On Mon, Sep 05, 2011 at 11:02:59PM +0800, Peter Zijlstra wrote:

On Sun, 2011-09-04 at 09:53 +0800, Wu Fengguang wrote:

quoted

+ * (o) bdi control lines
+ *
+ * The control lines for the global/bdi setpoints both stretch up to @limit.
+ * The below figure illustrates the main bdi control line with an auxiliary
+ * line extending it to @limit.
+ *
+ *   o
+ *     o
+ *       o                                      [o] main control line
+ *         o                                    [*] auxiliary control line
+ *           o
+ *             o
+ *               o
+ *                 o
+ *                   o
+ *                     o
+ *                       o--------------------- balance point, rate scale = 1
+ *                       | o
+ *                       |   o
+ *                       |     o
+ *                       |       o
+ *                       |         o
+ *                       |           o
+ *                       |             o------- connect point, rate scale = 1/2
+ *                       |               .*
+ *                       |                 .   *
+ *                       |                   .      *
+ *                       |                     .         *
+ *                       |                       .           *
+ *                       |                         .              *
+ *                       |                           .                 *
+ *  [--------------------+-----------------------------.--------------------*]
+ *  0              bdi_setpoint                    x_intercept           limit
+ *
+ * The auxiliary control line allows smoothly throttling bdi_dirty down to
+ * normal if it starts high in situations like
+ * - start writing to a slow SD card and a fast disk at the same time. The SD
+ *   card's bdi_dirty may rush to many times higher than bdi_setpoint.
+ * - the bdi dirty thresh drops quickly due to change of JBOD workload 

In light of the global control thing already having a hard stop at
limit, what's the point of the auxiliary line? Why not simply run the
bdi control between [0.5, 1.5] and leave it at that?

Good point! It helps remove one confusing concept.

This patch reduces the auxiliary control line to a flat y=0.25 line.
The comments will be further simplified, too.

Thanks,
Fengguang
---

 mm/page-writeback.c |   17 +++++------------
 1 file changed, 5 insertions(+), 12 deletions(-)

--- linux-next.orig/mm/page-writeback.c	2011-09-06 09:59:50.000000000 +0800
+++ linux-next/mm/page-writeback.c	2011-09-06 10:05:31.000000000 +0800

@@ -676,18 +676,11 @@ static unsigned long bdi_position_ratio(
 	span = (thresh - bdi_thresh + 8 * write_bw) * (u64)x >> 16;
 	x_intercept = bdi_setpoint + span;
 
-	span >>= 1;
-	if (unlikely(bdi_dirty > bdi_setpoint + span)) {
-		if (unlikely(bdi_dirty > limit))
-			return 0;
-		if (x_intercept < limit) {
-			x_intercept = limit;	/* auxiliary control line */
-			bdi_setpoint += span;
-			pos_ratio >>= 1;
-		}
-	}
-	pos_ratio *= x_intercept - bdi_dirty;
-	do_div(pos_ratio, x_intercept - bdi_setpoint + 1);
+	if (bdi_dirty < x_intercept - span / 4) {
+		pos_ratio *= x_intercept - bdi_dirty;
+		do_div(pos_ratio, x_intercept - bdi_setpoint + 1);
+	} else
+		pos_ratio /= 4;
 
 	return pos_ratio;
 }

--

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