Bug 1078437 - Prevent store buffer compaction from thrashing, r=terrence

--HG--
extra : rebase_source : e1e4096b7c975568471036868104a7e7ee05c9d6

js/src/gc/StoreBuffer.cpp

@@ -97,7 +97,7 @@ StoreBuffer::MonoTypeBuffer<T>::handleOverflow(StoreBuffer *owner)
          * trigger a minor collection.
          */
         compact(owner);
-        if (isAboutToOverflow())
+        if (isLowOnSpace())
             owner->setAboutToOverflow();
     } else {
          /*

js/src/gc/StoreBuffer.h

@@ -78,7 +78,18 @@ class StoreBuffer
     friend class mozilla::ReentrancyGuard;
 
     /* The size at which a block is about to overflow. */
-    static const size_t MinAvailableSize = (size_t)(LifoAllocBlockSize * 1.0 / 8.0);
+    static const size_t LowAvailableThreshold = (size_t)(LifoAllocBlockSize * 1.0 / 16.0);
+
+    /*
+     * If the space available in the store buffer hits the
+     * LowAvailableThreshold and gets compacted, but still doesn't have at
+     * least HighAvailableThreshold space available, then we will trigger a
+     * minor GC. HighAvailableThreshold should be set to provide enough space
+     * for the mutator to run for a while in between compactions. (If
+     * HighAvailableThreshold is too low, we will thrash and spend most of the
+     * time compacting. If it is too high, we will tenure things too early.)
+     */
+    static const size_t HighAvailableThreshold = (size_t)(LifoAllocBlockSize * 1.0 / 4.0);
 
     /*
      * This buffer holds only a single type of edge. Using this buffer is more
@@ -110,7 +121,11 @@ class StoreBuffer
         }
 
         bool isAboutToOverflow() const {
-            return !storage_->isEmpty() && storage_->availableInCurrentChunk() < MinAvailableSize;
+            return !storage_->isEmpty() && storage_->availableInCurrentChunk() < LowAvailableThreshold;
+        }
+
+        bool isLowOnSpace() const {
+            return !storage_->isEmpty() && storage_->availableInCurrentChunk() < HighAvailableThreshold;
         }
 
         void handleOverflow(StoreBuffer *owner);
@@ -189,7 +204,7 @@ class StoreBuffer
         }
 
         bool isAboutToOverflow() const {
-            return !storage_->isEmpty() && storage_->availableInCurrentChunk() < MinAvailableSize;
+            return !storage_->isEmpty() && storage_->availableInCurrentChunk() < LowAvailableThreshold;
         }
 
         /* Mark all generic edges. */