path: root/mm
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authorWu Fengguang <fengguang.wu@intel.com>2011-08-16 13:37:14 -0600
committerWu Fengguang <fengguang.wu@intel.com>2011-08-19 22:42:07 +0800
commitbb0822954aab7d23a3f902c2a103ee0242f6046e (patch)
tree3049962f0ecc05eea4b2b4ef5480b6708bc74ce7 /mm
parentLinux 3.1-rc2 (diff)
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 <shaohua.li@intel.com> Tested-by: Li Shaohua <shaohua.li@intel.com> Acked-by: Jan Kara <jack@suse.cz> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Diffstat (limited to 'mm')
1 files changed, 2 insertions, 13 deletions
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index d1960744f881..0e309cd1b5b9 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))
- /*
- * 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