Re: [PATCH 2/5] writeback: dirty position control

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

On Thu, Aug 25, 2011 at 02:00:58AM +0800, Vivek Goyal wrote:
> On Wed, Aug 24, 2011 at 08:12:58AM +0800, Wu Fengguang wrote:
> > > You somehow directly jump to  
> > > 
> > > 	balanced_rate = task_ratelimit_200ms * write_bw / dirty_rate
> > > 
> > > without explaining why following will not work.
> > > 
> > > 	balanced_rate_(i+1) = balance_rate(i) * write_bw / dirty_rate
> > 
> > Thanks for asking that, it's probably the root of confusions, so let
> > me answer it standalone.
> > 
> > It's actually pretty simple to explain this equation:
> > 
> >                                                write_bw
> >         balanced_rate = task_ratelimit_200ms * ----------       (1)
> >                                                dirty_rate
> > 
> > If there are N dd tasks, each task is throttled at task_ratelimit_200ms
> > for the past 200ms, we are going to measure the overall bdi dirty rate
> > 
> >         dirty_rate = N * task_ratelimit_200ms                   (2)
> > 
> > put (2) into (1) we get
> > 
> >         balanced_rate = write_bw / N                            (3)
> > 
> > So equation (1) is the right estimation to get the desired target (3).
> > 
> > 
> > As for
> > 
> >                                                   write_bw
> >         balanced_rate_(i+1) = balanced_rate_(i) * ----------    (4)
> >                                                   dirty_rate
> > 
> > Let's compare it with the "expanded" form of (1):
> > 
> >                                                               write_bw
> >         balanced_rate_(i+1) = balanced_rate_(i) * pos_ratio * ----------      (5)
> >                                                               dirty_rate
> > 
> > So the difference lies in pos_ratio.
> > 
> > Believe it or not, it's exactly the seemingly use of pos_ratio that
> > makes (5) independent(*) of the position control.
> > 
> > Why? Look at (4), assume the system is in a state
> > 
> > - dirty rate is already balanced, ie. balanced_rate_(i) = write_bw / N
> > - dirty position is not balanced, for example pos_ratio = 0.5
> > 
> > balance_dirty_pages() will be rate limiting each tasks at half the
> > balanced dirty rate, yielding a measured
> > 
> >         dirty_rate = write_bw / 2                               (6)
> > 
> > Put (6) into (4), we get
> > 
> >         balanced_rate_(i+1) = balanced_rate_(i) * 2
> >                             = (write_bw / N) * 2
> > 
> > That means, any position imbalance will lead to balanced_rate
> > estimation errors if we follow (4). Whereas if (1)/(5) is used, we
> > always get the right balanced dirty ratelimit value whether or not
> > (pos_ratio == 1.0), hence make the rate estimation independent(*) of
> > dirty position control.
> > 
> > (*) independent as in real values, not the seemingly relations in equation
> 
> Ok, I think I am beginning to see your point. Let me just elaborate on
> the example you gave.

Thank you very much :)

> Assume a system is completely balanced and a task is writing at 100MB/s
> rate.
> 
> write_bw = dirty_rate = 100MB/s, pos_ratio = 1; N=1
> 
> bdi->dirty_ratelimit = 100MB/s
> 
> Now another tasks starts dirtying the page cache on same bdi. Number of 
> dirty pages should go up pretty fast and likely position ratio feedback
> will kick in to reduce the dirtying rate. (rate based feedback does not
> kick in till next 200ms) and pos_ratio feedback seems to be instantaneous.

That's right. There must be some instantaneous feedback to react to
fast workload changes. With pos_ratio providing this capability, the
estimated balanced rate can take time to follow.

Note that pos_ratio by itself is enough to limit dirty pages within
the [freerun, limit] control scope. The cost of (temporarily) large
error in balanced rate is, task_ratelimit will be fluctuating much
more, due to the fact pos_ratio will depart from 1.0 (to the point it
can fully compensate for the rate errors) and dirty pages approaching
@freerun or @limit where the slope of pos_ratio goes sharp.

The correct estimation of balanced rate serves to drive pos_ratio back
to 1.0, where it has the most flat slope.

> Assume new pos_ratio is .5
> 
> So new throttle rate for both the tasks is 50MB/s.
> 
> bdi->dirty_ratelimit = 100MB/s (a feedback has not kicked in yet)
> task_ratelimit = bdi->dirty_ratelimit * pos_ratio = 100 *.5 = 50MB/s
> 
> Now lets say 200ms have passed and rate base feedback is reevaluated.
> 
> 						        write_bw	
> bdi->dirty_ratelimit_(i+1) = bdi->dirty_ratelimit_i * ---------
> 						        dirty_bw
> 
> bdi->dirty_ratelimit_(i+1) = 100 * 100/100 = 100MB/s
> 
> Ideally bdi->dirty_ratelimit should have now become 50MB/s as N=2 but 
> that did not happen. And reason being that there are two feedback control
> loops and pos_ratio loops reacts to imbalances much more quickly. Because
> previous loop has already reacted to the imbalance and reduced the
> dirtying rate of task, rate based loop does not try to adjust anything
> and thinks everything is just fine.

That's right.

> Things are fine in the sense that still dirty_rate == write_bw but
> system is not balanced in terms of number of dirty pages and pos_ratio=.5

Yes. The bad thing is, if the above equation (of pure rate feedback)
is used, the system is going to remain in that position-imbalanced
state forever, which is bad for the smoothness of task_ratelimit.

> So you are trying to make one feedback loop aware of second loop so that
> if second loop is unbalanced, first loop reacts to that as well and not
> just look at dirty_rate and write_bw. So refining new balanced rate by
> pos_ratio helps.
> 						      write_bw	
> bdi->dirty_ratelimit_(i+1) = bdi->dirty_ratelimit_i * --------- * pos_ratio
> 						      dirty_bw
> 
> Now if global dirty pages are imbalanced, balanced rate will still go
> down despite the fact that dirty_bw == write_bw. This will lead to
> further reduction in task dirty rate. Which in turn will lead to reduced
> number of dirty rate and should eventually lead to pos_ratio=1.

Right, that's a good alternative viewpoint to the below one.

  						  write_bw	
  bdi->dirty_ratelimit_(i+1) = task_ratelimit_i * ---------
  						  dirty_bw

(1) the periodic rate estimation uses that to refresh the balanced rate on every 200ms
(2) as long as the rate estimation is correct, pos_ratio is able to drive itself to 1.0

> A related question though I should have asked you this long back. How does
> throttling based on rate helps. Why we could not just work with two
> pos_ratios. One is gloabl postion ratio and other is bdi position ratio.
> And then throttle task gradually to achieve smooth throttling behavior.
> IOW, what property does rate provide which is not available just by
> looking at per bdi dirty pages. Can't we come up with bdi setpoint and
> limit the way you have done for gloabl setpoint and throttle tasks
> accordingly?

Good question. If we have no idea of the balanced rate at all, but
still want to limit dirty pages within the range [freerun, limit],
all we can do is to throttle the task at eg. 1TB/s at @freerun and
0 at @limit. Then you get a really sharp control line which will make
task_ratelimit fluctuate like mad...

So the balanced rate estimation is the key to get smooth task_ratelimit,
while pos_ratio is the ultimate guarantee for the dirty pages range.

Thanks,
Fengguang

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