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) ? -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
