Add UMA metrics for measuring number of coalesed events and their latency.

ui/base/x/x11_util.cc

Index: ui/base/x/x11_util.cc
diff --git a/ui/base/x/x11_util.cc b/ui/base/x/x11_util.cc
index 35e27636d94ecc64277e39bb17b6605d9166882c..844be0a80358aa63467493da1f476a127a9d0e80 100644
--- a/ui/base/x/x11_util.cc
+++ b/ui/base/x/x11_util.cc
@@ -28,6 +28,7 @@
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/singleton.h"
 #include "base/message_loop.h"
+#include "base/metrics/histogram.h"
 #include "base/string_number_conversions.h"
 #include "base/string_util.h"
 #include "base/stringprintf.h"
@@ -564,6 +565,7 @@ int CoalescePendingMotionEvents(const XEvent* xev,
   }
 #endif
 
+  base::TimeDelta first_coalesed_time;
   while (XPending(display)) {
     XEvent next_event;
     XPeekEvent(display, &next_event);
@@ -620,6 +622,9 @@ int CoalescePendingMotionEvents(const XEvent* xev,
         // Get the event and its cookie data.
         XNextEvent(display, last_event);
         XGetEventData(display, &last_event->xcookie);
+
+        if (num_coalesed == 0)
+          first_coalesed_time = ui::EventTimeFromNative(last_event);

[Rick Byers, 2013/04/05 15:30:35]

I think the first coalesced event is really xev, where this event here is the
2nd.  Eg. think of the case where we return 1 from this function - you could end
up recording a time of about 0, which would be wrong.

[varunjain, 2013/04/05 19:22:58]

Done.

         ++num_coalesed;
         continue;
       } else {
@@ -629,6 +634,20 @@ int CoalescePendingMotionEvents(const XEvent* xev,
     }
     break;
   }
+  UMA_HISTOGRAM_CUSTOM_COUNTS("Event.Latency.Browser.CoalesedCount",

[Rick Byers, 2013/04/05 15:30:35]

UMA_HISTOGRAM_COUNTS_10000 should be sufficient (50 buckets instead of 100
here).

[Rick Byers, 2013/04/05 15:30:35]

Event.Latency.Browser has an existing common meaning (and also Coalesced is
misspelled).  I'd find this simpler if we just called it "Event.CoalescedCount"
and "Event.CoalescedLatency" below.

[varunjain, 2013/04/05 19:22:58]

Done.

[varunjain, 2013/04/05 19:22:58]

Done.

+                              num_coalesed, 0, 10000, 100);
+  std::string name_for_event = base::StringPrintf(
+      "Event.Latency.Browser.Coalesed.%s",
+      (ui::IsMouseEvent(const_cast<XEvent*>(xev)) ? "Mouse" : "Touch"));

[Rick Byers, 2013/04/05 15:30:35]

Please add a CHECK that verifies all events here are either mouse or touch
events (or add a third 'other' category).

[Rick Byers, 2013/04/05 15:30:35]

If we're going to separate Mouse and Touch here, then I think we should also
separate it in the CoalescedCount too so we can compare them directly.  

[flackr, 2013/04/05 16:10:56]

Drive-by, wouldn't it be better / more efficient to switch between two const
strings rather than construct a string?

[varunjain, 2013/04/05 19:22:58]

Done.

[varunjain, 2013/04/05 19:22:58]

Done.

[varunjain, 2013/04/05 19:22:58]

Done.

+  base::TimeDelta delta = EventTimeForNow() - first_coalesed_time;

[Rick Byers, 2013/04/05 15:30:35]

The Event.Latency.Browser metrics already track the latency between event
generation timestamps and consumption in the browser.  Perhaps we should exclude
that time here to keep the measurements separate?  I.e. use last_event timestamp
instead of EventTimeForNow.

[varunjain, 2013/04/05 19:22:58]

Done.

+  base::HistogramBase* counter_for_type =
+      base::Histogram::FactoryGet(

[Rick Byers, 2013/04/05 15:30:35]

Isn't this fully dynamic histogram generation overkill here?  Eg. couldn't you
just do:
UMA_HISTOGRAM_COUNTS_10000(isMouse ? "Event.CoalescedLatency.Mouse" :
"Event.CoalescedLatency.Touch", delta)

[DaveMoore, 2013/04/05 16:46:15]

The macros don't support this...they get confused by having different strings
used. But you could have an if...else.. with two different
UMA_HISTOGRAM_COUNTS() calls.
On 2013/04/05 15:30:35, Rick Byers wrote:

[varunjain, 2013/04/05 19:22:58]

Done.

[varunjain, 2013/04/05 19:22:58]

Done.

+          name_for_event,
+          0,
+          1000000,
+          100,
+          base::HistogramBase::kUmaTargetedHistogramFlag);
+  counter_for_type->Add(delta.InMicroseconds());
   return num_coalesed;
 }
 #endif