Fix JS ratio computation on startup.

src/runtime-profiler.cc

Index: src/runtime-profiler.cc
diff --git a/src/runtime-profiler.cc b/src/runtime-profiler.cc
index 28755e3ebef77cd11c2beb460adede8460fd5b4d..028af49f12dffdee37ad34613668e2bb28c63ec3 100644
--- a/src/runtime-profiler.cc
+++ b/src/runtime-profiler.cc
@@ -130,9 +130,11 @@ RuntimeProfiler::RuntimeProfiler(Isolate* isolate)
       js_ratio_(0),
       sampler_window_position_(0),
       optimize_soon_list_(NULL),
-      state_window_position_(0) {
-  state_counts_[0] = kStateWindowSize;
-  state_counts_[1] = 0;
+      state_window_position_(0),
+      state_window_ticks_(0) {
+  state_counts_[IN_NON_JS_STATE] = kStateWindowSize;

[Vyacheslav Egorov (Chromium), 2011/04/09 17:00:21]

Wild idea: when going idle we stop profiler. Why don't we reset it's state when
we later restart it? 

We should experiment with this.

[Vitaly Repeshko, 2011/04/10 08:13:27]

I agree. As we discussed, we should revisit the heuristics behind Reset().

[Kasper Lund, 2011/04/11 06:10:06]

Before this change, we were intentionally being less aggressive in our
optimizations initially (slow start). It could definitely be that the rest of
our heuristics have changed so much that it doesn't matter anymore, but it would
be nice to verify that this doesn't force us to optimize too much stuff when
loading real web apps. 

Somehow the new code bothers me a bit. Because the state_counts_[IN_JS_STATE] is
initialized to zero, you can compute the JS ratio by dividing with
state_window_ticks_ but you cannot do the same for the non-JS ratio (initialized
to kStateWindowSize). Would it be cleaner if state_counts_[IN_NON_JS_STATE] was
initialized to zero too and only do state_counts_[old_state]-- when
state_window_ticks_ indicated a full window? That way you could also get rid of
the STATIC_ASSERT and the memset.

[Vitaly Repeshko, 2011/04/11 19:43:53]

The slow start is still here, because even if we optimize a lot, we spend quite
some time just compiling stuff and it's a separate VM state. So it's not like we
immediately jump to 100% of JS ratio.

I'm not quite happy with the window update code either. I'll work more on it.

+  state_counts_[IN_JS_STATE] = 0;
+  STATIC_ASSERT(IN_NON_JS_STATE == 0);
   memset(state_window_, 0, sizeof(state_window_));
   ClearSampleBuffer();
 }
@@ -344,8 +346,9 @@ void RuntimeProfiler::UpdateStateRatio(SamplerState current_state) {
   ASSERT(IsPowerOf2(kStateWindowSize));
   state_window_position_ = (state_window_position_ + 1) &
       (kStateWindowSize - 1);
+  state_window_ticks_ = Min(kStateWindowSize, state_window_ticks_ + 1);

[Vyacheslav Egorov (Chromium), 2011/04/09 17:00:21]

Our RuntimeProfiler is full of magic and very hard to grok. Lets start adding
comments. 

For example here you can mention that when we start profiling we don't have
enough ticks for full window (kStateWindowSize) so to calculate proper ratio we
have to use smaller window size equal to amount of ticks we got.

[Vitaly Repeshko, 2011/04/10 08:13:27]

It's not too late to make it readable :) Added a comment covering this aspect.

   NoBarrier_Store(&js_ratio_, state_counts_[IN_JS_STATE] * 100 /
-                  kStateWindowSize);
+                  state_window_ticks_);
 }
 #endif