On Thu, Apr 14, 2011 at 07:52:11AM +0800, Dave Chinner wrote: > On Thu, Apr 14, 2011 at 07:31:22AM +0800, Wu Fengguang wrote: > > On Thu, Apr 14, 2011 at 06:04:44AM +0800, Jan Kara wrote: > > > On Wed 13-04-11 16:59:41, Wu Fengguang wrote: > > > > Reduce the dampening for the control system, yielding faster > > > > convergence. The change is a bit conservative, as smaller values may > > > > lead to noticeable bdi threshold fluctuates in low memory JBOD setup. > > > > > > > > CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> > > > > CC: Richard Kennedy <richard@xxxxxxxxxxxxxxx> > > > > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> > > > Well, I have nothing against this change as such but what I don't like is > > > that it just changes magical +2 for similarly magical +0. It's clear that > > > > The patch tends to make the rampup time a bit more reasonable for > > common desktops. From 100s to 25s (see below). > > > > > this will lead to more rapid updates of proportions of bdi's share of > > > writeback and thread's share of dirtying but why +0? Why not +1 or -1? So > > > > Yes, it will especially be a problem on _small memory_ JBOD setups. > > Richard actually has requested for a much radical change (decrease by > > 6) but that looks too much. > > > > My team has a 12-disk JBOD with only 6G memory. The memory is pretty > > small as a server, but it's a real setup and serves well as the > > reference minimal setup that Linux should be able to run well on. > > FWIW, linux runs on a lot of low power NAS boxes with jbod and/or > raid setups that have <= 1GB of RAM (many of them run XFS), so even > your setup could be considered large by a significant fraction of > the storage world. Hence you need to be careful of optimising for > what you think is a "normal" server, because there simply isn't such > a thing.... Good point! This patch is likely to hurt a loaded 1GB 4-disk NAS box... I'll test the setup. I did test low memory setups -- but only on simple 1-disk cases. For example, when dirty thresh is lowered to 7MB, the dirty pages are fluctuating like mad within the controlled scope: http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/512M-2%25/xfs-4dd-1M-8p-435M-2%25-2.6.38-rc5-dt6+-2011-02-22-14-34/balance_dirty_pages-pages.png But still, it achieves 100% disk utilization http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/512M-2%25/xfs-4dd-1M-8p-435M-2%25-2.6.38-rc5-dt6+-2011-02-22-14-34/iostat-util.png and good IO throughput: http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/512M-2%25/xfs-4dd-1M-8p-435M-2%25-2.6.38-rc5-dt6+-2011-02-22-14-34/balance_dirty_pages-bandwidth.png And even better, less than 120ms writeback latencies: http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/512M-2%25/xfs-4dd-1M-8p-435M-2%25-2.6.38-rc5-dt6+-2011-02-22-14-34/balance_dirty_pages-pause.png Thanks, Fengguang -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
