Ensure traces with bad timers are not taken into accound in the startup report.

perf_insights/perf_insights/mappers/slice_cost.html

Index: perf_insights/perf_insights/mappers/slice_cost.html
diff --git a/perf_insights/perf_insights/mappers/slice_cost.html b/perf_insights/perf_insights/mappers/slice_cost.html
index 77d73953470fed1393f36712e9327785955ca608..acf63eb9b764deafca9a416f66dfac193181585b 100644
--- a/perf_insights/perf_insights/mappers/slice_cost.html
+++ b/perf_insights/perf_insights/mappers/slice_cost.html
@@ -90,7 +90,7 @@ tr.exportTo('pi.m', function() {
 
 
   function getSliceCostReport(model, threadGrouping, railTypeNameByGUID,
-      filterFunction) {
+      ignorePoorTimers, filterFunction) {
     var reduce = new pi.m.StreamingReducer(SliceCostInfo.asReduceTarget);
 
     function generateDomainCosts(slice) {
@@ -132,12 +132,18 @@ tr.exportTo('pi.m', function() {
       });
     }
 
+    var nbEvents = 0;
+    var nbSnappedEvents = 0;
     model.iterateAllEvents(function(event) {
       if (!(event instanceof tr.model.ThreadSlice))
         return;
       if (filterFunction && !filterFunction(event))
         return;
 
+      nbEvents++;
+      if (event.start === Math.floor(event.start))
+        nbSnappedEvents++;
+
       var threadSlice = event;
       if (threadSlice.title === 'V8.Execute') {
         generateDomainCosts(threadSlice);
@@ -161,6 +167,11 @@ tr.exportTo('pi.m', function() {
       reduce.push(key, sliceCostInfo);
     });
 
+    // If more than 90% of the events are snapped precisely on milliseconds
+    // boundary we got a trace with a poor timer, conditionally ignore it.

[nduca, 2015/10/29 21:43:12]

i think this patch should probably block on
https://github.com/catapult-project/catapult/issues/1694

you might be able to then create a feature in trace event importer that says
"when we dont find a metadata record for whether its highres or not, guess using
this heuristic and set the model field directly."

at that point, then this cl is just stopping generation of reports when its
lowres. which tbh might be an overall feature we want on the mapping engine
instead of being something that every mapper should do...

[beaudoin, 2015/10/30 14:09:53]

Wasn't sure how early you wanted to trim these traces since in the meeting you
sounded like you though low-rez traces were still useful for other reports. I
totally support making it a feature of the mapper.

+    if (ignorePoorTimers && nbSnappedEvents / nbEvents > 0.9)
+      return [];
+
     var sliceCostInfos = [];
     reduce.finalizeAndIterResults(function(key, sliceCostInfo) {
       sliceCostInfos.push(sliceCostInfo);