package melange

Atomic references.

See the examples below. See 'Memory model: The hard bits' chapter in the manual.

• since 4.12

Create an atomic reference that is alone on a cache line. It occupies 4-16x the memory of one allocated with make v. The primary purpose is to prevent false-sharing and the resulting performance degradation. When a CPU performs an atomic operation, it temporarily takes ownership of an entire cache line that contains the atomic reference. If multiple atomic references share the same cache line, modifying these disjoint memory regions simultaneously becomes impossible, which can create a bottleneck. Hence, as a general guideline, if an atomic reference is experiencing contention, assigning it its own cache line may enhance performance.

Examples

Basic Thread Coordination

A basic use case is to have global counters that are updated in a thread-safe way, for example to keep some sorts of metrics over IOs performed by the program. Another basic use case is to coordinate the termination of threads in a given program, for example when one thread finds an answer, or when the program is shut down by the user.

Here, for example, we're going to try to find a number whose hash satisfies a basic property. To do that, we'll run multiple threads which will try random numbers until they find one that works.

Of course the output below is a sample run and will change every time the program is run.

(* use for termination *)
let stop_all_threads = Atomic.make false

(* total number of individual attempts to find a number *)
let num_attempts = Atomic.make 0

(* find a number that satisfies [p], by... trying random numbers
   until one fits. *)
let find_number_where (p:int -> bool) =
  let rand = Random.State.make_self_init() in
  while not (Atomic.get stop_all_threads) do

    let n = Random.State.full_int rand max_int in
    ignore (Atomic.fetch_and_add num_attempts 1 : int);

    if p (Hashtbl.hash n) then (
      Printf.printf "found %d (hash=%d)\n%!" n (Hashtbl.hash n);
      Atomic.set stop_all_threads true; (* signal all threads to stop *)
    )
  done;;


(* run multiple domains to search for a [n] where [hash n <= 100] *)
let () =
  let criterion n = n <= 100 in
  let threads =
    Array.init 8
      (fun _ -> Domain.spawn (fun () -> find_number_where criterion))
  in
  Array.iter Domain.join threads;
  Printf.printf "total number of attempts: %d\n%!"
    (Atomic.get num_attempts) ;;

- : unit = ()
found 1651745641680046833 (hash=33)
total number of attempts: 30230350

Treiber Stack

Another example is a basic Treiber stack (a thread-safe stack) that can be safely shared between threads.

Note how both push and pop are recursive, because they attempt to swap the new stack (with one more, or one fewer, element) with the old stack. This is optimistic concurrency: each iteration of, say, push stack x gets the old stack l, and hopes that by the time it tries to replace l with x::l, nobody else has had time to modify the list. If the compare_and_set fails it means we were too optimistic, and must try again.

type 'a stack = 'a list Atomic.t

let rec push (stack: _ stack) elt : unit =
  let cur = Atomic.get stack in
  let success = Atomic.compare_and_set stack cur (elt :: cur) in
  if not success then
    push stack elt

let rec pop (stack: _ stack) : _ option =
  let cur = Atomic.get stack in
  match cur with
  | [] -> None
  | x :: tail ->
    let success = Atomic.compare_and_set stack cur tail in
    if success then Some x
    else pop stack

# let st = Atomic.make []
# push st 1
- : unit = ()
# push st 2
- : unit = ()
# pop st
- : int option = Some 2
# pop st
- : int option = Some 1
# pop st
- : int option = None