Bug 1506761 - Add a fraction threshold for collecting the nursery r=sfink

We collect the nursery in idle time if there is less than 256KB of space
remaining.  However when the nursery is small this doesn't make sense, so
add a percentage-based threshold to be used when the nursery is small.

Differential Revision: https://phabricator.services.mozilla.com/D20247

--HG--
extra : moz-landing-system : lando

js/public/GCAPI.h

@@ -283,6 +283,16 @@ typedef enum JSGCParamKey {
    * Pref: None
    */
   JSGC_PRETENURE_GROUP_THRESHOLD = 29,
+
+  /**
+   * Attempt to run a minor GC in the idle time if the free space falls
+   * below this percentage (from 0 to 99).
+   *
+   * Default: 25
+   * Pref: None
+   */
+  JSGC_NURSERY_FREE_THRESHOLD_FOR_IDLE_COLLECTION_PERCENT = 30,
+
 } JSGCParamKey;
 
 /*

js/src/gc/GC.cpp

@@ -347,6 +347,9 @@ static const bool CompactingEnabled = true;
 static const uint32_t NurseryFreeThresholdForIdleCollection =
     Nursery::NurseryChunkUsableSize / 4;
 
+/* JSGC_NURSERY_FREE_THRESHOLD_FOR_IDLE_COLLECTION_PERCENT */
+static const float NurseryFreeThresholdForIdleCollectionFraction = 0.25f;
+
 /* JSGC_PRETENURE_THRESHOLD */
 static const float PretenureThreashold = 0.6f;
 
@@ -1476,6 +1479,12 @@ bool GCSchedulingTunables::setParameter(JSGCParamKey key, uint32_t value,
       }
       nurseryFreeThresholdForIdleCollection_ = value;
       break;
+    case JSGC_NURSERY_FREE_THRESHOLD_FOR_IDLE_COLLECTION_PERCENT:
+      if (value == 0 || value > 100) {
+        return false;
+      }
+      nurseryFreeThresholdForIdleCollectionFraction_ = value / 100.0f;
+      break;
     case JSGC_PRETENURE_THRESHOLD: {
       // 100 disables pretenuring
       if (value == 0 || value > 100) {
@@ -1578,6 +1587,8 @@ GCSchedulingTunables::GCSchedulingTunables()
       maxEmptyChunkCount_(TuningDefaults::MaxEmptyChunkCount),
       nurseryFreeThresholdForIdleCollection_(
           TuningDefaults::NurseryFreeThresholdForIdleCollection),
+      nurseryFreeThresholdForIdleCollectionFraction_(
+          TuningDefaults::NurseryFreeThresholdForIdleCollectionFraction),
       pretenureThreshold_(TuningDefaults::PretenureThreashold),
       pretenureGroupThreshold_(TuningDefaults::PretenureGroupThreshold) {}
 
@@ -1661,6 +1672,10 @@ void GCSchedulingTunables::resetParameter(JSGCParamKey key,
       nurseryFreeThresholdForIdleCollection_ =
           TuningDefaults::NurseryFreeThresholdForIdleCollection;
       break;
+    case JSGC_NURSERY_FREE_THRESHOLD_FOR_IDLE_COLLECTION_PERCENT:
+      nurseryFreeThresholdForIdleCollectionFraction_ =
+          TuningDefaults::NurseryFreeThresholdForIdleCollectionFraction;
+      break;
     case JSGC_PRETENURE_THRESHOLD:
       pretenureThreshold_ = TuningDefaults::PretenureThreashold;
       break;

js/src/gc/Nursery.cpp

@@ -698,8 +698,35 @@ inline void js::Nursery::endProfile(ProfileKey key) {
 }
 
 bool js::Nursery::shouldCollect() const {
-  uint32_t threshold = tunables().nurseryFreeThresholdForIdleCollection();
-  return !isEmpty() && (minorGCRequested() || freeSpace() < threshold);
+  if (minorGCRequested()) {
+    return true;
+  }
+
+  bool belowBytesThreshold =
+      freeSpace() < tunables().nurseryFreeThresholdForIdleCollection();
+  bool belowFractionThreshold =
+      float(freeSpace()) / float(capacity()) <
+      tunables().nurseryFreeThresholdForIdleCollectionFraction();
+
+  // We want to use belowBytesThreshold when the nursery is sufficiently large,
+  // and belowFractionThreshold when it's small.
+  //
+  // When the nursery is small then belowBytesThreshold is a lower threshold
+  // (triggered earlier) than belowFractionThreshold.  So if the fraction
+  // threshold is true, the bytes one will be true also.  The opposite is true
+  // when the nursery is large.
+  //
+  // Therefore, by the time we cross the threshold we care about, we've already
+  // crossed the other one, and we can boolean AND to use either condition
+  // without encoding any "is the nursery big/small" test/threshold.  The point
+  // at which they cross is when the nursery is:  BytesThreshold /
+  // FractionThreshold large.
+  //
+  // With defaults that's:
+  //
+  //   1MB = 256KB / 0.25
+  //
+  return belowBytesThreshold && belowFractionThreshold;
 }
 
 static inline bool IsFullStoreBufferReason(JS::GCReason reason) {

js/src/gc/Scheduling.h

@@ -427,11 +427,14 @@ class GCSchedulingTunables {
 
   /*
    * JSGC_NURSERY_FREE_THRESHOLD_FOR_IDLE_COLLECTION
+   * JSGC_NURSERY_FREE_THRESHOLD_FOR_IDLE_COLLECTION_FRACTION
    *
    * Attempt to run a minor GC in the idle time if the free space falls
-   * below this threshold.
+   * below this threshold. The absolute threshold is used when the nursery is
+   * large and the percentage when it is small.  See Nursery::shouldCollect()
    */
   UnprotectedData<uint32_t> nurseryFreeThresholdForIdleCollection_;
+  UnprotectedData<float> nurseryFreeThresholdForIdleCollectionFraction_;
 
   /*
    * JSGC_PRETENURE_THRESHOLD
@@ -488,6 +491,9 @@ class GCSchedulingTunables {
   uint32_t nurseryFreeThresholdForIdleCollection() const {
     return nurseryFreeThresholdForIdleCollection_;
   }
+  float nurseryFreeThresholdForIdleCollectionFraction() const {
+    return nurseryFreeThresholdForIdleCollectionFraction_;
+  }
 
   bool attemptPretenuring() const { return pretenureThreshold_ < 1.0f; }
   float pretenureThreshold() const { return pretenureThreshold_; }