Changeset 56435 in webkit for trunk/JavaScriptCore/API

Timestamp:

Mar 24, 2010, 12:11:51 AM (15 years ago)

Author:

[email protected]

Message:

Bug 36519 - JSGlobalContextRelease is unnecessarily slow

Reviewed by Oliver Hunt.

Since [ ​http://trac.webkit.org/changeset/35917 ], calling
JSGlobalContextRelease always triggers a GC heap collection
(if not a full destroy). As per 35917's changelog "This is
only really necessary when the (JSGlobalObject's) last
reference is released, but there is no way to determine that,
and no harm in collecting slightly more often."

Well, we now know of cases of API clients who are harmed by
the performance penalty of collecting too often, so it's time
to add a way to determine whether a call to JSGlobalContextRelease
is removing the last protect from it's global object. If further
protects are retaining the global object (likely from other
JSGlobalContextRefs), then don't trigger a GC collection.

• API/JSContextRef.cpp:
• runtime/Collector.cpp:

(JSC::Heap::unprotect): return a boolean indicating that the value is now unprotected.

• runtime/Collector.h:
• wtf/HashCountedSet.h:

(WTF::::remove): return a boolean indicating whether the value was removed from the set.

File:

• trunk/JavaScriptCore/API/JSContextRef.cpp (modified) (1 diff)

trunk/JavaScriptCore/API/JSContextRef.cpp

@@ -123 +123 @@
 
     JSGlobalData& globalData = exec->globalData();
+    JSGlobalObject* dgo = exec->dynamicGlobalObject();
     IdentifierTable* savedIdentifierTable = setCurrentIdentifierTable(globalData.identifierTable);
 
-    gcUnprotect(exec->dynamicGlobalObject());
+    // One reference is held by JSGlobalObject, another added by JSGlobalContextRetain().
+    bool releasingContextGroup = globalData.refCount() == 2;
+    bool releasingGlobalObject = Heap::heap(dgo)->unprotect(dgo);
+    // If this is the last reference to a global data, it should also
+    // be the only remaining reference to the global object too!
+    ASSERT(!releasingContextGroup || releasingGlobalObject);
 
-    if (globalData.refCount() == 2) { // One reference is held by JSGlobalObject, another added by JSGlobalContextRetain().
-        // The last reference was released, this is our last chance to collect.
+    // An API 'JSGlobalContextRef' retains two things - a global object and a
+    // global data (or context group, in API terminology).
+    // * If this is the last reference to any contexts in the given context group,
+    //   call destroy on the heap (the global data is being  freed).
+    // * If this was the last reference to the global object, then unprotecting
+    //   it may  release a lot of GC memory - run the garbage collector now.
+    // * If there are more references remaining the the global object, then do nothing
+    //   (specifically that is more protects, which we assume come from other JSGlobalContextRefs).
+    if (releasingContextGroup)
         globalData.heap.destroy();
-    } else
+    else if (releasingGlobalObject)
         globalData.heap.collectAllGarbage();