On Mon, Mar 14, 2011 at 09:48:21PM +0100, Jan Kara wrote:

On Wed 09-03-11 19:07:31, Vivek Goyal wrote:

quoted

quoted

+static void balance_dirty_pages(struct address_space *mapping,
+				unsigned long write_chunk)
+{
+	struct backing_dev_info *bdi = mapping->backing_dev_info;
+	struct balance_waiter bw;
+	struct dirty_limit_state st;
+	int dirty_exceeded = check_dirty_limits(bdi, &st);
+
+	if (dirty_exceeded < DIRTY_MAY_EXCEED_LIMIT ||
+	    (dirty_exceeded == DIRTY_MAY_EXCEED_LIMIT &&
+	     !bdi_task_limit_exceeded(&st, current))) {
+		if (bdi->dirty_exceeded &&
+		    dirty_exceeded < DIRTY_MAY_EXCEED_LIMIT)
+			bdi->dirty_exceeded = 0;
 		/*
-		 * Increase the delay for each loop, up to our previous
-		 * default of taking a 100ms nap.
+		 * In laptop mode, we wait until hitting the higher threshold
+		 * before starting background writeout, and then write out all
+		 * the way down to the lower threshold.  So slow writers cause
+		 * minimal disk activity.
+		 *
+		 * In normal mode, we start background writeout at the lower
+		 * background_thresh, to keep the amount of dirty memory low.
 		 */
-		pause <<= 1;
-		if (pause > HZ / 10)
-			pause = HZ / 10;
+		if (!laptop_mode && dirty_exceeded == DIRTY_EXCEED_BACKGROUND)
+			bdi_start_background_writeback(bdi);
+		return;
 	}
 
-	/* Clear dirty_exceeded flag only when no task can exceed the limit */
-	if (!min_dirty_exceeded && bdi->dirty_exceeded)
-		bdi->dirty_exceeded = 0;
+	if (!bdi->dirty_exceeded)
+		bdi->dirty_exceeded = 1;

Will it make sense to move out bdi_task_limit_exceeded() check in a
separate if condition statement as follows. May be this is little
easier to read.

	if (dirty_exceeded < DIRTY_MAY_EXCEED_LIMIT) {
		if (bdi->dirty_exceeded)
			bdi->dirty_exceeded = 0;

		if (!laptop_mode && dirty_exceeded == DIRTY_EXCEED_BACKGROUND)
			bdi_start_background_writeback(bdi);

		return;
	}

	if (dirty_exceeded == DIRTY_MAY_EXCEED_LIMIT &&
	    !bdi_task_limit_exceeded(&st, current))
		return;   

  But then we have to start background writeback here as well. Which is
actually a bug in the original patch as well! So clearly your way is more
readable :) I'll change it. Thanks.

I was thinking about that starting of bdi writeback here. But I was
assuming that if we are here then we most likely have visited above
loop of < DIRTY_MAY_EXCEED_LIMIT and started background writeback.

Thanks
Vivek

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