On Tue, Aug 23, 2011 at 12:01:00PM +0200, 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.. Exactly. This is where it gets confusing and is bone of contention. > > So if you want completely separated feedback loops I would expect > something like: > > balance_rate_(i+1) = balance_rate_(i) * bw_ratio ; every 200ms > I agree. This makes sense. IOW. write_bw bdi->dirty_ratelimit_n = bdi->dirty_ratelimit_(n-1) * ------- dirty_rate > 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. I think you meant. "if we throttled too much, the dirty_rate will have dropped and the bw_ratio will be >1 causing the balance_rate to increase hence 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. > We I thought that this is evident that. task_ratelimit = balanced_rate * pos_ratio What is not evident to me is following. balanced_rate_(i+1) = task_ratelimit_200ms * pos_ratio. Instead, like you, I also thought that following is more obivious. balanced_rate_(i+1) = balanced_rate_(i) * pos_ratio Thanks Vivek -- 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
