Is ThreadLocal allocated in TLAB?

Question

I suppose, that ThreadLocal variables are allocated in Thread Local allocation Buffer(s) or TLABs, am I right ?

I was not successful in finding any document stating what exactly makes some class stored in TLAB. If you know some, please post a link.

Answer 1

I was not successfull to find any document stating what exactly makes some class stored in TLAB. If you know some, please post a link.

Actually, the explanation is right there in the blog post you lnked to:

A Thread Local Allocation Buffer (TLAB) is a region of Eden that is used for allocation by a single thread. It enables a thread to do object allocation using thread local top and limit pointers, which is faster than doing an atomic operation on a top pointer that is shared across threads.

Every thread allocates memory from its own chunk of Eden, the "Generation 0" part of the heap. Pretty much everything is stored in the TLAB for a period of time - quite possibly your ThreadLocals, too - but they get moved away from there after a gen0 garbage collection. TLABs are there to make allocations faster, not to make the memory unaccessible from other threads. A more accessible description from the same blog you linked to is A little thread privacy, please.

Answer 2

I'm pretty sure that this is up to the discretion of the JVM implementer. They could put the data in TLABs if they wanted to, or in a global table keyed by the thread ID. The Java Language Specification tends to be mute about these sorts of issues so that JVM authors can deploy Java on as many and as diverse platforms as possible.

Answer 3

i think only the pointer to it is, while the data itself resides in some other memory area. see http://blogs.oracle.com/jonthecollector/entry/the_real_thing and http://wikis.sun.com/display/MaxineVM/Threads#Threads-Threadlocalvariables

Answer 4

No. Here how it is: As of 1.4 each thread in Java has a field called threadLocals where the map is kept. Each threadLocal has an index to the structure, so it doesn't use hashCode(). Imagine an array and each ThreadLocal keep a slot index.

When the thread dies and there are no more references to it, the ThreadLocals are GC'd. Very simple idea.

You can implement your own ThreaLocal(s) by extending Thread and adding a field to hold the reference. Then cast the Thread to youw own class and take the data.

So it's not TLAB, it's still the heap like any other object.

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Historically there were implementations w/ static WeakHashMap which were very slow to access the data.

Answer 5

It is my understanding that TLAB is used for object allocation of all small to medium objects. Your ThreadLocal won't be allocated any differently.