From mboxrd@z Thu Jan 1 00:00:00 1970 From: Johannes Weiner Subject: Re: [PATCH 2/3] writeback: Record if the congestion was unnecessary Date: Thu, 26 Aug 2010 20:29:04 +0200 Message-ID: <20100826182904.GC6805@cmpxchg.org> References: <1282835656-5638-1-git-send-email-mel@csn.ul.ie> <1282835656-5638-3-git-send-email-mel@csn.ul.ie> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, Andrew Morton , Christian Ehrhardt , Wu Fengguang , Jan Kara , linux-kernel@vger.kernel.org To: Mel Gorman Return-path: Received: from f0.cmpxchg.org ([85.214.51.133]:49263 "EHLO cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753554Ab0HZS3M (ORCPT ); Thu, 26 Aug 2010 14:29:12 -0400 Content-Disposition: inline In-Reply-To: <1282835656-5638-3-git-send-email-mel@csn.ul.ie> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Thu, Aug 26, 2010 at 04:14:15PM +0100, Mel Gorman wrote: > If congestion_wait() is called when there is no congestion, the caller > will wait for the full timeout. This can cause unreasonable and > unnecessary stalls. There are a number of potential modifications that > could be made to wake sleepers but this patch measures how serious the > problem is. It keeps count of how many congested BDIs there are. If > congestion_wait() is called with no BDIs congested, the tracepoint will > record that the wait was unnecessary. I am not convinced that unnecessary is the right word. On a workload without any IO (i.e. no congestion_wait() necessary, ever), I noticed the VM regressing both in time and in reclaiming the right pages when simply removing congestion_wait() from the direct reclaim paths (the one in __alloc_pages_slowpath and the other one in do_try_to_free_pages). So just being stupid and waiting for the timeout in direct reclaim while kswapd can make progress seemed to do a better job for that load. I can not exactly pinpoint the reason for that behaviour, it would be nice if somebody had an idea. So personally I think it's a good idea to get an insight on the use of congestion_wait() [patch 1] but I don't agree with changing its behaviour just yet, or judging its usefulness solely on whether it correctly waits for bdi congestion.