Re: [PATCH 4/4] writeback: reduce per-bdi dirty threshold ramp up time

[Wu Fengguang <fengguang.wu@intel.com>]

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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@chello.nl>
> > > > > > CC: Richard Kennedy <richard@rsk.demon.co.uk>
> > > > > > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > > > >   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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