This patch seems optional and won't improve things noticeably. Even if we break out of the loop, the task will quickly return to balance_dirty_pages() as long as the bdi is dirty_exceeded. So I'd like to drop this patch for now. Thanks, Fengguang On Mon, Dec 13, 2010 at 10:47:03PM +0800, Wu, Fengguang wrote: > Tests show that bdi_thresh may take minutes to ramp up on a typical > desktop. The time should be improvable but cannot be eliminated totally. > So when (background_thresh + dirty_thresh)/2 is reached and > balance_dirty_pages() starts to throttle the task, it will suddenly find > the (still low and ramping up) bdi_thresh is exceeded _excessively_. Here > we definitely don't want to stall the task for one minute (when it's > writing to USB stick). So introduce an alternative way to break out of > the loop when the bdi dirty/write pages has dropped by a reasonable > amount. > > It will at least pause for one loop before trying to break out. > > The break is designed mainly to help the single task case. The break > threshold is time for writing 125ms data, so that when the task slept > for MAX_PAUSE=200ms, it will have good chance to break out. For NFS > there may be only 1-2 completions of large COMMIT per second, in which > case the task may still get stuck for 1s. > > Note that this opens the chance that during normal operation, a huge > number of slow dirtiers writing to a really slow device might manage to > outrun bdi_thresh. But the risk is pretty low. > > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> > --- > mm/page-writeback.c | 19 +++++++++++++++++++ > 1 file changed, 19 insertions(+) > > --- linux-next.orig/mm/page-writeback.c 2010-12-13 21:46:16.000000000 +0800 > +++ linux-next/mm/page-writeback.c 2010-12-13 21:46:16.000000000 +0800 > @@ -693,6 +693,7 @@ static void balance_dirty_pages(struct a > long nr_dirty; > long bdi_dirty; /* = file_dirty + writeback + unstable_nfs */ > long avg_dirty; /* smoothed bdi_dirty */ > + long bdi_prev_dirty = 0; > unsigned long background_thresh; > unsigned long dirty_thresh; > unsigned long bdi_thresh; > @@ -749,6 +750,24 @@ static void balance_dirty_pages(struct a > > bdi_update_bandwidth(bdi, start_time, bdi_dirty, bdi_thresh); > > + /* > + * bdi_thresh takes time to ramp up from the initial 0, > + * especially for slow devices. > + * > + * It's possible that at the moment dirty throttling starts, > + * bdi_dirty = nr_dirty > + * = (background_thresh + dirty_thresh) / 2 > + * >> bdi_thresh > + * Then the task could be blocked for many seconds to flush all > + * the exceeded (bdi_dirty - bdi_thresh) pages. So offer a > + * complementary way to break out of the loop when 125ms worth > + * of dirty pages have been cleaned during our pause time. > + */ > + if (nr_dirty <= dirty_thresh && > + bdi_prev_dirty - bdi_dirty > (long)bdi->write_bandwidth / 8) > + break; > + bdi_prev_dirty = bdi_dirty; > + > avg_dirty = bdi->avg_dirty; > if (avg_dirty < bdi_dirty || avg_dirty > task_thresh) > avg_dirty = bdi_dirty; > -- 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
