Change chrome from statically enabling high resolution timers on windows

to enabling them dynamically - only when the application really needs
them.

I am working on some test cases for this, and will add them.  But wanted
to send out the concept for review.

In this implementation, I modify the message loop to detect when the
application has requested high resolution timers.  Note that there are
multiple MessageLoops active in a single process.  After a period of
time, we simply shut it off again.    We could have set a timer or
kept a count of active timers, or any number of more complex algorithms.
But I think this algorithm is very simple and good enough.  If an
application continues needing high resolution timers for more than 1s,
we'll turn the high-resolution timers back on again.

One last change - since we've implemented the clamp at 4ms, there isn't
a lot of point to our use of 1ms for timeBeginPeriod.  I've modified
that to 2 (which is half of 4ms, our target minimal interval).

BUG=46531
TEST=MessageLoop.HighResolutionTimers


Review URL: http://codereview.chromium.org/2822035

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@51102 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 18 insertions, 0 deletions

diff --git a/base/message_loop.h b/base/message_loop.h
index 8b4be7c..8deacec 100644
--- a/base/message_loop.h
+++ b/base/message_loop.h
@@ -287,6 +287,20 @@ class MessageLoop : public base::MessagePump::Delegate {
   typedef base::MessagePumpForUI::Observer Observer;
 #endif
 
+  // Returns true if the message loop has high resolution timers enabled.
+  // Provided for testing.
+  bool high_resolution_timers_enabled() {
+#if defined(OS_WIN)
+    return !high_resolution_timer_expiration_.is_null();
+#else
+    return true;
+#endif
+  }
+
+  // When we go into high resolution timer mode, we will stay in hi-res mode
+  // for at least 1s.
+  static const int kHighResolutionTimerModeLeaseTimeMs = 1000;
+
   //----------------------------------------------------------------------------
  protected:
   struct RunState {
@@ -454,6 +468,10 @@ class MessageLoop : public base::MessagePump::Delegate {
 
   RunState* state_;
 
+#if defined(OS_WIN)
+  base::TimeTicks high_resolution_timer_expiration_;
+#endif
+
   // The next sequence number to use for delayed tasks.
   int next_sequence_num_;