Bug 1304205 - Increase slice time for longer running CCs. r=smaug

If a CC takes too long (around 50 slices) or gets interrupted by a GC,
we have to finish it synchronously, which can cause a big pause. This
patch tries to avoid that by eagerly increasing the slice budget the
longer a CC goes on. It linearly increases the slice time from 5ms to
40ms as we approach the halfway point of a CC (1 second), matching GC
pauses, and then leaves it at 40ms.

MozReview-Commit-ID: 8TKZ0ZuxsUA

--HG--
extra : rebase_source : 2c46e56ecaa47242177d8cce53a208f08f5cabe2

dom/base/nsJSEnvironment.cpp

@@ -1477,9 +1477,20 @@ nsJSContext::RunCycleCollectorSlice()
       TimeStamp now = TimeStamp::Now();
 
       // Only run a limited slice if we're within the max running time.
-      if (TimeBetween(gCCStats.mBeginTime, now) < kMaxICCDuration) {
-        float sliceMultiplier = std::max(TimeBetween(gCCStats.mEndSliceTime, now) / (float)kICCIntersliceDelay, 1.0f);
-        budget = js::SliceBudget(js::TimeBudget(kICCSliceBudget * sliceMultiplier));
+      uint32_t runningTime = TimeBetween(gCCStats.mBeginTime, now);
+      if (runningTime < kMaxICCDuration) {
+        // Try to make up for a delay in running this slice.
+        float sliceDelayMultiplier = TimeBetween(gCCStats.mEndSliceTime, now) / (float)kICCIntersliceDelay;
+        float delaySliceBudget = kICCSliceBudget * sliceDelayMultiplier;
+
+        // Increase slice budgets up to |maxLaterSlice| as we approach
+        // half way through the ICC, to avoid large sync CCs.
+        float percentToHalfDone = std::min(2.0f * runningTime / kMaxICCDuration, 1.0f);
+        const float maxLaterSlice = 40.0f;
+        float laterSliceBudget = maxLaterSlice * percentToHalfDone;
+
+        budget = js::SliceBudget(js::TimeBudget(std::max({delaySliceBudget,
+                  laterSliceBudget, (float)kICCSliceBudget})));
       }
     }
   }