domain.mli

(**************************************************************************)
(*                                                                        *)
(*                                 OCaml                                  *)
(*                                                                        *)
(*      KC Sivaramakrishnan, Indian Institute of Technology, Madras       *)
(*                 Stephen Dolan, University of Cambridge                 *)
(*                   Tom Kelly, OCaml Labs Consultancy                    *)
(*                                                                        *)
(*   Copyright 2019 Indian Institute of Technology, Madras                *)
(*   Copyright 2014 University of Cambridge                               *)
(*   Copyright 2021 OCaml Labs Consultancy Ltd                            *)
(*                                                                        *)
(*   All rights reserved.  This file is distributed under the terms of    *)
(*   the GNU Lesser General Public License version 2.1, with the          *)
(*   special exception on linking described in the file LICENSE.          *)
(*                                                                        *)
(**************************************************************************)

[@@@alert unstable
    "The Domain interface may change in incompatible ways in the future."
]

(** Domains.

    See 'Parallel programming' chapter in the manual.

    @since 5.0 *)

type !'a t
(** A domain of type ['a t] runs independently, eventually producing a
    result of type 'a, or an exception *)

val spawn : (unit -> 'a) -> 'a t
(** [spawn f] creates a new domain that runs in parallel with the
    current domain.

    @raise Failure if the program has insufficient resources to create another
    domain. *)

val join : 'a t -> 'a
(** [join d] blocks until domain [d] runs to completion. If [d] results in a
    value, then that is returned by [join d]. If [d] raises an uncaught
    exception, then that is re-raised by [join d]. *)

type id = private int
(** Domains have unique integer identifiers *)

val get_id : 'a t -> id
(** [get_id d] returns the identifier of the domain [d] *)

val self : unit -> id
(** [self ()] is the identifier of the currently running domain *)

val before_first_spawn : (unit -> unit) -> unit
(** [before_first_spawn f] registers [f] to be called before the first domain
    is spawned by the program. The functions registered with
    [before_first_spawn] are called on the main (initial) domain. The functions
    registered with [before_first_spawn] are called in 'first in, first out'
    order: the oldest function added with [before_first_spawn] is called first.

    @raise Invalid_argument if the program has already spawned a domain. *)

val at_exit : (unit -> unit) -> unit
(** [at_exit f] registers [f] to be called when the current domain exits. Note
    that [at_exit] callbacks are domain-local and only apply to the calling
    domain. The registered functions are called in 'last in, first out' order:
    the function most recently added with [at_exit] is called first. An example:

    {[
let temp_file_key = Domain.DLS.new_key (fun _ ->
  let tmp = snd (Filename.open_temp_file "" "") in
  Domain.at_exit (fun () -> close_out_noerr tmp);
  tmp)
    ]}

    The snippet above creates a key that when retrieved for the first
    time will open a temporary file and register an [at_exit] callback
    to close it, thus guaranteeing the descriptor is not leaked in
    case the current domain exits. *)

val cpu_relax : unit -> unit
(** If busy-waiting, calling cpu_relax () between iterations
    will improve performance on some CPU architectures *)

val is_main_domain : unit -> bool
(** [is_main_domain ()] returns true if called from the initial domain. *)

val recommended_domain_count : unit -> int
(** The recommended maximum number of domains which should be running
    simultaneously (including domains already running).

    The value returned is at least [1]. *)

val self_index : unit -> int
(** The index of the current domain. It is an integer unique among
    currently-running domains, in the interval [0; N-1] where N is the
    peak number of domains running simultaneously so far.

    The index of a terminated domain may be reused for a new
    domain. Use [(Domain.self () :> int)] instead for an identifier
    unique among all domains ever created by the program.

    @since 5.3
*)

module DLS : sig
(** Domain-local Storage *)

    type 'a key
    (** Type of a DLS key *)

    val new_key : ?split_from_parent:('a -> 'a) -> (unit -> 'a) -> 'a key
    (** [new_key f] returns a new key bound to initialiser [f] for accessing
,        domain-local variables.

        If [split_from_parent] is not provided, the value for a new
        domain will be computed on-demand by the new domain: the first
        [get] call will call the initializer [f] and store that value.

        {b Warning.} [f] may be called several times if another call
        to [get] occurs during initialization on the same domain. Only
        the 'first' value computed will be used, the other now-useless
        values will be discarded. Your initialization function should
        support this situation, or contain logic to detect this case
        and fail.

        If [split_from_parent] is provided, spawning a domain will
        derive the child value (for this key) from the parent
        value. This computation happens in the parent domain and it
        always happens, regardless of whether the child domain will
        use it.
        If the splitting function is expensive or requires
        child-side computation, consider using ['a Lazy.t key]:

        {[
        let init () = ...

        let split_from_parent parent_value =
          ... parent-side computation ...;
          lazy (
            ... child-side computation ...
          )

        let key = Domain.DLS.new_key ~split_from_parent init

        let get () = Lazy.force (Domain.DLS.get key)
        ]}

        In this case a part of the computation happens on the child
        domain; in particular, it can access [parent_value]
        concurrently with the parent domain, which may require
        explicit synchronization to avoid data races.
    *)

    val get : 'a key -> 'a
    (** [get k] returns [v] if a value [v] is associated to the key [k] on
        the calling domain's domain-local state. Sets [k]'s value with its
        initialiser and returns it otherwise. *)

    val set : 'a key -> 'a -> unit
    (** [set k v] updates the calling domain's domain-local state to associate
        the key [k] with value [v]. It overwrites any previous values associated
        to [k], which cannot be restored later. *)

end