On Thu, Apr 14, 2011 at 06:36:22PM +0800, Richard Kennedy wrote: > On Thu, 2011-04-14 at 08:23 +0800, Wu Fengguang wrote: > > 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 > > > > I'm only testing on a desktop with 2 drives. I use a simple test to > write 2gb to sda then 2gb to sdb while recording the threshold values. > On 2.6.39-rc3, after the 2nd write starts it take approx 90 seconds for > sda's threshold value to drop from its maximum to minimum and sdb's to > rise from min to max. So this seems much too slow for normal desktop > workloads. Yes. > I haven't tested with this patch on 2.6.39-rc3 yet, but I'm just about > to set that up. It will sure help, but the problem is now the low-memory NAS servers.. Fortunately my patchset could make the dirty pages ramp up much more fast than the ramp up speed of the per-bdi threshold, and is also less sensitive to the fluctuations of per-bdi thresholds in JBOD setup. In fact my main concern in the low-memory NAS setup is how to prevent disk from going idle from time to time due to bdi dirty pages running low. The fluctuations of per-bdi thresholds in this case is no longer relevant for me. I end up adding a rule to throttle the task less when the bdi is running low of dirty pages. I find that the vanilla kernel also has this problem. > I know it's difficult to pick one magic number to fit every case, but I > don't see any easy way to make this more adaptive. We could make this > calculation take account of more things, but I don't know what. > > > Nice graphs :) BTW do you know what's causing that 10 second (1/10 Hz) > fluctuation in write bandwidth? and does this change effect that in any > way? In fact each filesystems is fluctuating in its unique way. For example, ext4, 4 dd http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/512M-2%25/ext4-4dd-1M-8p-435M-2%25-2.6.38-rc5-dt6+-2011-02-22-14-49/balance_dirty_pages-bandwidth.png btrfs, 4 dd http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/512M-2%25/btrfs-4dd-1M-8p-435M-2%25-2.6.38-rc5-dt6+-2011-02-22-15-03/balance_dirty_pages-bandwidth.png btrfs, 1 dd http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/512M-2%25/btrfs-1dd-1M-8p-435M-2%25-2.6.38-rc5-dt6+-2011-02-22-14-56/balance_dirty_pages-bandwidth.png I'm not sure about the exact root cause, but it's more or less related to the fluctuations of IO completion events. For example, the "written" curve is not a strictly straight line: http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/512M-2%25/btrfs-1dd-1M-8p-435M-2%25-2.6.38-rc5-dt6+-2011-02-22-14-56/global_dirtied_written.png Thanks, Fengguang -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
