[PATCH block 1/2] writeback, cgroup: Adjust WB_FRN_TIME_CUT_DIV to accelerate foreign inode switching

[Tejun Heo <tj@kernel.org>]

WB_FRN_TIME_CUT_DIV is used to tell the foreign inode detection logic
to ignore short writeback rounds to prevent getting confused by a
burst of short writebacks.  The parameter is currently 2 meaning that
anything smaller than half of the running average writback duration
will be ignored.

This is unnecessarily aggressive.  The detection logic uses 16 history
slots and is already reasonably protected against some short bursts
confusing it and the current parameter can lead to tens of seconds of
missed detection depending on the writeback pattern.

Let's change the parameter to 8, so that it only ignores writeback
with are smaller than 12.5% of the current running average.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 fs/fs-writeback.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -227,7 +227,7 @@ static void wb_wait_for_completion(struc
 /* parameters for foreign inode detection, see wb_detach_inode() */
 #define WB_FRN_TIME_SHIFT	13	/* 1s = 2^13, upto 8 secs w/ 16bit */
 #define WB_FRN_TIME_AVG_SHIFT	3	/* avg = avg * 7/8 + new * 1/8 */
-#define WB_FRN_TIME_CUT_DIV	2	/* ignore rounds < avg / 2 */
+#define WB_FRN_TIME_CUT_DIV	8	/* ignore rounds < avg / 8 */
 #define WB_FRN_TIME_PERIOD	(2 * (1 << WB_FRN_TIME_SHIFT))	/* 2s */
 
 #define WB_FRN_HIST_SLOTS	16	/* inode->i_wb_frn_history is 16bit */