Thread local heap

The Garbage Collector (GC) maintains areas of the heap for fast object allocation.

The heap is subject to concurrent access by all the threads that are running in the JVM. Therefore, it must be protected by a resource lock so that one thread can complete updates to the heap before another thread is allowed in. Access to the heap is therefore single-threaded. However, the GC also maintains areas of the heap as thread caches or thread local heap (TLH). These TLHs are areas of the heap that are allocated as a single large object, marked non-collectable, and allocated to a thread. The thread can now sub allocate from the TLH objects that are below a defined size. No heap lock is needed which means that allocation is very fast and efficient. When a cache becomes full, a thread returns the TLH to the main heap and grabs another chunk for a new cache.

A TLH is not subject to a garbage collection cycle; it is a reference that is dedicated to a thread.