cc: add PaintTimeCounter to keep track of per frame paint time

cc/paint_time_counter.h

Index: cc/paint_time_counter.h
diff --git a/cc/paint_time_counter.h b/cc/paint_time_counter.h
new file mode 100644
index 0000000000000000000000000000000000000000..a7698f35bc23cc68fcea68f7fd7424ab014c529d
--- /dev/null
+++ b/cc/paint_time_counter.h
@@ -0,0 +1,40 @@
+// Copyright 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CC_PAINT_TIME_COUNTER_H_
+#define CC_PAINT_TIME_COUNTER_H_
+
+#include "base/basictypes.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/time.h"
+#include "cc/ring_buffer.h"
+
+namespace cc {
+
+// Maintains a history of paint times for each frame
+class PaintTimeCounter {
+ public:
+  static scoped_ptr<PaintTimeCounter> create();
+
+  size_t HistorySize() const { return ring_buffer_.BufferSize(); }

[egraether, 2013/01/16 19:31:56]

We decided to use size_t for the buffer size in the RingBuffer
(https://codereview.chromium.org/11817011).

+
+  // n = 0 returns the oldest and
+  // n = PaintTimeHistorySize() - 1 the most recent paint time.
+  base::TimeDelta GetPaintTimeOfRecentFrame(const size_t& n) const;

[egraether, 2013/01/16 19:31:56]

Returns TimeDelta now.

+
+  void SavePaintTime(const base::TimeDelta& total_paint_time);

[egraether, 2013/01/16 19:31:56]

New paint times are saved as TimeDelta.

+  void GetMinAndMaxPaintTimeInMilliseconds(double* min, double* max) const;

[egraether, 2013/01/16 19:31:56]

Changed the name to indicate the measurement unit.

[reveman, 2013/01/16 19:56:04]

why isn't this using TimeDelta for min/max?

[egraether, 2013/01/16 22:14:13]

Mostly because milliseconds with type double is the only real usecase. The
method is used in this patch:
https://codereview.chromium.org/11820014/patch/19001/20001

paintTimeCounter->GetMinAndMaxPaintTimeInMilliseconds(&m_paintTimeGraph.min,
&m_paintTimeGraph.max);
(m_paintTimeGraph is a private structure holding a few double values)

Changing this call to TimeDelta would result in something like this:

base::TimeDelta min, max;
paintTimeCounter->GetMinAndMaxPaintTime(&min, &max);
m_paintTimeGraph.min = min.InMillisecondsF();
m_paintTimeGraph.max = max.InMillisecondsF();

So I thought lets keep it simple stupid and just got with double in
milliseconds.

[reveman, 2013/01/16 22:48:04]

I'd prefer if you used TimeDelta consistently throughout the public
PaintTimeCounter API. You'll need something like what you mentioned above to
convert the value from GetPaintTimeOfRecentFrame() in HeadsUpDisplayLayerImpl
anyhow, right?

+
+ private:
+  PaintTimeCounter();
+
+  RingBuffer<base::TimeDelta, 80> ring_buffer_;

[egraether, 2013/01/16 19:31:56]

Changed type to TimeDelta.

+  base::TimeDelta last_total_paint_time_;
+
+  DISALLOW_COPY_AND_ASSIGN(PaintTimeCounter);
+};
+
+}  // namespace cc
+
+#endif  // CC_PAINT_TIME_COUNTER_H_