Implement tick events compression in a log file.

src/log.cc

Index: src/log.cc
diff --git a/src/log.cc b/src/log.cc
index ee1ebb05fdc3d1633d5aaa8508b78ed292a84687..2a8c455a24b2686515ad9226b9a54e62b6587da3 100644
--- a/src/log.cc
+++ b/src/log.cc
@@ -303,6 +303,7 @@ VMState* Logger::current_state_ = NULL;
 VMState Logger::bottom_state_(EXTERNAL);
 SlidingStateWindow* Logger::sliding_state_window_ = NULL;
 const char** Logger::log_events_ = NULL;
+int Logger::tick_repeat_count_ = 0;
 
 #define DECLARE_LONG_EVENT(ignore1, long_name, ignore2) long_name,
 const char* kLongLogEventsNames[Logger::NUMBER_OF_LOG_EVENTS] = {
@@ -848,6 +849,28 @@ void Logger::TickEvent(TickSample* sample, bool overflow) {
       msg.Append(",0x%" V8PRIxPTR, sample->stack[i]);
     }
   }
+  // In case if log compression is disabled, the flow is straightforward,
+  // because both StoreInCompressor and RetrieveCompressedPrevious do nothing
+  // and just return 'true'. Otherwise, to perform compression of repeated
+  // tick events, instead of the current event, the previous one is written.
+  if (!msg.StoreInCompressor()) {
+    // Current message repeats the previous one, don't write it.
+    ++tick_repeat_count_;

[Søren Thygesen Gjesse, 2009/06/11 11:13:56]

Maybe we want to report something once in a while even if events are the same
e.g after 1s (X repeated events depending on the sample interval)?

[Mikhail Naganov, 2009/06/11 14:07:12]

I think this is not needed. Overflow of a counter will only happen after about
24 days. Also, as I've noticed, in Chromium even idle application doesn't stuck
in the same place for a while. Every incoming event causes it to resume
execution, so PC or SP periodically changes.

+    return;
+  } else {
+    bool has_previous_message;
+    if (tick_repeat_count_ > 0) {
+      EmbeddedVector<char, 20> prefix;
+      OS::SNPrintF(prefix, "%s,%d,",
+                   log_events_[REPEAT_META_EVENT],
+                   tick_repeat_count_ + 1);
+      tick_repeat_count_ = 0;
+      has_previous_message = msg.RetrieveCompressedPrevious(prefix.start());
+    } else {
+      has_previous_message = msg.RetrieveCompressedPrevious();
+    }
+    if (!has_previous_message) return;
+  }
   msg.Append('\n');
   msg.WriteToLogFile();
 }