On Wed 07-09-11 01:27:38, Jan Kara wrote:

On Tue 06-09-11 17:47:10, Peter Zijlstra wrote:

quoted

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

quoted

 /*
+ * 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? 

  He does 2^{log_2(x)/2} which, if done in real numbers arithmetics, would
result in x^{1/2}. Given the integer arithmetics, it might be twice as
small but still it's some approximation...

  Ah, now I realized that you probably meant to ask why does he use sqrt
and not some other function... Sorry for the noise.

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR

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