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

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

On Sat, Apr 16, 2011 at 06:13:14AM +0800, Jan Kara wrote:
> On Fri 15-04-11 22:37:11, Wu Fengguang wrote:
> > On Fri, Apr 15, 2011 at 11:43:00AM +0800, Wu Fengguang wrote:
> > > On Fri, Apr 15, 2011 at 02:16:09AM +0800, Jan Kara wrote:
> > > > On Thu 14-04-11 23:14:25, 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).
> > > >   Thanks for the test but I cannot find out from the numbers you provided
> > > > how much did the per-bdi thresholds fluctuate in this low memory NAS case?
> > > > You can gather current bdi threshold from /sys/kernel/debug/bdi/<dev>/stats
> > > > so it shouldn't be hard to get the numbers...
> > > 
> > > Hi Jan, attached are your results w/o this patch. The "bdi goal" (gray
> > > line) is calculated as (bdi_thresh - bdi_thresh/8) and is fluctuating
> > > all over the place.. and average wkB/s is only 49MB/s..
> > 
> > I got the numbers for vanilla kernel: XFS can do 57MB/s and 63MB/s in
> > the two runs.  There are large fluctuations in the attached graphs, too.
>   Hmm, so the graphs from previous email are with longer "proportion
> period (without patch we discuss here)" and graphs from this email are
> with it?

All graphs for vanilla and your IO-less kernels are collected without
this patch.

I only showed in previous email how my IO-less kernel works with and
without this patch, and the conclusion is, it's not sensitive to it
and is working fine in both cases.

> > To summary it up, for a 1GB mem, 4 disks JBOD setup, running 1 dd per
> > disk:
> > 
> > vanilla: 57MB/s, 63MB/s
> > Jan:     49MB/s, 103MB/s
> > Wu:      76MB/s, 80MB/s
> > 
> > The balance_dirty_pages-task-bw-jan.png and
> > balance_dirty_pages-pages-jan.png shows very unfair allocation of
> > dirty pages and throughput among the disks...
>   Fengguang, can we please stay on topic? It's good to know that throughput
> fluctuates so much with my patches (although not that surprising seeing the
> fluctuations of bdi limits) but for the sake of this patch throughput
> numbers with different balance_dirty_pages() implementations do not seem
> that interesting.  What is interesting (at least to me) is how this
> particular patch changes fluctuations of bdi thresholds (fractions) in
> vanilla kernel. In the graphs, I can see only bdi goal - that is the
> per-bdi threshold we have in balance_dirty_pages() am I right? And it is
> there for only a single device, right?

bdi_goal = bdi_thresh * 7/8. They are close. So by looking at the bdi
goal curve, you get the idea how bdi_thresh fluctuates over time.

balance_dirty_pages-pages-jan.png looks very like the single device
situation, because the bdi goal is so high! But that's exactly the
problem: the first bdi is consuming most dirty pages quota and run at
full speed, while the other bdi's run mostly idle.  You can confirm
the imbalance in balance_dirty_pages-task-bw-jan.png and iostat.

Looks similar to the problem described here:

https://lkml.org/lkml/2010/12/5/6

> Anyway either with or without the patch, bdi thresholds are jumping rather
> wildly if I'm interpreting the graphs right. Hmm, which is not that surprising
> given that in ideal case we should have about 0.5s worth of writeback for
> each disk in the page cache. So with your patch the period for proportion
> estimation is also just about 0.5s worth of page writeback which is
> understandably susceptible to fluctuations. Thinking about it, the original
> period of 4*"dirty limit" on your machine is about 2.5 GB which is about
> 50s worth of writeback on that machine so it is in match with your
> observation that it takes ~100s for bdi threshold to climb up.
> 
> So what is a takeaway from this for me is that scaling the period
> with the dirty limit is not the right thing. If you'd have 4-times more
> memory, your choice of "dirty limit" as the period would be as bad as
> current 4*"dirty limit". What would seem like a better choice of period
> to me would be to have the period in an order of a few seconds worth of
> writeback. That would allow the bdi limit to scale up reasonably fast when
> new bdi starts to be used and still not make it fluctuate that much
> (hopefully).

Yes it's good to make it more bandwidth and time wise.  I'll be glad
if you can improve the algorithm :)

Thanks,
Fengguang

> Looking at math in lib/proportions.c, nothing really fundamental requires
> that each period has the same length. So it shouldn't be hard to actually
> create proportions calculator that would have timer triggered periods -
> simply whenever the timer fires, we would declare a new period. The only
> things which would be broken by this are (t represents global counter of
> events):
> a) counting of periods as t/period_len - we would have to maintain global
> period counter but that's trivial
> b) trick that we don't do t=t/2 for each new period but rather use
> period_len/2+(t % (period_len/2)) when calculating fractions - again we
> would have to bite the bullet and divide the global counter when we declare
> new period but again it's not a big deal in our case.
> 
> Peter what do you think about this? Do you (or anyone else) think it makes
> sense?
> 
> 								Honza
> -- 
> Jan Kara <jack@suse.cz>
> SUSE Labs, CR

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