From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail202.messagelabs.com (mail202.messagelabs.com [216.82.254.227]) by kanga.kvack.org (Postfix) with SMTP id 8702D6B0071 for ; Tue, 22 Jun 2010 18:46:12 -0400 (EDT) Date: Wed, 23 Jun 2010 08:45:51 +1000 From: Dave Chinner Subject: Re: [PATCH RFC] mm: Implement balance_dirty_pages() through waiting for flusher thread Message-ID: <20100622224551.GS7869@dastard> References: <1276797878-28893-1-git-send-email-jack@suse.cz> <20100618060901.GA6590@dastard> <20100621233628.GL3828@quack.suse.cz> <20100622054409.GP7869@dastard> <20100621231416.904c50c7.akpm@linux-foundation.org> <20100622100924.GQ7869@dastard> <20100622131745.GB3338@quack.suse.cz> <20100622135234.GA11561@localhost> <20100622143124.GA15235@infradead.org> <20100622143856.GG3338@quack.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100622143856.GG3338@quack.suse.cz> Sender: owner-linux-mm@kvack.org To: Jan Kara Cc: Christoph Hellwig , Wu Fengguang , Andrew Morton , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, peterz@infradead.org List-ID: On Tue, Jun 22, 2010 at 04:38:56PM +0200, Jan Kara wrote: > On Tue 22-06-10 10:31:24, Christoph Hellwig wrote: > > On Tue, Jun 22, 2010 at 09:52:34PM +0800, Wu Fengguang wrote: > > > 2) most writeback will be submitted by one per-bdi-flusher, so no worry > > > of cache bouncing (this also means the per CPU counter error is > > > normally bounded by the batch size) > > > > What counter are we talking about exactly? Once balanance_dirty_pages > The new per-bdi counter I'd like to introduce. > > > stops submitting I/O the per-bdi flusher thread will in fact be > > the only thing submitting writeback, unless you count direct invocations > > of writeback_single_inode. > Yes, I agree that the per-bdi flusher thread should be the only thread > submitting lots of IO (there is direct reclaim or kswapd if we change > direct reclaim but those should be negligible). So does this mean that > also I/O completions will be local to the CPU running per-bdi flusher > thread? Because the counter is incremented from the I/O completion > callback. 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. 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. Cheers, Dave. -- Dave Chinner david@fromorbit.com -- 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