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

 /*
+ * After a task dirtied this many pages, balance_dirty_pages_ratelimited_nr()
+ * will look to see if it needs to start dirty throttling.
+ *
+ * If dirty_poll_interval is too low, big NUMA machines will call the expensive
+ * global_page_state() too often. So scale it near-sqrt to the safety margin
+ * (the number of pages we may dirty without exceeding the dirty limits).
+ */
+static unsigned long dirty_poll_interval(unsigned long dirty,
+                                        unsigned long thresh)
+{
+       if (thresh > dirty)
+               return 1UL << (ilog2(thresh - dirty) >> 1);
+
+       return 1;
+}

Where does that sqrt come from? 

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