writeback: reduce per-bdi dirty threshold ramp up time

Andrew,
References: <[email protected]>
Content-Disposition: inline; filename=writeback-speedup-per-bdi-threshold-ramp-up.patch

Reduce the dampening for the control system, yielding faster convergence.

Currently it converges at a snail's pace for slow devices (in order of minutes). For really fast storage, the convergence speed should be fine.

It makes sense to make it reasonably fast for typical desktops.

After patch, it converges in ~10 seconds for 60MB/s writes and 4GB mem. So expect ~1s for a fast 600MB/s storage under 4GB mem, or ~4s under 16GB mem, which seems reasonable.

$ while true; do grep BdiDirtyThresh /debug/bdi/8:0/stats; sleep 1; done
BdiDirtyThresh:            0 kB
BdiDirtyThresh:       118748 kB
BdiDirtyThresh:       214280 kB
BdiDirtyThresh:       303868 kB
BdiDirtyThresh:       376528 kB
BdiDirtyThresh:       411180 kB
BdiDirtyThresh:       448636 kB
BdiDirtyThresh:       472260 kB
BdiDirtyThresh:       490924 kB
BdiDirtyThresh:       499596 kB
BdiDirtyThresh:       507068 kB
...
DirtyThresh:          530392 kB

---
 mm/page-writeback.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -125,7 +125,7 @@ static int calc_period_shift(void)
 	else
 		dirty_total = (vm_dirty_ratio * determine_dirtyable_memory()) /
 				100;
-	return 2 + ilog2(dirty_total - 1);
+	return ilog2(dirty_total - 1) - 1;
 }
 
 /*

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