From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Wed, 13 Oct 2004 15:44:52 +1000 From: Nathan Scott Subject: Page cache write performance issue Message-ID: <20041013054452.GB1618@frodo> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Sender: owner-linux-mm@kvack.org Return-Path: To: Andrew Morton , Nick Piggin Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-xfs@oss.sgi.com List-ID: Hi guys, I've noticed the following performance regression from between 2.6.8.1 and 2.6.9-rc. It seems to have a very pronounced affect on both ext2 and xfs. - single thread, writing (what should be) straight into the page cache, file size 1/2 of memory size (500MB vs 1GB), writes are in 1K chunks, most of memory is free, machine was just booted; - on 2.6.8 (and earlier 2.6 releases) I can typically get ~50MB/sec on this machine doing this; (or better with larger I/O sizes, but thats not the point here) - on 2.4.28-pre (and all 2.4 releases) I can typically get ~70MB/sec, presumably writeback kicks in earlier on 2.6; OK, I guess we can live with that... probably some tradeoff is being made there in the VM; - on 2.6.9-rc I can only get _4_MB/sec (ext2 or xfs); writeback commences very quickly, CPU utilisation drops way down (from 100% to <10%)... looks like we go slower cos we're initiating I/O almost from the start. Now if I bump up /proc/sys/vm/dirty_background_ratio and /proc/sys/vm/dirty_ratio from 40 to 80, I see the expected performance again (actually, I see the 2.4 performance, so the poorer early-2.6 numbers were probably due to I/O commencing at the tail end of all the writes, due to 50% being more than 40% :). But 2.6.8 had the same default dirty writeout ratios (40) as 2.6.9-rc does, didn't it? So, any ideas what happened to 2.6.9? Whats the rationale for commencing writeout earlier in 2.6 (even when there's so much free memory available)? Any chance we can get the defaults set to something much larger in the wake of the other 2.6.9 VM changes, so we don't regress here? thanks! -- Nathan -- 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: aart@kvack.org