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? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href