onSpinWait​() method of Thread class - Java 9

Question

While learning Java 9 features I came across a new method of Thread class, called onSpinWait​. As per javadocs, this method is used for this:

Indicates that the caller is momentarily unable to progress, until the occurrence of one or more actions on the part of other activities.

Can someone help me understand this method giving a real-life example or scenario?

Answer 1

It's the same (and probably compiles to) as the x86 opcode PAUSE and equivalent the Win32 macro YieldProcessor, GCC's __mm_pause() and the C# method Thread.SpinWait

It's a very weakened form of yielding: it tells your CPU that you are in a loop that may burn many CPU-cycles waiting for something to happen (busy-waiting).

This way, The CPU can assign more resources to other threads, without actually loading the OS scheduler and dequeuing a ready-to-run thread (which may be expensive).

A common use for that is spin-locking, when you know the contention on a shared memory is very infrequent or finishes very quickly, a spinlock may preform better than an ordinary lock.

Pseudo code for such can look like:

int state = 0; //1 - locked, 0 - unlocked

routine lock:
    while state.cas(new_value=1, wanted_value=0) == false //if state is 0 (unlocked), store 1 (locked) and return true, otherwise just return false.
       yield

routine unlock:
    atomic_store(state,0)

yield can be implemented with Thread.onSpinWait(), hinting that while trying to lock the lock, the CPU can give more resources to other threads.

This technique of yielding is extremely common and popular when implementing a lock-free algorithm, since most of them depend on busy-waiting (which is implemented almost always as an atomic compare-and-swap loop). this has every real-world use you can imagine.