High resolution timer fix for Windows

base/message_loop/message_pump.h

Index: base/message_loop/message_pump.h
diff --git a/base/message_loop/message_pump.h b/base/message_loop/message_pump.h
index 4dc501b660839e0a56b74778abc883864396847d..74a2206bb8bd1b747d180e39de0587b51477ab93 100644
--- a/base/message_loop/message_pump.h
+++ b/base/message_loop/message_pump.h
@@ -41,6 +41,10 @@ class BASE_EXPORT MessagePump : public NonThreadSafe {
     // Called from within Run just before the message pump goes to sleep.
     // Returns true to indicate that idle work was done.
     virtual bool DoIdleWork() = 0;
+
+    // Called when the pump is about to sleep. The delegate should update the
+    // timer granularity to the appropriate value for the pending tasks.
+    virtual void UpdateTimerGranularity() {}

[darin (slow to review), 2014/07/18 04:07:59]

why don't you just use DoIdleWork for this? it is always called before sleeping.

[jamesr, 2014/07/18 04:45:37]

it's also called before not sleeping (i.e. we have an idle work task that posts
more tasks) and we really should only do this when we're really about to sleep.
we could do it in MessageLoop::DoIdleWork just before returning false, but imho
that ties this a little too deeply to how the pump works. updating the
granularity changes the behavior of the sleep call so i like having it be as
close to possible as the sleep call

[darin (slow to review), 2014/07/18 04:55:54]

I don't think that's quite right. In that case, DoIdleWork returns true,
and we avoid waiting at all. We always completely drain the work queue of
work that can be done before sleeping, and if we do any work, then we
assume we could plausibly have more work to do.
^^^ Yes, just before returning false seems like the right time.
I don't think it is tying us too closely. Looking at the comment for
DoIdleWork, it seems to describe exactly what we are looking for here...
notification that the pump is about to sleep. The comment describing
UpdateTimerGranularity says almost exactly the same thing.

[jamesr, 2014/07/18 06:18:33]

Good points.  I agree doing this at the end of DoIdleWork() would be better.

   };
 
   MessagePump();