VideoTrackRecorder: limit VP9 cpu usage and quality

content/renderer/media/video_track_recorder.cc

Index: content/renderer/media/video_track_recorder.cc
diff --git a/content/renderer/media/video_track_recorder.cc b/content/renderer/media/video_track_recorder.cc
index 82ae3ea9ba7dc9ed25e19f9277f1e0fd758de253..0f6657f7f1512675609c5a4fa51174b1b1b91486 100644
--- a/content/renderer/media/video_track_recorder.cc
+++ b/content/renderer/media/video_track_recorder.cc
@@ -260,9 +260,8 @@ void VideoTrackRecorder::VpxEncoder::ConfigureEncoding(const gfx::Size& size) {
 
   const vpx_codec_iface_t* interface =
       use_vp9_ ? vpx_codec_vp9_cx() : vpx_codec_vp8_cx();
-  const vpx_codec_err_t result = vpx_codec_enc_config_default(interface,
-                                                              &codec_config_,
-                                                              0 /* reserved */);
+  vpx_codec_err_t result =
+      vpx_codec_enc_config_default(interface, &codec_config_, 0 /* reserved */);
   DCHECK_EQ(VPX_CODEC_OK, result);
 
   DCHECK_EQ(320u, codec_config_.g_w);
@@ -285,6 +284,17 @@ void VideoTrackRecorder::VpxEncoder::ConfigureEncoding(const gfx::Size& size) {
 
     // DCHECK that the profile selected by default is I420 (magic number 0).
     DCHECK_EQ(0u, codec_config_.g_profile);
+
+    // Request the lowest-CPU usage that VP9 supports. Values greater than 0
+    // will increase encoder speed at the expense of quality. Note that this is
+    // configured via the same parameter as for VP8.
+    const int kCpuUsage = 6;
+    result = vpx_codec_control(encoder_.get(), VP8E_SET_CPUUSED, kCpuUsage);

[miu, 2016/02/25 20:22:09]

6 is actually a high-end setting (4 being the highest, IIRC).  Most users
machines will not be able to encode in real-time at this setting.  If your goal
is to reduce CPU usage, you probably will want to set this to 12, but then
you'll take a quality hit.

(In media/cast code, we actually micro-manage this setting, instead of using the
codec's built-in logic, to make the trade-off for each frame.)

Also, I noticed you're not setting |codec_config_.gthreads|, so perhaps you're
not leveraging multi-threaded encode right now?  For media/cast, we set this to
ceil(num processor cores / 2).

[mcasas, 2016/02/26 01:34:32]

Let's go for a compromise of 10. Users can still use
the bitrate parameter.
It's done in l.327.

+    DLOG_IF(WARNING, VPX_CODEC_OK != result) << "Failed to set CPUUSED";
+
+    // Clamping the quantizer constrains the worst-case quality and CPU usage.
+    codec_config_.rc_min_quantizer = 20;

[miu, 2016/02/25 20:22:09]

When you clamp the quantizer values this way, the encoder can and will use
waaaay more bandwidth than your target bandwidth setting (could be 5X or more!).
 It's generally best to keep these at their defaults.

[mcasas, 2016/02/26 01:34:32]

Done.

+    codec_config_.rc_max_quantizer = 30;
   } else {
     // VP8 always produces frames instantaneously.
     DCHECK_EQ(0u, codec_config_.g_lag_in_frames);