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authorWu Fengguang2011-08-16 13:37:14 -0600
committerWu Fengguang2011-08-19 22:42:07 +0800
commitbb0822954aab7d23a3f902c2a103ee0242f6046e (patch)
tree3049962f0ecc05eea4b2b4ef5480b6708bc74ce7 /include
parent93ee7a9340d64f20295aacc3fb6a22b759323280 (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 'include')
-rw-r--r--include/linux/writeback.h11
1 files changed, 0 insertions, 11 deletions
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index f1bfa12ea246..2b8963ff0f35 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -12,15 +12,6 @@
*
* (thresh - thresh/DIRTY_FULL_SCOPE, thresh)
*
- * The 1/16 region above the global dirty limit will be put to maximum pauses:
- *
- * (limit, limit + limit/DIRTY_MAXPAUSE_AREA)
- *
- * The 1/16 region above the max-pause region, dirty exceeded bdi's will be put
- * to loops:
- *
- * (limit + limit/DIRTY_MAXPAUSE_AREA, limit + limit/DIRTY_PASSGOOD_AREA)
- *
* Further beyond, all dirtier tasks will enter a loop waiting (possibly long
* time) for the dirty pages to drop, unless written enough pages.
*
@@ -31,8 +22,6 @@
*/
#define DIRTY_SCOPE 8
#define DIRTY_FULL_SCOPE (DIRTY_SCOPE / 2)
-#define DIRTY_MAXPAUSE_AREA 16
-#define DIRTY_PASSGOOD_AREA 8
/*
* 4MB minimal write chunk size