From bb0822954aab7d23a3f902c2a103ee0242f6046e Mon Sep 17 00:00:00 2001 From: Wu Fengguang Date: Tue, 16 Aug 2011 13:37:14 -0600 Subject: squeeze max-pause area and drop pass-good area Revert the pass-good area introduced in ffd1f609ab10 ("writeback: introduce max-pause and pass-good dirty limits") and make the max-pause area smaller and safe. This fixes ~30% performance regression in the ext3 data=writeback fio_mmap_randwrite_64k/fio_mmap_randrw_64k test cases, where there are 12 JBOD disks, on each disk runs 8 concurrent tasks doing reads+writes. Using deadline scheduler also has a regression, but not that big as CFQ, so this suggests we have some write starvation. The test logs show that - the disks are sometimes under utilized - global dirty pages sometimes rush high to the pass-good area for several hundred seconds, while in the mean time some bdi dirty pages drop to very low value (bdi_dirty << bdi_thresh). Then suddenly the global dirty pages dropped under global dirty threshold and bdi_dirty rush very high (for example, 2 times higher than bdi_thresh). During which time balance_dirty_pages() is not called at all. So the problems are 1) The random writes progress so slow that they break the assumption of the max-pause logic that "8 pages per 200ms is typically more than enough to curb heavy dirtiers". 2) The max-pause logic ignored task_bdi_thresh and thus opens the possibility for some bdi's to over dirty pages, leading to (bdi_dirty >> bdi_thresh) and then (bdi_thresh >> bdi_dirty) for others. 3) The higher max-pause/pass-good thresholds somehow leads to the bad swing of dirty pages. The fix is to allow the task to slightly dirty over task_bdi_thresh, but no way to exceed bdi_dirty and/or global dirty_thresh. Tests show that it fixed the JBOD regression completely (both behavior and performance), while still being able to cut down large pause times in balance_dirty_pages() for single-disk cases. Reported-by: Li Shaohua Tested-by: Li Shaohua Acked-by: Jan Kara Signed-off-by: Wu Fengguang --- mm/page-writeback.c | 15 ++------------- 1 file changed, 2 insertions(+), 13 deletions(-) (limited to 'mm') diff --git a/mm/page-writeback.c b/mm/page-writeback.c index d1960744f88..0e309cd1b5b 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -754,21 +754,10 @@ static void balance_dirty_pages(struct address_space *mapping, * 200ms is typically more than enough to curb heavy dirtiers; * (b) the pause time limit makes the dirtiers more responsive. */ - if (nr_dirty < dirty_thresh + - dirty_thresh / DIRTY_MAXPAUSE_AREA && + if (nr_dirty < dirty_thresh && + bdi_dirty < (task_bdi_thresh + bdi_thresh) / 2 && time_after(jiffies, start_time + MAX_PAUSE)) break; - /* - * pass-good area. When some bdi gets blocked (eg. NFS server - * not responding), or write bandwidth dropped dramatically due - * to concurrent reads, or dirty threshold suddenly dropped and - * the dirty pages cannot be brought down anytime soon (eg. on - * slow USB stick), at least let go of the good bdi's. - */ - if (nr_dirty < dirty_thresh + - dirty_thresh / DIRTY_PASSGOOD_AREA && - bdi_dirty < bdi_thresh) - break; /* * Increase the delay for each loop, up to our previous -- cgit v1.2.3-70-g09d2