Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753884AbZIWBpK (ORCPT ); Tue, 22 Sep 2009 21:45:10 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753655AbZIWBpJ (ORCPT ); Tue, 22 Sep 2009 21:45:09 -0400 Received: from mga03.intel.com ([143.182.124.21]:39600 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753563AbZIWBpI (ORCPT ); Tue, 22 Sep 2009 21:45:08 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.44,434,1249282800"; d="scan'208";a="190579578" Date: Wed, 23 Sep 2009 09:45:00 +0800 From: Wu Fengguang To: Andrew Morton Cc: Chris Mason , Peter Zijlstra , "Li, Shaohua" , "linux-kernel@vger.kernel.org" , "richard@rsk.demon.co.uk" , "jens.axboe@oracle.com" Subject: Re: regression in page writeback Message-ID: <20090923014500.GA11076@localhost> References: <1253601612.8439.274.camel@twins> <20090922080505.GB9192@localhost> <1253606965.8439.281.camel@twins> <20090922082427.GA24888@localhost> <1253608335.8439.283.camel@twins> <20090922155259.GL10825@think> <20090923002220.GA6382@localhost> <20090922175452.d66400dd.akpm@linux-foundation.org> <20090923011758.GC6382@localhost> <20090922182832.28e7f73a.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090922182832.28e7f73a.akpm@linux-foundation.org> 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: 2281 Lines: 55 On Wed, Sep 23, 2009 at 09:28:32AM +0800, Andrew Morton wrote: > On Wed, 23 Sep 2009 09:17:58 +0800 Wu Fengguang wrote: > > > On Wed, Sep 23, 2009 at 08:54:52AM +0800, Andrew Morton wrote: > > > On Wed, 23 Sep 2009 08:22:20 +0800 Wu Fengguang wrote: > > > > > > > Jens' per-bdi writeback has another improvement. In 2.6.31, when > > > > superblocks A and B both have 100000 dirty pages, it will first > > > > exhaust A's 100000 dirty pages before going on to sync B's. > > > > > > That would only be true if someone broke 2.6.31. Did they? > > > > > > SYSCALL_DEFINE0(sync) > > > { > > > wakeup_pdflush(0); > > > sync_filesystems(0); > > > sync_filesystems(1); > > > if (unlikely(laptop_mode)) > > > laptop_sync_completion(); > > > return 0; > > > } > > > > > > the sync_filesystems(0) is supposed to non-blockingly start IO against > > > all devices. It used to do that correctly. But people mucked with it > > > so perhaps it no longer does. > > > > I'm referring to writeback_inodes(). Each invocation of which (to sync > > 4MB) will do the same iteration over superblocks A => B => C ... So if > > A has dirty pages, it will always be served first. > > > > So if wbc->bdi == NULL (which is true for kupdate/background sync), it > > will have to first exhaust A before going on to B and C. > > But that works OK. We fill the first device's queue, then it gets > congested and sync_sb_inodes() does nothing and we advance to the next > queue. So in common cases "exhaust" is a bit exaggerated, but A does receive much more opportunity than B. Computation resources for IO submission are unbalanced for A, and there are pointless overheads in rechecking A. > If a device has more than a queue's worth of dirty data then we'll > probably leave some of that dirty memory un-queued, so there's some > lack of concurrency in that situation. Good insight. That possibly explains one major factor of the performance gains of Jens' per-bdi writeback. Thanks, Fengguang -- 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/