguang > --- > mm/readahead.c | 22 ++++++++++++++++++++++ > 1 file changed, 22 insertions(+) > > --- linux.orig/mm/readahead.c 2010-02-26 10:11:41.000000000 +0800 > +++ linux/mm/readahead.c 2010-02-26 10:11:55.000000000 +0800 > @@ -7,6 +7,28 @@ > * Initial version. > */ > > +/* > + * Notes on readahead size. > + * > + * The default max readahead size is VM_MAX_READAHEAD=512k, > + * which can be changed by user with boot time parameter "readahead=" > + * or runtime interface "/sys/devices/virtual/bdi/default/read_ahead_kb". > + * The latter normally only takes effect in future for hot added devices. > + * > + * The effective max readahead size for each block device can be accessed with > + * 1) the `blockdev` command > + * 2) /sys/block/sda/queue/read_ahead_kb > + * 3) /sys/devices/virtual/bdi/$(env stat -c '%t:%T' /dev/sda)/read_ahead_kb > + * > + * They are typically initialized with the global default size, however may be > + * auto scaled down for small devices in add_disk(). NFS, software RAID, btrfs > + * etc. have special rules to setup their default readahead size. > + * > + * The mmap read-around size typically equals with readahead size, with an > + * extra limit proportional to system memory size. For example, a 64MB box > + * will have a 64KB read-around size limit, 128MB mem => 128KB limit, etc. > + */ > + Great. I was confused among so many ways to control read ahead size. This documentation helps a lot. Vivek > #include > #include > #include -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/