[boost] Question about boost::thread::yield for Win32

[Russell Hind]

Is yield intended to always yield to another thread if one can run? 
Because  the code for yield is

void thread::yield()
{
#if defined(BOOST_HAS_WINTHREADS)
    Sleep(0);
#elif defined(BOOST_HAS_PTHREADS)
#   if defined(BOOST_HAS_SCHED_YIELD)
    int res = 0;
    res = sched_yield();
    assert(res == 0);
#   elif defined(BOOST_HAS_PTHREAD_YIELD)
    int res = 0;
    res = pthread_yield();
    assert(res == 0);
#   else
    xtime xt;
    xtime_get(&xt, TIME_UTC);
    sleep(xt);
#   endif
#elif defined(BOOST_HAS_MPTASKS)
    MPYield();
#endif
}
Taken from the main CVS.

Sleep(0) on Win32 will only yield to another thread of equal or higher 
priority, not to lower priority threads.

In boost::detail::lightweight_mutex::scoped_lock, it is mentioned that 
Sleep(1) will get around.  Is the behaviour of Sleep(0) the intended use 
of yield?

        explicit scoped_lock(lightweight_mutex & m): m_(m)
        {
            while( InterlockedExchange(&m_.l_, 1) )
            {
                // Note: changed to Sleep(1) from Sleep(0).
                // According to MSDN, Sleep(0) will never yield
                // to a lower-priority thread, whereas Sleep(1)
                // will. Performance seems not to be affected.
                Sleep(1);
            }
        }
(I don't actually use yield yet, so currently have no preference for 
either, but just wondered what the intended use of yield was)

Thanks

Russell

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