[Tracing] Integrate GC object statistics with tracing.

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index 38c32befdb16f1b02f5097d5fadb14a836074793..1311be91a823bd956ec9bd3ea38152f44a0c5af8 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -2246,6 +2246,13 @@ void MarkCompactCollector::RecordObjectStats() {
     ObjectStatsVisitor visitor(heap(), heap()->live_object_stats_,
                                heap()->dead_object_stats_);
     VisitAllObjects(&visitor);
+    std::stringstream live, dead;
+    heap()->live_object_stats_->Dump(live);
+    heap()->dead_object_stats_->Dump(dead);
+    TRACE_EVENT_INSTANT2(TRACE_DISABLED_BY_DEFAULT("v8.gc_stats"),

[ssid, 2016/09/29 21:45:31]

Do we really need a new category? can we just use "v8.gc"?

[lpy, 2016/09/29 22:45:42]

We can use v8.gc, but, v8.gc will add a bunch of other trace events, which will
bring too many overhead, I want to avoid it.

[fmeawad, 2016/09/30 00:44:05]

I think gc_stats is fine.
But I wonder if TRACE_EVENT_INSTANT is the right choice here. I am OK with it
for now.

[lpy, 2016/09/30 18:21:58]

Acknowledged.

+                         "V8.GC_Objects_Stats", TRACE_EVENT_SCOPE_THREAD,
+                         "live", TRACE_STR_COPY(live.str().c_str()), "dead",
+                         TRACE_STR_COPY(dead.str().c_str()));
     if (FLAG_trace_gc_object_stats) {
       heap()->live_object_stats_->PrintJSON("live");
       heap()->dead_object_stats_->PrintJSON("dead");