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 -- 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
