Messages in this thread | | | Subject | Re: [PATCH 2/5] writeback: dirty position control | From | Peter Zijlstra <> | Date | Tue, 23 Aug 2011 12:01:00 +0200 |
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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 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.
(*) 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
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.
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
> 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.
> There is fundamentally no dependency between balanced_rate_(i+1) and > balanced_rate_i/task_ratelimit_200ms: the balanced_rate estimation > only asks for _whatever_ CONSTANT task ratelimit to be executed for > 200ms, then it get the balanced rate from the dirty_rate feedback.
How can there not be a relation between balance_rate_(i+1) and balance_rate_(i) ?
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