Fix JS ratio computation on startup.

src/runtime-profiler.cc

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@@ skipping 112 matching lines @@
 
 RuntimeProfiler::RuntimeProfiler(Isolate* isolate)
     : isolate_(isolate),
       sampler_threshold_(kSamplerThresholdInit),
       sampler_threshold_size_factor_(kSamplerThresholdSizeFactorInit),
       sampler_ticks_until_threshold_adjustment_(
         kSamplerTicksBetweenThresholdAdjustment),
       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();
 }
 
 
 bool RuntimeProfiler::IsEnabled() {
   return V8::UseCrankshaft() && FLAG_opt;
 }
 
 
@@ skipping 191 matching lines @@
 
 #ifdef ENABLE_LOGGING_AND_PROFILING
 void RuntimeProfiler::UpdateStateRatio(SamplerState current_state) {
   SamplerState old_state = state_window_[state_window_position_];
   state_counts_[old_state]--;
   state_window_[state_window_position_] = current_state;
   state_counts_[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
 
 
 void RuntimeProfiler::NotifyTick() {
 #ifdef ENABLE_LOGGING_AND_PROFILING
   // Record state sample.
   SamplerState state = IsSomeIsolateInJS()
       ? IN_JS_STATE
       : IN_NON_JS_STATE;
@@ skipping 110 matching lines @@
     } else {
       return RuntimeProfiler::WaitForSomeIsolateToEnterJS();
     }
   }
 #endif
   return false;
 }
 
 
 } }  // namespace v8::internal