Actually, now that I think about, most thread local stuff wouldn't be freed automatically either, since it's still allocated from a common pool of memory, and interleaved allocations would still prevent memory blocks from being returned to the OS. As far as behavior goes, assuming you aren't explicitly checking thread IDs (which should be meaningless to worker threads anyway; the tasks are assigned arbitrarily), clearing out the thread local storage when a thread exits would be equivalent to ending one worker thread and starting a new one.