From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail190.messagelabs.com (mail190.messagelabs.com [216.82.249.51]) by kanga.kvack.org (Postfix) with ESMTP id 658416B004D for ; Fri, 9 Oct 2009 17:31:37 -0400 (EDT) Date: Fri, 9 Oct 2009 14:31:24 -0700 From: Andrew Morton Subject: Re: [PATCH] mm: make VM_MAX_READAHEAD configurable Message-Id: <20091009143124.1241a6bc.akpm@linux-foundation.org> In-Reply-To: <20091009122952.GI9228@kernel.dk> References: <1255087175-21200-1-git-send-email-ehrhardt@linux.vnet.ibm.com> <1255090830.8802.60.camel@laptop> <20091009122952.GI9228@kernel.dk> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Jens Axboe Cc: Peter Zijlstra , Ehrhardt Christian , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Martin Schwidefsky , Wu Fengguang List-ID: On Fri, 9 Oct 2009 14:29:52 +0200 Jens Axboe wrote: > On Fri, Oct 09 2009, Peter Zijlstra wrote: > > On Fri, 2009-10-09 at 13:19 +0200, Ehrhardt Christian wrote: > > > From: Christian Ehrhardt > > > > > > On one hand the define VM_MAX_READAHEAD in include/linux/mm.h is just a default > > > and can be configured per block device queue. > > > On the other hand a lot of admins do not use it, therefore it is reasonable to > > > set a wise default. > > > > > > This path allows to configure the value via Kconfig mechanisms and therefore > > > allow the assignment of different defaults dependent on other Kconfig symbols. > > > > > > Using this, the patch increases the default max readahead for s390 improving > > > sequential throughput in a lot of scenarios with almost no drawbacks (only > > > theoretical workloads with a lot concurrent sequential read patterns on a very > > > low memory system suffer due to page cache trashing as expected). > > > > Why can't this be solved in userspace? > > > > Also, can't we simply raise this number if appropriate? Wu did some > > read-ahead trashing detection bits a long while back which should scale > > the read-ahead window back when we're low on memory, not sure that ever > > made it in, but that sounds like a better option than having different > > magic numbers for each platform. > > Agree, making this a config option (and even defaulting to a different > number because of an arch setting) is crazy. Given the (increasing) level of disparity between different kinds of storage devices, having _any_ default is crazy. Would be better to make some sort of vaguely informed guess at runtime, based upon the characteristics of the device. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org