This will noticeably reduce the fluctuaions of pause time when there are 100+ concurrent dirtiers. The more parallel dirtiers (1 dirtier => 4 dirtiers), the smaller bandwidth each dirtier will share (bdi_bandwidth => bdi_bandwidth/4), the less gap to the dirty limit ((C-A) => (C-B)), the less stable the pause time will be (given the same fluctuation of bdi_dirty). For example, if A drifts to A', its pause time may drift from 5ms to 6ms, while B to B' may drift from 50ms to 90ms. It's much larger fluctuations in relative ratio as well as absolute time. Fig.1 before patch, gap (C-B) is too low to get smooth pause time throttle_bandwidth_A = bdi_bandwidth .........o | o <= A' | o | o | o | o throttle_bandwidth_B = bdi_bandwidth / 4 .....|...........o | | o <= B' ----------------------------------------------+-----------+---o A B C The solution is to lower the slope of the throttle line accordingly, which makes B stabilize at some point more far away from C. Fig.2 after patch throttle_bandwidth_A = bdi_bandwidth .........o | o <= A' | o | o lowered max throttle bandwidth for B ===> * o | * o throttle_bandwidth_B = bdi_bandwidth / 4 .............* o | | * o ----------------------------------------------+-------+-------o A B C Note that C is actually different points for 1-dirty and 4-dirtiers cases, but for easy graphing, we move them together. Signed-off-by: Wu Fengguang --- mm/page-writeback.c | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) --- linux-next.orig/mm/page-writeback.c 2010-12-13 21:46:14.000000000 +0800 +++ linux-next/mm/page-writeback.c 2010-12-13 21:46:15.000000000 +0800 @@ -587,6 +587,7 @@ static void balance_dirty_pages(struct a unsigned long background_thresh; unsigned long dirty_thresh; unsigned long bdi_thresh; + unsigned long task_thresh; unsigned long long bw; unsigned long period; unsigned long pause = 0; @@ -616,7 +617,7 @@ static void balance_dirty_pages(struct a break; bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh, nr_dirty); - bdi_thresh = task_dirty_limit(current, bdi_thresh); + task_thresh = task_dirty_limit(current, bdi_thresh); /* * In order to avoid the stacked BDI deadlock we need @@ -638,14 +639,23 @@ static void balance_dirty_pages(struct a bdi_update_bandwidth(bdi, start_time, bdi_dirty, bdi_thresh); - if (bdi_dirty >= bdi_thresh || nr_dirty > dirty_thresh) { + if (bdi_dirty >= task_thresh || nr_dirty > dirty_thresh) { pause = MAX_PAUSE; goto pause; } + /* + * When bdi_dirty grows closer to bdi_thresh, it indicates more + * concurrent dirtiers. Proportionally lower the max throttle + * bandwidth. This will resist bdi_dirty from approaching to + * close to task_thresh, and help reduce fluctuations of pause + * time when there are lots of dirtiers. + */ bw = bdi->write_bandwidth; - bw = bw * (bdi_thresh - bdi_dirty); + do_div(bw, bdi_thresh / BDI_SOFT_DIRTY_LIMIT + 1); + + bw = bw * (task_thresh - bdi_dirty); do_div(bw, bdi_thresh / TASK_SOFT_DIRTY_LIMIT + 1); period = HZ * pages_dirtied / ((unsigned long)bw + 1) + 1; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/