Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935043Ab0BZC3N (ORCPT ); Thu, 25 Feb 2010 21:29:13 -0500 Received: from mga02.intel.com ([134.134.136.20]:39462 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S935018Ab0BZC3M (ORCPT ); Thu, 25 Feb 2010 21:29:12 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.49,543,1262592000"; d="scan'208";a="599321389" Date: Fri, 26 Feb 2010 10:29:07 +0800 From: Wu Fengguang To: Christian Ehrhardt Cc: Andrew Morton , Jens Axboe , Matt Mackall , Chris Mason , Peter Zijlstra , Clemens Ladisch , Olivier Galibert , Vivek Goyal , Nick Piggin , Linux Memory Management List , "linux-fsdevel@vger.kernel.org" , LKML , Rik van Riel Subject: Re: [PATCH 05/15] readahead: limit readahead size for small memory systems Message-ID: <20100226022907.GA22226@localhost> References: <20100224031001.026464755@intel.com> <20100224031054.307027163@intel.com> <4B869682.9010709@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4B869682.9010709@linux.vnet.ibm.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4360 Lines: 103 On Thu, Feb 25, 2010 at 11:25:54PM +0800, Christian Ehrhardt wrote: > > > 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 > > Signed-off-by: Wu Fengguang > > 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. OK. > 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. In fact I'd expect a 64MB box to also benefit from 512kb readahead :) > Therefore I would recommend a table like: > >=256MB mem => 512KB readahead size > 128MB mem => 128KB readahead size > 32MB mem => 32KB readahead size (minimal) So, I'm fed up with compromising the read-ahead size with read-around size. There is no good to introduce a read-around size to confuse the user though. Instead, I'll introduce a read-around size limit _on top of_ the readahead size. This will allow power users to adjust read-ahead/read-around size at the same time, while saving the low end from unnecessary memory pressure :) I made the assumption that low end users have no need to request a large read-around size. Thanks, Fengguang --- 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. 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 read-ahead size proportional to available system memory, ie. 512MB mem => 512KB readahead size 128MB mem => 128KB readahead size 32MB mem => 32KB readahead size CC: Matt Mackall CC: Christian Ehrhardt Signed-off-by: Wu Fengguang --- mm/filemap.c | 2 +- mm/readahead.c | 22 ++++++++++++++++++++++ 2 files changed, 23 insertions(+), 1 deletion(-) --- linux.orig/mm/filemap.c 2010-02-26 10:04:28.000000000 +0800 +++ linux/mm/filemap.c 2010-02-26 10:08:33.000000000 +0800 @@ -1431,7 +1431,7 @@ static void do_sync_mmap_readahead(struc /* * mmap read-around */ - ra_pages = max_sane_readahead(ra->ra_pages); + ra_pages = min(ra->ra_pages, roundup_pow_of_two(totalram_pages / 1024)); if (ra_pages) { ra->start = max_t(long, 0, offset - ra_pages/2); ra->size = ra_pages; -- 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/