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

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Aug 23, 2011 at 10:15:04PM +0800, Wu Fengguang wrote:
> On Tue, Aug 23, 2011 at 06:01:00PM +0800, Peter Zijlstra wrote:
> > On Tue, 2011-08-23 at 11:40 +0800, Wu Fengguang wrote:
> > > - not a factor at all for updating balanced_rate (whether or not we do (2))
> > >   well, in this concept: the balanced_rate formula inherently does not
> > >   derive the balanced_rate_(i+1) from balanced_rate_i. Rather it's
> > >   based on the ratelimit executed for the past 200ms:
> > > 
> > >           balanced_rate_(i+1) = task_ratelimit_200ms * bw_ratio
> > 
> > Ok, this is where it all goes funny..
> > 
> > So if you want completely separated feedback loops I would expect
> 
> If call it feedback loops, then it's a series of independent feedback
> loops of depth 1.  Because each balanced_rate is a fresh estimation
> dependent solely on
> 
> - writeout bandwidth
> - N, the number of dd tasks
> 
> in the past 200ms.
> 
> As long as a CONSTANT ratelimit (whatever value it is) is executed in
> the past 200ms, we can get the same balanced_rate.
> 
>         balanced_rate = CONSTANT_ratelimit * write_bw / dirty_rate
> 
> The resulted balanced_rate is independent of how large the CONSTANT
> ratelimit is, because if we start with a doubled CONSTANT ratelimit,
> we'll see doubled dirty_rate and result in the same balanced_rate. 
> 
> In that manner, balance_rate_(i+1) is not really depending on the
> value of balance_rate_(i): whatever balance_rate_(i) is, we are going
> to get the same balance_rate_(i+1) if not considering estimation
> errors. Note that the estimation errors mainly come from the
> fluctuations in dirty_rate.
> 
> That may well be what's already in your mind, just that we disagree
> about the terms ;)
> 
> > something like:
> > 
> > 	balance_rate_(i+1) = balance_rate_(i) * bw_ratio   ; every 200ms
> > 
> > The former is a complete feedback loop, expressing the new value in the
> > old value (*) with bw_ratio as feedback parameter; if we throttled too
> > much, the dirty_rate will have dropped and the bw_ratio will be <1
> > causing the balance_rate to drop increasing the dirty_rate, and vice
> > versa.
> 
> In principle, the bw_ratio works that way. However since
> balance_rate_(i) is not the exact _executed_ ratelimit in
> balance_dirty_pages().
> 
> > (*) which is the form I expected and why I thought your primary feedback
> > loop looked like: rate_(i+1) = rate_(i) * pos_ratio * bw_ratio
>  
> Because the executed ratelimit was rate_(i) * pos_ratio.
> 
> > With the above balance_rate is an independent variable that tracks the
> > write bandwidth. Now possibly you'd want a low-pass filter on that since
> > your bw_ratio is a bit funny in the head, but that's another story.
> 
> Yeah.
> 
> > Then when you use the balance_rate to actually throttle tasks you apply
> > your secondary control steering the dirty page count, yielding:
> > 
> > 	task_rate = balance_rate * pos_ratio
> 
> Right. Note the above formula is not a derived one, but an original
> one that later leads to pos_ratio showing up in the calculation of
> balanced_rate.
> 
> > >   and task_ratelimit_200ms happen to can be estimated from
> > > 
> > >           task_ratelimit_200ms ~= balanced_rate_i * pos_ratio
> > 
> > >   We may alternatively record every task_ratelimit executed in the
> > >   past 200ms and average them all to get task_ratelimit_200ms. In this
> > >   way we take the "superfluous" pos_ratio out of sight :) 
> > 
> > Right, so I'm not at all sure that makes sense, its not immediately
> > evident that <task_ratelimit> ~= balance_rate * pos_ratio. Nor is it
> > clear to me why your primary feedback loop uses task_ratelimit_200ms at
> > all. 
> 
> task_ratelimit is used and hence defined to be (balance_rate * pos_ratio)
> by balance_dirty_pages(). So this is an original formula:
> 
>         task_ratelimit = balance_rate * pos_ratio
> 
> task_ratelimit_200ms is also used as an original data source in
> 
>         balanced_rate = task_ratelimit_200ms * write_bw / dirty_rate
> 

I think above calculates to.

 task_ratelimit = balanced_rate * pos_ratio
or
 task_ratelimit = task_ratelimit_200ms * write_bw / dirty_rate * pos_ratio
or
 task_ratelimit = balance_rate * pos_ratio  * write_bw / dirty_rate * pos_ratio
or
								    2
 task_ratelimit = balance_rate * write_bw / dirty_rate * (pos_ratio)

And the question is why not.

 task_ratelimit = prev-balance_rate * write_bw / dirty_rate * pos_ratio

Which sounds intutive as comapred to former one.

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
Vivek