JavaScript Shared Memory — Quiz

Review

1. What makes SharedArrayBuffer different from ArrayBuffer?

   SharedArrayBuffer provides shared memory storage and cannot be detached like an ordinary ArrayBuffer.
2. What can a TypedArray do with a SharedArrayBuffer?

   TypedArrays can be views over SharedArrayBuffer storage.
3. What does Atomics.add return?

   The read-modify-write Atomics.add operation returns the previous value.
4. Why use Atomics.exchange?

   exchange replaces the element as one atomic operation and returns the previous value.
5. What does compareExchange test?

   compareExchange performs a conditional atomic replacement and returns the previous value.
6. What is Atomics.load for?

   Atomics.load performs an atomic read of a supported shared-memory element.
7. What does an Atomics bitwise operation change?

   Atomics bitwise methods perform atomic read-modify-write operations on supported integer shared views.
8. What does Atomics.sub return?

   Atomics.sub returns the previous value before applying the subtraction.
9. What does Atomics.isLockFree report?

   isLockFree reports whether operations of a given byte size are expected to be lock-free by the implementation.
10. What can Atomics.wait do?

    Atomics.wait can block an agent waiting on a shared integer location; its use depends on the execution context.
11. What does Atomics.notify do?

    Atomics.notify dispatches wake-ups to agents waiting on a shared memory location.
12. Which views are intended for Atomics operations?

    Atomics operations use supported integer TypedArray views over shared memory; ordinary Arrays are not shared-memory cells.
13. What is growable SharedArrayBuffer?

    A growable SharedArrayBuffer can increase its byte length up to its maxByteLength when the host/runtime supports the growth operation.
14. Can a SharedArrayBuffer become detached?

    ECMAScript specifies that SharedArrayBuffer instances are never detached.
15. Why use Atomics for a shared counter?

    Atomics.add provides an indivisible read-modify-write operation for a supported shared-memory cell; it does not automatically design the whole synchronization protocol.
16. Why is the return value of Atomics.sub useful?

    Atomic read-modify-write methods return the value observed before the update, which can be used when coordinating shared state.
17. What is Atomics.pause intended to do?

    Atomics.pause is a spin-loop hint and has no observable behavior other than timing.
18. What does Atomics.pause return?

    The specification says Atomics.pause returns undefined.
19. Why should a lesson feature-detect Atomics.pause?

    Feature detection keeps runnable examples valid when the executing runtime does not yet implement the method.
20. Which statement is correct about Atomics.pause versus Atomics.wait?

    Atomics.wait is a synchronization primitive that can suspend the agent, while Atomics.pause is a non-observable timing hint for spinning.