tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index baeb7c392f20f2864bbc00451fc1c5eeefdf86d6..4a5e2f93b53d1a3ac922449161e19799b87d3a86 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -12684,6 +12684,22 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Renderer.IsScrollableArea">

[Alexei Svitkine (slow), 2013/09/03 19:54:49]

Add enum="Boolean" here and below.

[hartmanng, 2013/09/03 20:23:07]

Done.

+  <summary>
+    Total count of the number of RenderLayers which are scrollable areas. This
+    gets sampled at most once per RenderLayer, the first time a RenderLayer
+    becomes scrollable.

[Alexei Svitkine (slow), 2013/09/03 19:54:49]

It may be useful to also log the "non-scrollable" case, so that you have a
baseline to compare this too - e.g. it would let you answer "what percentage of
layers become scrollable". (You can do this in the same histogram with a value
of 0).

Otherwise, I'm not sure how you're looking to make sense of the absolute
number...

[hartmanng, 2013/09/03 20:23:07]

The goal I have in mind is finding the ratio of scrollable areas that we're able
to promote to stacking containers. Only scrollable areas can become stacking
containers.

So the idea was to just count the absolute number of scrollable areas, and the
absolute number of stacking containers, and then the ratio between those two
numbers is what we're really interested in.

Hopefully that makes more sense, but do let me know if you still think I'm going
about this the wrong way :)

[Alexei Svitkine (slow), 2013/09/03 20:25:51]

Ah, that makes sense. In that case, I think it would be better to combine them
into a single histogram with different enum values for the two cases - so that
they can easily be compared together.

[hartmanng, 2013/09/03 23:34:33]

Yeah, that makes a lot of sense - thanks for the suggestion! (Done).

I also added a third bucket on the histogram, which maintains a count of the
number of RenderLayers that use composited scrolling. At first, this will be the
same as the count of the ones that become stacking containers (which is why I
wasn't going to add it right now at first). Now that we're combining it all into
one histogram, though, I might as well just add this bucket now too. Soon (after
we land a couple patches in the next week) we'll also be interested in the ratio
of scrollable areas : composited layers.

+  </summary>
+</histogram>
+
+<histogram name="Renderer.NeedsToBeStackingContainer">
+  <summary>
+    Total count of the number of RenderLayers which need to be promoted to
+    stacking containers. This is sampled at most once per RenderLayer, when the
+    RenderLayer first needs to become a stacking container.
+  </summary>
+</histogram>
+
 <histogram name="Renderer.PixelIncreaseFromTransitions">
   <summary>
     A lower-bound on the percentage increase in memory that would result from