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, 28 insertions, 3 deletions

diff --git a/base/time.h b/base/time.h
index 6e9a8a4..98a711d 100644
--- a/base/time.h
+++ b/base/time.h
@@ -254,9 +254,27 @@ class Time {
   static Time FromFileTime(FILETIME ft);
   FILETIME ToFileTime() const;
 
-  // Enable or disable Windows high resolution timer. For more details
-  // see comments in time_win.cc. Returns true on success.
-  static bool UseHighResolutionTimer(bool use);
+  // The minimum time of a low resolution timer.  This is basically a windows
+  // constant of ~15.6ms.  While it does vary on some older OS versions, we'll
+  // treat it as static across all windows versions.
+  static const int kMinLowResolutionThresholdMs = 16;
+
+  // Enable or disable Windows high resolution timer. If the high resolution
+  // timer is not enabled, calls to ActivateHighResolutionTimer will fail.
+  // When disabling the high resolution timer, this function will not cause
+  // the high resolution timer to be deactivated, but will prevent future
+  // activations.
+  // Must be called from the main thread.
+  // For more details see comments in time_win.cc.
+  static void EnableHighResolutionTimer(bool enable);
+
+  // Activates or deactivates the high resolution timer based on the |activate|
+  // flag.  If the HighResolutionTimer is not Enabled (see
+  // EnableHighResolutionTimer), this function will return false.  Otherwise
+  // returns true.
+  // All callers to activate the high resolution timer must eventually call
+  // this function to deactivate the high resolution timer.
+  static bool ActivateHighResolutionTimer(bool activate);
 #endif
 
   // Converts an exploded structure representing either the local time or UTC
@@ -370,6 +388,13 @@ class Time {
   // platform-dependent epoch.
   static const int64 kTimeTToMicrosecondsOffset;
 
+#if defined(OS_WIN)
+  // Indicates whether fast timers are usable right now.  For instance,
+  // when using battery power, we might elect to prevent high speed timers
+  // which would draw more power.
+  static bool high_resolution_timer_enabled_;
+#endif
+
   // Time in microseconds in UTC.
   int64 us_;
 };