Using tooling from bug 186698, loading can and running "notifyutil -p com.apple.WebKit.dumpHashTableCapacity" shows: Wasted capacity: 244368 bytes (used 148848 of 393216 bytes, utilization: 37.85%) - 1 allocations 1 0x1166cc265 WTF::HashTable<JSC::PrototypeKey, WTF::KeyValuePair<JSC::PrototypeKey, JSC::Weak<JSC::Structure> >, WTF::KeyValuePairKeyExtractor<WTF::KeyValuePair<JSC::PrototypeKey, JSC::Weak<JSC::Structure> > >, JSC::PrototypeKeyHash, WTF::HashMap<JSC::PrototypeKey, JSC::Weak<JSC::Structure>, JSC::PrototypeKeyHash, WTF::HashTraits<JSC::PrototypeKey>, WTF::HashTraits<JSC::Weak<JSC::Structure> > >::KeyValuePairTraits, WTF::HashTraits<JSC::PrototypeKey> >::HashTable() 2 0x1166cc245 WTF::HashMap<JSC::PrototypeKey, JSC::Weak<JSC::Structure>, JSC::PrototypeKeyHash, WTF::HashTraits<JSC::PrototypeKey>, WTF::HashTraits<JSC::Weak<JSC::Structure> > >::HashMap() 3 0x1166cc1a5 WTF::HashMap<JSC::PrototypeKey, JSC::Weak<JSC::Structure>, JSC::PrototypeKeyHash, WTF::HashTraits<JSC::PrototypeKey>, WTF::HashTraits<JSC::Weak<JSC::Structure> > >::HashMap() 4 0x1166cc15b JSC::WeakGCMap<JSC::PrototypeKey, JSC::Structure, JSC::PrototypeKeyHash, WTF::HashTraits<JSC::PrototypeKey> >::WeakGCMap(JSC::VM&) 5 0x1166cc10d JSC::WeakGCMap<JSC::PrototypeKey, JSC::Structure, JSC::PrototypeKeyHash, WTF::HashTraits<JSC::PrototypeKey> >::WeakGCMap(JSC::VM&) 6 0x1166cc0dd JSC::StructureCache::StructureCache(JSC::VM&) 7 0x1166bffed JSC::StructureCache::StructureCache(JSC::VM&) 8 0x1166bc42f JSC::VM::VM(JSC::VM::VMType, JSC::HeapType)
<rdar://problem/41189110>