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: Tue, 22 Jun 2010 20:09:24 +1000 Message-ID: <20100622100924.GQ7869@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> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Kara , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, hch@infradead.org, peterz@infradead.org, wfg@mail.ustc.edu.cn To: Andrew Morton Return-path: Content-Disposition: inline In-Reply-To: <20100621231416.904c50c7.akpm@linux-foundation.org> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org On Mon, Jun 21, 2010 at 11:14:16PM -0700, Andrew Morton wrote: > On Tue, 22 Jun 2010 15:44:09 +1000 Dave Chinner wrote: > > > > > And so on. This isn't necessarily bad - we'll throttle for longer > > > > than we strictly need to - but the cumulative counter resolution > > > > error gets worse as the number of CPUs doing IO completion grows. > > > > Worst case ends up at for (num cpus * 31) + 1 pages of writeback for > > > > just the first waiter. For an arbitrary FIFO queue of depth d, the > > > > worst case is more like d * (num cpus * 31 + 1). > > > Hmm, I don't see how the error would depend on the FIFO depth. > > > > It's the cumulative error that depends on the FIFO depth, not the > > error seen by a single waiter. > > Could use the below to basically eliminate the inaccuracies. > > Obviously things might get a bit expensive in certain threshold cases > but with some hysteresis that should be manageable. That seems a lot more... unpredictable than modifying the accounting to avoid cumulative errors. > + /* Check to see if rough count will be sufficient for comparison */ > + if (abs(count - rhs) > (percpu_counter_batch*num_online_cpus())) { Also, that's a big margin when we are doing equality matches for every page IO completion. If we a large CPU count machine where per-cpu counters actually improve performance (say 16p) then we're going to be hitting the slow path for the last 512 pages of every waiter. Hence I think the counter sum is compared too often to scale with this method of comparison. 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