Re: [PATCH 5/5] writeback: IO-less balance_dirty_pages()

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

On Sat, Aug 20, 2011 at 03:00:37AM +0800, Vivek Goyal wrote:
> On Fri, Aug 19, 2011 at 10:54:06AM +0800, Wu Fengguang wrote:
> > Hi Vivek,
> > 
> > > > +		base_rate = bdi->dirty_ratelimit;
> > > > +		pos_ratio = bdi_position_ratio(bdi, dirty_thresh,
> > > > +					       background_thresh, nr_dirty,
> > > > +					       bdi_thresh, bdi_dirty);
> > > > +		if (unlikely(pos_ratio == 0)) {
> > > > +			pause = MAX_PAUSE;
> > > > +			goto pause;
> > > >  		}
> > > > +		task_ratelimit = (u64)base_rate *
> > > > +					pos_ratio >> RATELIMIT_CALC_SHIFT;
> > > 
> > > Hi Fenguaang,
> > > 
> > > I am little confused here. I see that you have already taken pos_ratio
> > > into account in bdi_update_dirty_ratelimit() and wondering why to take
> > > that into account again in balance_diry_pages().
> > > 
> > > We calculated the pos_rate and balanced_rate and adjusted the
> > > bdi->dirty_ratelimit accordingly in bdi_update_dirty_ratelimit().
> > 
> > Good question. There are some inter-dependencies in the calculation,
> > and the dependency chain is the opposite to the one in your mind:
> > balance_dirty_pages() used pos_ratio in the first place, so that
> > bdi_update_dirty_ratelimit() have to use pos_ratio in the calculation
> > of the balanced dirty rate, too.
> > 
> > Let's return to how the balanced dirty rate is estimated. Please pay
> > special attention to the last paragraphs below the "......" line.
> > 
> > Start by throttling each dd task at rate
> > 
> >         task_ratelimit = task_ratelimit_0                               (1)
> >                          (any non-zero initial value is OK)
> > 
> > After 200ms, we measured
> > 
> >         dirty_rate = # of pages dirtied by all dd's / 200ms
> >         write_bw   = # of pages written to the disk / 200ms
> > 
> > For the aggressive dd dirtiers, the equality holds
> > 
> >         dirty_rate == N * task_rate
> >                    == N * task_ratelimit
> >                    == N * task_ratelimit_0                              (2)
> > Or     
> >         task_ratelimit_0 = dirty_rate / N                               (3)
> > 
> > Now we conclude that the balanced task ratelimit can be estimated by
> > 
> >         balanced_rate = task_ratelimit_0 * (write_bw / dirty_rate)      (4)
> > 
> > Because with (2) and (3), (4) yields the desired equality (1):
> > 
> >         balanced_rate == (dirty_rate / N) * (write_bw / dirty_rate)
> >                       == write_bw / N
> 
> Hi Fengguang,
> 
> Following is my understanding. Please correct me where I got it wrong.
> 
> Ok, I think I follow till this point. I think what you are saying is
> that following is our goal in a stable system.
> 
> 	task_ratelimit = write_bw/N				(6)
> 
> So we measure the write_bw of a bdi over a period of time and use that
> as feedback loop to modify bdi->dirty_ratelimit which inturn modifies
> task_ratelimit and hence we achieve the balance. So we will start with
> some arbitrary task limit say task_ratelimit_0, and modify that limit
> over a period of time based on our feedback loop to achieve a balanced
> system. And following seems to be the formula.
> 					    write_bw
> 	task_ratelimit = task_ratelimit_0 * ------- 		(7)
> 					    dirty_rate
> 
> Now I also understand that by using (2) and (3), you proved that
> how (7) will lead to (6) and that is our deisred goal. 

That's right.

> > 
> > .............................................................................
> > 
> > Now let's revisit (1). Since balance_dirty_pages() chooses to execute
> > the ratelimit
> > 
> >         task_ratelimit = task_ratelimit_0
> >                        = dirty_ratelimit * pos_ratio                    (5)
> > 
> 
> So balance_drity_pages() chose to take into account pos_ratio() also
> because for various reason like just taking into account only bandwidth
> variation as feedback was not sufficient. So we also took pos_ratio
> into account which in-trun is dependent on gloabal dirty pages and per
> bdi dirty_pages/rate.

That's right so far. balance_drity_pages() needs to do dirty position
control, so used formula (5).

> So we refined the formula for calculating a tasks's effective rate
> over a period of time to following.
> 					    write_bw
> 	task_ratelimit = task_ratelimit_0 * ------- * pos_ratio		(9)
> 					    dirty_rate
> 

That's not true. It should still be formula (7) when
balance_drity_pages() considers pos_ratio.

> > Put (5) into (4), we get the final form used in
> > bdi_update_dirty_ratelimit()
> > 
> >         balanced_rate = (dirty_ratelimit * pos_ratio) * (write_bw / dirty_rate)
> > 
> > So you really need to take (dirty_ratelimit * pos_ratio) as a single entity.
> 
> Now few questions.
> 
> - What is dirty_ratelimit in formula above?

It's bdi->dirty_ratelimit.

> - Is it wrong to understand the issue in following manner.
> 
>   bdi->dirty_ratelimit is tracking write bandwidth variation on the bdi
>   and effectively tracks write_bw/N.
> 
>   bdi->dirty_ratelimit = write_bw/N

Yes. Strictly speaking, the target value is (note the "==")

        bdi->dirty_ratelimit == write_bw/N

>   or 
> 
> 					    		  write_bw
>   bdi->dirty_ratelimit = previous_bdi->dirty_ratelimit * -------------    (10)
> 					     		  dirty_rate

Both (9) and (10) are not true. The right form is

                                                                     write_bw
balanced_rate = whatever_ratelimit_executed_in_balance_dirty_pages * ----------
                                                                     dirty_rate

where

whatever_ratelimit_executed_in_balance_dirty_pages ~= bdi->dirty_ratelimit * pos_ratio
bdi->dirty_ratelimit ~= balanced_rate

>  Hence a tasks's balanced rate from (9) and (10) is.
> 
>  task_ratelimit = bdi->dirty_ratelimit * pos_ratio		(11)
> So my understanding about (10) and (11) is wrong? if no, then question
> comes that

(11) in itself is right. It's the exact form used in code.
 
> bdi->dirty_ratelimit is supposed to be keeping track of 
> write bandwidth variations only.

Yes in a stable workload. Besides, if the number of dd tasks (N)
changed, dirty_ratelimit will adapt to new value (write_bw / N).

> And in turn task ratelimit will be
> driven by both bandwidth varation as well as pos_ratio variation.

That's right.
 
> But you seem to be doing following.
> 
>  bdi->dirty_ratelimit = adjust based on a cobination of bandwidth feedback
> 		        and pos_ratio feedback. 
> 
>  task_ratelimit = bdi->dirty_ratelimit * pos_ratio		(12)
> 
> So my question is that when task_ratelimit is finally being adjusted 
> based on pos_ratio feedback, why bdi->dirty_ratelimit also needs to
> take that into account.

In _concept_, bdi->dirty_ratelimit only depends on
whatever_ratelimit_executed_in_balance_dirty_pages.

Then, we try to estimate the latter with formula

whatever_ratelimit_executed_in_balance_dirty_pages ~= bdi->dirty_ratelimit * pos_ratio

That is the main reason we want to limit the step size of bdi->dirty_ratelimit:
otherwise the above estimation will have big errors if bdi->dirty_ratelimit
has changed a lot during the past 200ms.

That's also the reason balanced_rate will have larger errors when
close to @limit: because there pos_ratio drops _quickly_ to 0, hence
the regular fluctuations in dirty pages will result in big
fluctuations in the _relative_ value of pos_ratio.

> I know you have tried explaining it, but sorry, I did not get it. May
> be give it another shot in a layman's terms and I might understand it.

Sorry for that. I can explain if you have more questions :)

Thanks,
Fengguang

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