RFC: dirty_ratio back to 40%

[Larry Woodman <lwoodman@redhat.com>]

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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