On Thu, Apr 14, 2011 at 11:14:24PM +0800, Wu Fengguang wrote: > On Thu, Apr 14, 2011 at 08:23:02AM +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. > > Just did a comparison of the IO-less patches' performance with and > without this patch. I hardly notice any differences besides some more > bdi goal fluctuations in the attached graphs. The write throughput is > a bit large with this patch (80MB/s vs 76MB/s), however the delta is > within the even larger stddev range (20MB/s). > > The basic conclusion is, my IO-less patchset is very insensible to the > bdi threshold fluctuations. In this kind of low memory case, just take > care to stop the bdi pages from dropping too low and you get good > performance. (well, the disks are still not 100% utilized at times...) > Fluctuations in disk throughput and dirty rate and virtually > everything are unavoidable due to the low memory situation. Yeah the fluctuations in the dirty rate are worse than memory bounty situations, however is still a lot better than what vanilla kernel can provide. The attached graphs are collected with this patch. They show <=20ms pause times and not all that straight but nowhere bumpy progresses. Thanks, Fengguang
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