From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Chinner Subject: Re: [PATCH RFC] mm: Implement balance_dirty_pages() through waiting for flusher thread Date: Thu, 24 Jun 2010 09:42:37 +1000 Message-ID: <20100623234237.GA23223@dastard> References: <20100622131745.GB3338@quack.suse.cz> <20100622135234.GA11561@localhost> <20100622143124.GA15235@infradead.org> <20100622143856.GG3338@quack.suse.cz> <20100622224551.GS7869@dastard> <20100623013426.GA6706@localhost> <20100623030604.GM6590@dastard> <20100623032213.GA13068@localhost> <20100623060319.GN6590@dastard> <20100623062540.GA25103@localhost> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Kara , Christoph Hellwig , Andrew Morton , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" , "peterz@infradead.org" To: Wu Fengguang Return-path: Received: from bld-mail16.adl2.internode.on.net ([150.101.137.101]:45019 "EHLO mail.internode.on.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1754093Ab0FWXm5 (ORCPT ); Wed, 23 Jun 2010 19:42:57 -0400 Content-Disposition: inline In-Reply-To: <20100623062540.GA25103@localhost> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Wed, Jun 23, 2010 at 02:25:40PM +0800, Wu Fengguang wrote: > On Wed, Jun 23, 2010 at 02:03:19PM +0800, Dave Chinner wrote: > > On Wed, Jun 23, 2010 at 11:22:13AM +0800, Wu Fengguang wrote: > > > On Wed, Jun 23, 2010 at 11:06:04AM +0800, Dave Chinner wrote: > > > > On Wed, Jun 23, 2010 at 09:34:26AM +0800, Wu Fengguang wrote: > > > > > On Wed, Jun 23, 2010 at 06:45:51AM +0800, Dave Chinner wrote: > > > > > > By default we set QUEUE_FLAG_SAME_COMP, which means we hand > > > > > > completions back to the submitter CPU during blk_complete_request(). > > > > > > Completion processing is then handled by a softirq on the CPU > > > > > > selected for completion processing. > > > > > > > > > > Good to know about that, thanks! > > > > > > > > > > > This was done, IIRC, because it provided some OLTP benchmark 1-2% > > > > > > better results. It can, however, be turned off via > > > > > > /sys/block//queue/rq_affinity, and there's no guarantee that > > > > > > the completion processing doesn't get handled off to some other CPU > > > > > > (e.g. via a workqueue) so we cannot rely on this completion > > > > > > behaviour to avoid cacheline bouncing. > > > > > > > > > > If rq_affinity does not work reliably somewhere in the IO completion > > > > > path, why not trying to fix it? > > > > > > > > Because completion on the submitter CPU is not ideal for high > > > > bandwidth buffered IO. > > > > > > Yes there may be heavy post-processing for read data, however for writes > > > it is mainly the pre-processing that costs CPU? > > > > Could be either - delayed allocation requires significant pre-processing > > for allocation. Avoiding this by using preallocation just > > moves the processing load to IO completion which needs to issue > > transactions to mark the region written. > > Good point, thanks. > > > > So perfect rq_affinity > > > should always benefit write IO? > > > > No, because the flusher thread gets to be CPU bound just writing > > pages, allocating blocks and submitting IO. It might take 5-10GB/s > > to get there (say a million dirty pages a second being processed by > > a single CPU), but that's the sort of storage subsystem XFS is > > capable of driving. IO completion time for such a workload is > > significant, too, so putting that on the same CPU as the flusher > > thread will slow things down by far more than gain from avoiding > > cacheline bouncing. > > So super fast storage is going to demand multiple flushers per bdi. > And once we run multiple flushers for one bdi, it will again be > beneficial to schedule IO completion to the flusher CPU :) Yes - that is where we want to get to with XFS. But we don't have multiple bdi-flusher thread support yet for any filesystem, so I think it will be a while before the we can ignore this issue... Cheers, Dave.> -- Dave Chinner david@fromorbit.com