RFC: dirty_ratio back to 40%

From: Larry Woodman
Date: Thu May 20 2010 - 07:25:18 EST

We've seen multiple performance regressions linked to the lower(20%)
dirty_ratio. When performing enough IO to overwhelm the background flush daemons the percent of dirty pagecache memory quickly climbs to the new/lower dirty_ratio value of 20%. At that point all writing processes are forced to stop and write dirty pagecache pages back to disk. This causes performance regressions in several benchmarks as well as causing
a noticeable overall sluggishness. We all know that the dirty_ratio is
an integrity vs performance trade-off but the file system journaling
will cover any devastating effects in the event of a system crash.

Increasing the dirty_ratio to 40% will regain the performance loss seen
in several benchmarks. Whats everyone think about this???


diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index ef27e73..645a462 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -78,7 +78,7 @@ int vm_highmem_is_dirtyable;
* The generator of dirty data starts writeback at this percentage
-int vm_dirty_ratio = 20;
+int vm_dirty_ratio = 40;

* vm_dirty_bytes starts at 0 (disabled) so that it is a function of

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