tor-browser

commit ed5ff27d216e350a643938b194d4107fe76bb256
parent 6c6bf22d0f4dbc52e07bfc254b8dfc19a5c72f9b
Author: Karl Tomlinson <karlt+@karlt.net>
Date:   Mon,  3 Nov 2025 21:24:22 +0000

Bug 1996520 Increase SystemClockDriver underrun bankruptcy threshold r=pehrsons

so that rendered time can remain close to clock time even on systems where
thread wake-up is 30ms late.

Differential Revision: https://phabricator.services.mozilla.com/D270451

Diffstat:
M
dom/media/GraphDriver.cpp
 | 
8
++++----
M
dom/media/GraphDriver.h
 | 
14
+++++++++++++-

2 files changed, 17 insertions(+), 5 deletions(-)
diff --git a/dom/media/GraphDriver.cpp b/dom/media/GraphDriver.cpp
@@ -245,10 +245,10 @@ TimeDuration SystemClockDriver::NextIterationWaitDuration() {
     mTargetIterationTimeStamp += IterationDuration();
   }
   TimeDuration timeout = mTargetIterationTimeStamp - now;
-  if (timeout < TimeDuration::FromMilliseconds(-MEDIA_GRAPH_TARGET_PERIOD_MS)) {
-    // Rendering has fallen so far behind that the entire next rendering
-    // period has already passed.  Don't try to catch up again, but instead
-    // try to render at consistent time intervals from now.
+  if (timeout <
+      TimeDuration::FromMilliseconds(-SYSTEM_CLOCK_BANKRUPTCY_THRESHOLD_MS)) {
+    // Don't try to catch up because rendering has fallen so far behind.
+    // Instead try to render at consistent time intervals from now.
     LOG(LogLevel::Warning, ("%p: Global underrun detected", Graph()));
     mTargetIterationTimeStamp = now;
   }
diff --git a/dom/media/GraphDriver.h b/dom/media/GraphDriver.h
@@ -36,7 +36,19 @@ namespace mozilla {
  * We try to run the control loop at this rate.
  */
 static const int MEDIA_GRAPH_TARGET_PERIOD_MS = 10;
-
+/**
+ * The SystemClockDriver does not necessarily wake up as precisely as an
+ * AudioCallbackDriver.  SleepConditionVariableSRW() has been observed to wake
+ * almost 30ms late on Windows 10 2009 systems, which implies a lower timer
+ * resolution than the 15.6ms default system-wide timer resolution in Windows.
+ * https://download.microsoft.com/download/3/0/2/3027d574-c433-412a-a8b6-5e0a75d5b237/timer-resolution.docx
+ *
+ * Allow a SystemClockDriver to try to catch up when rendering is up to this
+ * many milliseconds late, so that rendered time is close to clock time.  When
+ * later than this threshold, SystemClockDriver will declare bankruptcy and
+ * re-sync target render time with a new clock time.
+ */
+static const int SYSTEM_CLOCK_BANKRUPTCY_THRESHOLD_MS = 30;
 /**
  * After starting a fallback driver, wait this long before attempting to re-init
  * the audio stream the first time.