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

[Peter Zijlstra]

On Wed, 2011-08-24 at 14:00 -0400, Vivek Goyal wrote:
> 
> Ok, I think I am beginning to see your point. Let me just elaborate on
> the example you gave.
> 
> 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.
> 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.
> 
> 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
> 
> 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.


Ok so this argument makes sense, is there some formalism to describe
such systems where such things are more evident?


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