On Sun, 19 Jun 2011 23:01:12 +0800 Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote: > The max-pause limit helps to keep the sleep time inside > balance_dirty_pages() within 200ms. The 200ms max sleep means per task > rate limit of 8pages/200ms=160KB/s, which normally is enough to stop > dirtiers from continue pushing the dirty pages high, unless there are > a sufficient large number of slow dirtiers (ie. 500 tasks doing 160KB/s > will still sum up to 80MB/s, reaching the write bandwidth of a slow disk). > > The pass-good limit helps to let go of the good bdi's in the presence of > a blocked bdi (ie. NFS server not responding) or slow USB disk which for > some reason build up a large number of initial dirty pages that refuse > to go away anytime soon. The hard-wired numbers and hard-wired assumptions about device speeds shouldn't be here at all. They will be sub-optimal (and sometimes extremely so) for all cases. They will become wronger over time. Or less wrong, depending upon which way they were originally wrong. > + dirty_thresh = hard_dirty_limit(dirty_thresh); > + if (nr_dirty < dirty_thresh + dirty_thresh / DIRTY_MAXPAUSE && > + jiffies - start_time > MAX_PAUSE) > + break; > + if (nr_dirty < dirty_thresh + dirty_thresh / DIRTY_PASSGOOD && > + bdi_dirty < bdi_thresh) > + break; It appears that despite their similarity, DIRTY_MAXPAUSE is a dimensionless value whereas the units of MAX_PAUSE is jiffies. Perhaps more care in naming choices would clarify things like this. The first comparison might be clearer if it used time_after(). Both statements need comments explaining what they do and *why they do it*. -- 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
