Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755374Ab0G0Nfb (ORCPT ); Tue, 27 Jul 2010 09:35:31 -0400 Received: from gir.skynet.ie ([193.1.99.77]:57568 "EHLO gir.skynet.ie" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752809Ab0G0Nf3 (ORCPT ); Tue, 27 Jul 2010 09:35:29 -0400 Date: Tue, 27 Jul 2010 14:35:13 +0100 From: Mel Gorman To: Wu Fengguang Cc: "linux-kernel@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" , Dave Chinner , Chris Mason , Nick Piggin , Rik van Riel , Johannes Weiner , Christoph Hellwig , KAMEZAWA Hiroyuki , KOSAKI Motohiro , Andrew Morton , Andrea Arcangeli Subject: Re: [PATCH 8/8] vmscan: Kick flusher threads to clean pages when reclaim is encountering dirty pages Message-ID: <20100727133513.GZ5300@csn.ul.ie> References: <1279545090-19169-1-git-send-email-mel@csn.ul.ie> <1279545090-19169-9-git-send-email-mel@csn.ul.ie> <20100726072832.GB13076@localhost> <20100726092616.GG5300@csn.ul.ie> <20100726112709.GB6284@localhost> <20100726125717.GS5300@csn.ul.ie> <20100726131008.GE11947@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20100726131008.GE11947@localhost> User-Agent: Mutt/1.5.17+20080114 (2008-01-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3566 Lines: 81 On Mon, Jul 26, 2010 at 09:10:08PM +0800, Wu Fengguang wrote: > On Mon, Jul 26, 2010 at 08:57:17PM +0800, Mel Gorman wrote: > > On Mon, Jul 26, 2010 at 07:27:09PM +0800, Wu Fengguang wrote: > > > > > > @@ -933,13 +934,16 @@ keep_dirty: > > > > > > VM_BUG_ON(PageLRU(page) || PageUnevictable(page)); > > > > > > } > > > > > > > > > > > > + /* > > > > > > + * If reclaim is encountering dirty pages, it may be because > > > > > > + * dirty pages are reaching the end of the LRU even though > > > > > > + * the dirty_ratio may be satisified. In this case, wake > > > > > > + * flusher threads to pro-actively clean some pages > > > > > > + */ > > > > > > + wakeup_flusher_threads(laptop_mode ? 0 : nr_dirty + nr_dirty / 2); > > > > > > > > > > Ah it's very possible that nr_dirty==0 here! Then you are hitting the > > > > > number of dirty pages down to 0 whether or not pageout() is called. > > > > > > > > > > > > > True, this has been fixed to only wakeup flusher threads when this is > > > > the file LRU, dirty pages have been encountered and the caller has > > > > sc->may_writepage. > > > > > > OK. > > > > > > > > Another minor issue is, the passed (nr_dirty + nr_dirty / 2) is > > > > > normally a small number, much smaller than MAX_WRITEBACK_PAGES. > > > > > The flusher will sync at least MAX_WRITEBACK_PAGES pages, this is good > > > > > for efficiency. > > > > > And it seems good to let the flusher write much more > > > > > than nr_dirty pages to safeguard a reasonable large > > > > > vmscan-head-to-first-dirty-LRU-page margin. So it would be enough to > > > > > update the comments. > > > > > > > > > > > > > Ok, the reasoning had been to flush a number of pages that was related > > > > to the scanning rate but if that is inefficient for the flusher, I'll > > > > use MAX_WRITEBACK_PAGES. > > > > > > It would be better to pass something like (nr_dirty * N). > > > MAX_WRITEBACK_PAGES may be increased to 128MB in the future, which is > > > obviously too large as a parameter. When the batch size is increased > > > to 128MB, the writeback code may be improved somehow to not exceed the > > > nr_pages limit too much. > > > > > > > What might be a useful value for N? 1.5 appears to work reasonably well > > to create a window of writeback ahead of the scanner but it's a bit > > arbitrary. > > I'd recommend N to be a large value. It's no longer relevant now since > we'll call the flusher to sync some range containing the target page. > The flusher will then choose an N large enough (eg. 4MB) for efficient > IO. It needs to be a large value, otherwise the vmscan code will > quickly run into dirty pages again.. > Ok, I took the 4MB at face value to be a "reasonable amount that should not cause congestion". The end result is #define MAX_WRITEBACK (4194304UL >> PAGE_SHIFT) #define WRITEBACK_FACTOR (MAX_WRITEBACK / SWAP_CLUSTER_MAX) static inline long nr_writeback_pages(unsigned long nr_dirty) { return laptop_mode ? 0 : min(MAX_WRITEBACK, (nr_dirty * WRITEBACK_FACTOR)); } nr_writeback_pages(nr_dirty) is what gets passed to wakeup_flusher_threads(). Does that seem sensible? -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- 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/