Bug 30922 – ThreadIdentifier abstraction is unnecessarily slow

• Component: JavaScriptCore
• Status: RESOLVED DUPLICATE

[Jens Alfke]

Threading.h uses a 'ThreadIdentifier' type to abstract away platform-specific thread types like pthread_t. But the implementation of this type for pthreads, found in ThreadingPthreads.cpp, is suboptimal.  This is causing measurable performance impact in Chrome*; I haven't checked Safari.

ThreadIdentifiers are consecutive integers which are mapped from pthread_t's via a HashMap. This means that looking up the identifier of a thread (including the current thread) means first acquiring a mutex, then a hashtable lookup. And looking up a thread by its identifier requires a mutex plus a linear scan of the table.

I can think of two much faster ways to do this:
(1) Define ThreadIdentifier as a typedef of pthread_t, eliminating the need for the mapping altogether. Drawback is that pthread_t is probably a pointer, making it 64 bits wide in a 64-bit process, which increases the size of a ThreadIdentifier. Does this matter? We probably don't store enough of these to make a difference.
(2) Use a pthread_t[ ] array instead of a dictionary. The ThreadIdentifier is just the array index where that pthread_t is found. This makes lookup quite fast. If the array is fixed-size then no locking is needed, you can do an atomic check-and-test to append to the array. Drawback is that the array is fixed-size and thread IDs are never removed, so the array could overflow if a lot of threads are created.
(3) Use pthread_getspecific et al to associate the identifier. I don't know how well this performs compared to what exists.

Option 1 seems like a winner to me. It's very fast and allows at least a page of code to be ripped out of ThreadingPthreads.cpp.

* Here's some analysis from Shark, running the Dromaeo core DOM benchmarks, focused on the V8 GC entry point (so it shows up as 100%). Note that 9.6% of the GC time is spent in WTF::currentThread, becuase DOMData::ensureDeref has to check whether the object is being released on its native thread or not.
	0.0%	100.0%	Chromium Framework	v8::internal::Heap::CollectGarbage(int, v8::internal::AllocationSpace)
	0.0%	100.0%	Chromium Framework	 v8::internal::Heap::PerformGarbageCollection(v8::internal::AllocationSpace, v8::internal::GarbageCollector, v8::internal::GCTracer*)
	10.8%	63.8%	Chromium Framework	  v8::internal::GlobalHandles::PostGarbageCollectionProcessing()
	0.2%	41.6%	Chromium Framework	   WebCore::DOMDataStore::weakDOMObjectCallback(v8::Persistent<v8::Value>, void*)
	1.6%	40.4%	Chromium Framework	    void WebCore::DOMData::handleWeakObject<void>(WebCore::DOMDataStore::DOMWrapperMapType, v8::Handle<v8::Object>, void*)
	7.1%	19.7%	Chromium Framework	     WebCore::DOMData::ensureDeref(WebCore::V8ClassIndex::V8WrapperType, void*)
	0.3%	9.6%	Chromium Framework	      WTF::currentThread()
	5.7%	9.1%	Chromium Framework	       _ZN3WTFL25identifierByPthreadHandleERKP17_opaque_pthread_t

[Mark Rowe (bdash)]

*** This bug has been marked as a duplicate of bug 25348 ***