Hello Fengguang,

On Fri 25-03-11 21:44:11, Wu Fengguang wrote:

On Wed, Mar 23, 2011 at 05:43:14AM +0800, Jan Kara wrote:

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  Hello Fengguang,

On Fri 18-03-11 22:30:01, Wu Fengguang wrote:

quoted

On Wed, Mar 09, 2011 at 06:31:10AM +0800, Jan Kara wrote:

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  Hello,

  I'm posting second version of my IO-less balance_dirty_pages() patches. This
is alternative approach to Fengguang's patches - much simpler I believe (only
300 lines added) - but obviously I does not provide so sophisticated control.

Well, it may be too early to claim "simplicity" as an advantage, until
you achieve the following performance/feature comparability (most of
them are not optional ones). AFAICS this work is kind of heavy lifting
that will consume a lot of time and attention. You'd better find some
more fundamental needs before go on the reworking.

(1)  latency
(2)  fairness
(3)  smoothness
(4)  scalability
(5)  per-task IO controller
(6)  per-cgroup IO controller (TBD)
(7)  free combinations of per-task/per-cgroup and bandwidth/priority controllers
(8)  think time compensation
(9)  backed by both theory and tests
(10) adapt pause time up on 100+ dirtiers
(11) adapt pause time down on low dirty pages 
(12) adapt to new dirty threshold/goal
(13) safeguard against dirty exceeding
(14) safeguard against device queue underflow

  I think this is a misunderstanding of my goals ;). My main goal is to
explore, how far we can get with a relatively simple approach to IO-less
balance_dirty_pages(). I guess what I have is better than the current
balance_dirty_pages() but it sure does not even try to provide all the
features you try to provide.

OK.

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I'm thinking about tweaking ratelimiting logic to reduce latencies in some
tests, possibly add compensation when we waited for too long in
balance_dirty_pages() (e.g. because of bumpy IO completion) but that's
about it...

Basically I do this so that we can compare and decide whether what my
simple approach offers is OK or whether we want some more complex solution
like your patches...

Yeah, now both results are on the website. Let's see whether they are
acceptable for others.

  Yes. BTW, I think we'll discuss this at LSF so it would be beneficial if
we both prepared a fairly short explanation of our algorithm and some
summary of the measured results. I think it would be good to keep each of
us below 5 minutes so that we don't bore the audience - people will ask for
details where they are interested... What do you think?

I'll try to run also your patches on my setup to see how they work :) V6
from your website is the latest version, isn't it?

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The basic idea (implemented in the third patch) is that processes throttled
in balance_dirty_pages() wait for enough IO to complete. The waiting is
implemented as follows: Whenever we decide to throttle a task in
balance_dirty_pages(), task adds itself to a list of tasks that are throttled
against that bdi and goes to sleep waiting to receive specified amount of page
IO completions. Once in a while (currently HZ/10, in patch 5 the interval is
autotuned based on observed IO rate), accumulated page IO completions are
distributed equally among waiting tasks.

This waiting scheme has been chosen so that waiting time in
balance_dirty_pages() is proportional to
  number_waited_pages * number_of_waiters.
In particular it does not depend on the total number of pages being waited for,
thus providing possibly a fairer results.

When there comes no IO completion in 1 second (normal in NFS), the
tasks will all get stuck. It is fixable based on your v2 code base
(detailed below), however will likely bring the same level of
complexity as the base bandwidth solution.

  I have some plans how to account for bumpy IO completion when we wait for
a long time and then get completion of much more IO than we actually need.
But in case where processes use all the bandwidth and the latency of the
device is high, sure they take the penalty and have to wait for a long time
in balance_dirty_pages().

No, I don't think it's good to block for long time in
balance_dirty_pages(). This seems to be our biggest branch point.

  I agree we should not block for several seconds under normal load but
when something insane like 1000 dds is running, I don't think it's a big
problem :)

And actually the NFS traces you pointed to originally seem to be different
problem, in fact not directly related to what balance_dirty_pages() does...
And with local filesystem the results seem to be reasonable (although there
are some longer sleeps in your JBOD measurements I don't understand yet).

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The results for different bandwidths fio load is interesting. There are 8
threads dirtying pages at 1,2,4,..,128 MB/s rate. Due to different task
bdi dirty limits, what happens is that three most aggresive tasks get
throttled so they end up at bandwidths 24, 26, and 30 MB/s and the lighter
dirtiers run unthrottled.

The base bandwidth based throttling can do better and provide almost
perfect fairness, because all tasks writing to one bdi derive their
own throttle bandwidth based on the same per-bdi base bandwidth. So
the heavier dirtiers will converge to equal dirty rate and weight.

  So what do you consider a perfect fairness in this case and are you sure
it is desirable? I was thinking about this and I'm not sure...

Perfect fairness could be 1, 2, 4, 8, N, N, N MB/s, where

        N = (write_bandwidth - 1 - 2 - 4 - 8) / 3.

I guess its usefulness is largely depending on the user space
applications.  Most of them should not be sensible to it.

  I see, that makes some sense although it makes it advantageous to split
heavy dirtier task into two less heavy dirtiers which is a bit strange. But
as you say, precise results here probably do not matter much.

						Have a nice weekend

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR

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