GetVSyncParameters only once every 5 seconds on Linux

ui/gl/gl_surface_glx.cc

Index: ui/gl/gl_surface_glx.cc
diff --git a/ui/gl/gl_surface_glx.cc b/ui/gl/gl_surface_glx.cc
index b131b380a61dbcaf2b257b0a18a6844732164200..d0300b860d108edfef98a6795c52de6cee64c51a 100644
--- a/ui/gl/gl_surface_glx.cc
+++ b/ui/gl/gl_surface_glx.cc
@@ -56,6 +56,17 @@ bool g_glx_get_msc_rate_oml_supported = false;
 
 bool g_glx_sgi_video_sync_supported = false;
 
+#if defined(OS_LINUX)

[Ken Russell (switch to Gerrit), 2014/09/11 18:37:34]

Are the OS_LINUX #ifdefs really necessary? This entire file should mainly
(only?) be used on Linux -- is Chrome OS the other consumer?

Could a different constant be chosen on Linux vs. non-Linux? See below.

[brianderson, 2014/09/11 18:50:29]

I was under the impression the CrOS used this too. Trying to verify.

+// See crbug.com/373489
+// On Linux, querying the vsync parameters might burn CPU for up to an
+// entire vsync, so we only query periodically to reduce CPU usage.
+// 5 seconds is chosen somewhat abitrarily as a balance between:
+//  a) Drift in the phase of our signal.
+//  b) Potential janks from periodically pegging the CPU.
+static const base::TimeDelta kGetVSyncParametersMinPeriod =
+    base::TimeDelta::FromSeconds(5);
+#endif
+
 class OMLSyncControlVSyncProvider
     : public gfx::SyncControlVSyncProvider {
  public:
@@ -190,7 +201,6 @@ class SGIVideoSyncProviderThreadShim {
 
       if (!context_ || cancel_vsync_flag_.IsSet())
         return;
-
       glXMakeCurrent(display_, window_, context_);
 
       unsigned int retrace_count = 0;
@@ -257,6 +267,13 @@ class SGIVideoSyncVSyncProvider
 
   virtual void GetVSyncParameters(
       const VSyncProvider::UpdateVSyncCallback& callback) OVERRIDE {
+#if defined(OS_LINUX)

[Ken Russell (switch to Gerrit), 2014/09/11 18:37:34]

Instead of an #ifdef here, could this be written like:
  if (kGetVsyncParametersMinPeriod > 0) {
    ...
  }

and set it to 0 if it shouldn't be applied?

[brianderson, 2014/09/11 18:50:29]

Done.

+    base::TimeTicks now = base::TimeTicks::Now();
+    if ((now - last_get_vsync_parameters_time_) < kGetVSyncParametersMinPeriod)
+      return;
+    last_get_vsync_parameters_time_ = now;
+#endif
+
     // Only one outstanding request per surface.
     if (!pending_callback_) {
       pending_callback_.reset(
@@ -292,6 +309,10 @@ class SGIVideoSyncVSyncProvider
   base::CancellationFlag* cancel_vsync_flag_;
   base::Lock* vsync_lock_;
 
+#if defined(OS_LINUX)
+  base::TimeTicks last_get_vsync_parameters_time_;
+#endif
+
   DISALLOW_COPY_AND_ASSIGN(SGIVideoSyncVSyncProvider);
 };