On Tue, Sep 06, 2011 at 10:22:48PM +0800, Peter Zijlstra wrote:

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

quoted

+++ linux-next/mm/page-writeback.c      2011-08-31 14:40:58.000000000 +0800

@@ -1067,6 +1067,9 @@ static void balance_dirty_pages(struct a
                                     nr_dirty, bdi_thresh, bdi_dirty,
                                     start_time);
 
+               if (unlikely(!dirty_exceeded && bdi_async_underrun(bdi)))
+                       break;
+
                dirty_ratelimit = bdi->dirty_ratelimit;
                pos_ratio = bdi_position_ratio(bdi, dirty_thresh,
                                               background_thresh, nr_dirty,

So dirty_exceeded looks like:


1109                 dirty_exceeded = (bdi_dirty > bdi_thresh) ||
1110                                   (nr_dirty > dirty_thresh);

Would it make sense to write it as:

	if (nr_dirty > dirty_thresh || 
	    (nr_dirty > freerun && bdi_dirty > bdi_thresh))
		dirty_exceeded = 1;

So that we don't actually throttle bdi thingies when we're still in the
freerun area?

Sounds not necessary -- (nr_dirty > freerun) is implicitly true
because there is a big break early in the loop:

        if (nr_dirty > freerun)
                break;

Thanks,
Fengguang

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