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

[Wu Fengguang]

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
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