(*  Title:      Pure/Concurrent/future.ML

    Author:     Makarius

Value-oriented parallelism via futures and promises.  See also

http://www4.in.tum.de/~wenzelm/papers/parallel-isabelle.pdf

http://www4.in.tum.de/~wenzelm/papers/parallel-ml.pdf

Notes:

  * Futures are similar to delayed evaluation, i.e. delay/force is

    generalized to fork/join.  The idea is to model parallel

    value-oriented computations (not communicating processes).

  * Forked futures are evaluated spontaneously by a farm of worker

    threads in the background; join resynchronizes the computation and

    delivers results (values or exceptions).

  * The pool of worker threads is limited, usually in correlation with

    the number of physical cores on the machine.  Note that allocation

    of runtime resources may be distorted either if workers yield CPU

    time (e.g. via system sleep or wait operations), or if non-worker

    threads contend for significant runtime resources independently.

    There is a limited number of replacement worker threads that get

    activated in certain explicit wait conditions.

  * Future tasks are organized in groups, which are block-structured.

    When forking a new new task, the default is to open an individual

    subgroup, unless some common group is specified explicitly.

    Failure of one group member causes peer and subgroup members to be

    interrupted eventually.  Interrupted tasks that lack regular

    result information, will pick up parallel exceptions from the

    cumulative group context (as Par_Exn).

  * Future task groups may be canceled: present and future group

    members will be interrupted eventually.

  * Promised "passive" futures are fulfilled by external means.  There

    is no associated evaluation task, but other futures can depend on

    them via regular join operations.

*)

signature FUTURE =

sig

  type task = Task_Queue.task

  type group = Task_Queue.group

  val new_group: group option -> group

  val worker_task: unit -> task option

  val worker_group: unit -> group option

  val the_worker_group: unit -> group

  val worker_subgroup: unit -> group

  val worker_context: string -> group -> ('a -> 'b) -> 'a -> 'b

  type 'a future

  val task_of: 'a future -> task

  val peek: 'a future -> 'a Exn.result option

  val is_finished: 'a future -> bool

  val ML_statistics: bool Unsynchronized.ref

  val interruptible_task: ('a -> 'b) -> 'a -> 'b

  val cancel_group: group -> unit

  val cancel: 'a future -> unit

  val error_msg: Position.T -> (serial * string) * string option -> unit

  val identify_result: Position.T -> 'a Exn.result -> 'a Exn.result

  type params = {name: string, group: group option, deps: task list, pri: int, interrupts: bool}

  val default_params: params

  val forks: params -> (unit -> 'a) list -> 'a future list

  val fork: (unit -> 'a) -> 'a future

  val join_results: 'a future list -> 'a Exn.result list

  val join_result: 'a future -> 'a Exn.result

  val joins: 'a future list -> 'a list

  val join: 'a future -> 'a

  val join_tasks: task list -> unit

  val value_result: 'a Exn.result -> 'a future

  val value: 'a -> 'a future

  val cond_forks: params -> (unit -> 'a) list -> 'a future list

  val map: ('a -> 'b) -> 'a future -> 'b future

  val promise_group: group -> (unit -> unit) -> 'a future

  val promise: (unit -> unit) -> 'a future

  val fulfill_result: 'a future -> 'a Exn.result -> unit

  val fulfill: 'a future -> 'a -> unit

  val group_snapshot: group -> task list

  val terminate: group -> unit

  val shutdown: unit -> unit

end;

structure Future: FUTURE =

struct

(** future values **)

type task = Task_Queue.task;

type group = Task_Queue.group;

val new_group = Task_Queue.new_group;

(* identifiers *)

local

  val tag = Universal.tag () : task option Universal.tag;

in

  fun worker_task () = the_default NONE (Thread.getLocal tag);

  fun setmp_worker_task task f x = setmp_thread_data tag (worker_task ()) (SOME task) f x;

end;

val worker_group = Option.map Task_Queue.group_of_task o worker_task;

fun the_worker_group () =

  (case worker_group () of

    SOME group => group

  | NONE => raise Fail "Missing worker thread context");

fun worker_subgroup () = new_group (worker_group ());

fun worker_context name group f x =

  setmp_worker_task (Task_Queue.new_task group name NONE) f x;

fun worker_joining e =

  (case worker_task () of

    NONE => e ()

  | SOME task => Task_Queue.joining task e);

fun worker_waiting deps e =

  (case worker_task () of

    NONE => e ()

  | SOME task => Task_Queue.waiting task deps e);

(* datatype future *)

type 'a result = 'a Exn.result Single_Assignment.var;

datatype 'a future = Future of

 {promised: bool,

  task: task,

  result: 'a result};

fun task_of (Future {task, ...}) = task;

fun result_of (Future {result, ...}) = result;

fun peek x = Single_Assignment.peek (result_of x);

fun is_finished x = is_some (peek x);

(** scheduling **)

(* synchronization *)

val scheduler_event = ConditionVar.conditionVar ();

val work_available = ConditionVar.conditionVar ();

val work_finished = ConditionVar.conditionVar ();

local

  val lock = Mutex.mutex ();

in

fun SYNCHRONIZED name = Simple_Thread.synchronized name lock;

fun wait cond = (*requires SYNCHRONIZED*)

  Multithreading.sync_wait NONE NONE cond lock;

fun wait_timeout timeout cond = (*requires SYNCHRONIZED*)

  Multithreading.sync_wait NONE (SOME (Time.+ (Time.now (), timeout))) cond lock;

fun signal cond = (*requires SYNCHRONIZED*)

  ConditionVar.signal cond;

fun broadcast cond = (*requires SYNCHRONIZED*)

  ConditionVar.broadcast cond;

end;

(* global state *)

val queue = Unsynchronized.ref Task_Queue.empty;

val next = Unsynchronized.ref 0;

val scheduler = Unsynchronized.ref (NONE: Thread.thread option);

val canceled = Unsynchronized.ref ([]: group list);

val do_shutdown = Unsynchronized.ref false;

val max_workers = Unsynchronized.ref 0;

val max_active = Unsynchronized.ref 0;

val worker_trend = Unsynchronized.ref 0;

val status_ticks = Unsynchronized.ref 0;

val last_round = Unsynchronized.ref Time.zeroTime;

val next_round = seconds 0.05;

datatype worker_state = Working | Waiting | Sleeping;

val workers = Unsynchronized.ref ([]: (Thread.thread * worker_state Unsynchronized.ref) list);

fun count_workers state = (*requires SYNCHRONIZED*)

  fold (fn (_, state_ref) => fn i => if ! state_ref = state then i + 1 else i) (! workers) 0;

(* status *)

val ML_statistics = Unsynchronized.ref false;

fun report_status () = (*requires SYNCHRONIZED*)

  if ! ML_statistics then

    let

      val {ready, pending, running, passive} = Task_Queue.status (! queue);

      val total = length (! workers);

      val active = count_workers Working;

      val waiting = count_workers Waiting;

      val stats =

       [("now", Markup.print_real (Time.toReal (Time.now ()))),

        ("tasks_ready", Markup.print_int ready),

        ("tasks_pending", Markup.print_int pending),

        ("tasks_running", Markup.print_int running),

        ("tasks_passive", Markup.print_int passive),

        ("workers_total", Markup.print_int total),

        ("workers_active", Markup.print_int active),

        ("workers_waiting", Markup.print_int waiting)] @

        ML_Statistics.get ();

    in Output.try_protocol_message (Markup.ML_statistics :: stats) "" end

  else ();

(* cancellation primitives *)

fun cancel_now group = (*requires SYNCHRONIZED*)

  let

    val running = Task_Queue.cancel (! queue) group;

    val _ = running |> List.app (fn thread =>

      if Simple_Thread.is_self thread then ()

      else Simple_Thread.interrupt_unsynchronized thread);

  in running end;

fun cancel_all () = (*requires SYNCHRONIZED*)

  let

    val (groups, threads) = Task_Queue.cancel_all (! queue);

    val _ = List.app Simple_Thread.interrupt_unsynchronized threads;

  in groups end;

fun cancel_later group = (*requires SYNCHRONIZED*)

 (Unsynchronized.change canceled (insert Task_Queue.eq_group group);

  broadcast scheduler_event);

fun interruptible_task f x =

  (if Multithreading.available then

    Multithreading.with_attributes

      (if is_some (worker_task ())

       then Multithreading.private_interrupts

       else Multithreading.public_interrupts)

      (fn _ => f x)

   else interruptible f x)

  before Multithreading.interrupted ();

(* worker threads *)

fun worker_exec (task, jobs) =

  let

    val group = Task_Queue.group_of_task task;

    val valid = not (Task_Queue.is_canceled group);

    val ok =

      Task_Queue.running task (fn () =>

        setmp_worker_task task (fn () =>

          fold (fn job => fn ok => job valid andalso ok) jobs true) ());

    val _ =

      if ! Multithreading.trace >= 2 then

        Output.try_protocol_message (Markup.task_statistics :: Task_Queue.task_statistics task) ""

      else ();

    val _ = SYNCHRONIZED "finish" (fn () =>

      let

        val maximal = Unsynchronized.change_result queue (Task_Queue.finish task);

        val test = Exn.capture Multithreading.interrupted ();

        val _ =

          if ok andalso not (Exn.is_interrupt_exn test) then ()

          else if null (cancel_now group) then ()

          else cancel_later group;

        val _ = broadcast work_finished;

        val _ = if maximal then () else signal work_available;

      in () end);

  in () end;

fun worker_wait active cond = (*requires SYNCHRONIZED*)

  (case AList.lookup Thread.equal (! workers) (Thread.self ()) of

    SOME state =>

     (state := (if active then Waiting else Sleeping);

      wait cond;

      state := Working)

  | NONE => ignore (wait cond));

fun worker_next () = (*requires SYNCHRONIZED*)

  if length (! workers) > ! max_workers then

    (Unsynchronized.change workers (AList.delete Thread.equal (Thread.self ()));

     signal work_available;

     NONE)

  else if count_workers Working > ! max_active then

    (worker_wait false work_available; worker_next ())

  else

    (case Unsynchronized.change_result queue (Task_Queue.dequeue (Thread.self ())) of

      NONE => (worker_wait false work_available; worker_next ())

    | some => (signal work_available; some));

fun worker_loop name =

  (case SYNCHRONIZED name (fn () => worker_next ()) of

    NONE => ()

  | SOME work => (worker_exec work; worker_loop name));

fun worker_start name = (*requires SYNCHRONIZED*)

  Unsynchronized.change workers (cons (Simple_Thread.fork false (fn () => worker_loop name),

    Unsynchronized.ref Working));

(* scheduler *)

fun scheduler_next () = (*requires SYNCHRONIZED*)

  let

    val now = Time.now ();

    val tick = Time.<= (Time.+ (! last_round, next_round), now);

    val _ = if tick then last_round := now else ();

    (* runtime status *)

    val _ =

      if tick then Unsynchronized.change status_ticks (fn i => i + 1) else ();

    val _ =

      if tick andalso ! status_ticks mod (if ! Multithreading.trace >= 1 then 2 else 10) = 0

      then report_status () else ();

    val _ =

      if forall (Thread.isActive o #1) (! workers) then ()

      else

        let

          val (alive, dead) = List.partition (Thread.isActive o #1) (! workers);

          val _ = workers := alive;

        in

          Multithreading.tracing 0 (fn () =>

            "SCHEDULER: disposed " ^ string_of_int (length dead) ^ " dead worker threads")

        end;

    (* worker pool adjustments *)

    val max_active0 = ! max_active;

    val max_workers0 = ! max_workers;

    val m = if ! do_shutdown then 0 else Multithreading.max_threads_value ();

    val _ = max_active := m;

    val mm =

      if ! do_shutdown then 0

      else Int.min (Int.max (count_workers Working + 2 * count_workers Waiting, m), 4 * m);

    val _ =

      if tick andalso mm > ! max_workers then

        Unsynchronized.change worker_trend (fn w => if w < 0 then 0 else w + 1)

      else if tick andalso mm < ! max_workers then

        Unsynchronized.change worker_trend (fn w => if w > 0 then 0 else w - 1)

      else ();

    val _ =

      if mm = 0 orelse ! worker_trend > 50 orelse ! worker_trend < ~50 then

        max_workers := mm

      else if ! worker_trend > 5 andalso ! max_workers < 2 * m orelse ! max_workers = 0 then

        max_workers := Int.min (mm, 2 * m)

      else ();

    val missing = ! max_workers - length (! workers);

    val _ =

      if missing > 0 then

        funpow missing (fn () =>

          ignore (worker_start ("worker " ^ string_of_int (Unsynchronized.inc next)))) ()

      else ();

    val _ =

      if ! max_active = max_active0 andalso ! max_workers = max_workers0 then ()

      else signal work_available;

    (* canceled groups *)

    val _ =

      if null (! canceled) then ()

      else

       (Multithreading.tracing 1 (fn () =>

          string_of_int (length (! canceled)) ^ " canceled groups");

        Unsynchronized.change canceled (filter_out (null o cancel_now));

        signal work_available);

    (* delay loop *)

    val _ = Exn.release (wait_timeout next_round scheduler_event);

    (* shutdown *)

    val _ = if Task_Queue.all_passive (! queue) then do_shutdown := true else ();

    val continue = not (! do_shutdown andalso null (! workers));

    val _ = if continue then () else (report_status (); scheduler := NONE);

    val _ = broadcast scheduler_event;

  in continue end

  handle exn =>

    if Exn.is_interrupt exn then

     (Multithreading.tracing 1 (fn () => "SCHEDULER: Interrupt");

      List.app cancel_later (cancel_all ());

      signal work_available; true)

    else reraise exn;

fun scheduler_loop () =

 (while

    Multithreading.with_attributes

      (Multithreading.sync_interrupts Multithreading.public_interrupts)

      (fn _ => SYNCHRONIZED "scheduler" (fn () => scheduler_next ()))

  do (); last_round := Time.zeroTime);

fun scheduler_active () = (*requires SYNCHRONIZED*)

  (case ! scheduler of NONE => false | SOME thread => Thread.isActive thread);

fun scheduler_check () = (*requires SYNCHRONIZED*)

 (do_shutdown := false;

  if scheduler_active () then ()

  else scheduler := SOME (Simple_Thread.fork false scheduler_loop));

(** futures **)

(* cancel *)

fun cancel_group_unsynchronized group = (*requires SYNCHRONIZED*)

  let

    val _ = if null (cancel_now group) then () else cancel_later group;

    val _ = signal work_available;

    val _ = scheduler_check ();

  in () end;

fun cancel_group group =

  SYNCHRONIZED "cancel_group" (fn () => cancel_group_unsynchronized group);

fun cancel x = cancel_group (Task_Queue.group_of_task (task_of x));

(* results *)

fun error_msg pos ((serial, msg), exec_id) =

  Position.setmp_thread_data pos (fn () =>

    let val id = Position.get_id pos in

      if is_none id orelse is_none exec_id orelse id = exec_id

      then Output.error_msg' (serial, msg) else ()

    end) ();

fun identify_result pos res =

  (case res of

    Exn.Exn exn =>

      let val exec_id =

        (case Position.get_id pos of

          NONE => []

        | SOME id => [(Markup.exec_idN, id)])

      in Exn.Exn (Par_Exn.identify exec_id exn) end

  | _ => res);

fun assign_result group result res =

  let

    val _ = Single_Assignment.assign result res

      handle exn as Fail _ =>

        (case Single_Assignment.peek result of

          SOME (Exn.Exn e) => reraise (if Exn.is_interrupt e then e else exn)

        | _ => reraise exn);

    val ok =

      (case the (Single_Assignment.peek result) of

        Exn.Exn exn =>

          (SYNCHRONIZED "cancel" (fn () => Task_Queue.cancel_group group exn); false)

      | Exn.Res _ => true);

  in ok end;

(* future jobs *)

fun future_job group interrupts (e: unit -> 'a) =

  let

    val result = Single_Assignment.var "future" : 'a result;

    val pos = Position.thread_data ();

    fun job ok =

      let

        val res =

          if ok then

            Exn.capture (fn () =>

              Multithreading.with_attributes

                (if interrupts

                 then Multithreading.private_interrupts else Multithreading.no_interrupts)

                (fn _ => Position.setmp_thread_data pos e ())) ()

          else Exn.interrupt_exn;

      in assign_result group result (identify_result pos res) end;

  in (result, job) end;

(* fork *)

type params = {name: string, group: group option, deps: task list, pri: int, interrupts: bool};

val default_params: params = {name = "", group = NONE, deps = [], pri = 0, interrupts = true};

fun forks ({name, group, deps, pri, interrupts}: params) es =

  if null es then []

  else

    let

      val grp =

        (case group of

          NONE => worker_subgroup ()

        | SOME grp => grp);

      fun enqueue e queue =

        let

          val (result, job) = future_job grp interrupts e;

          val (task, queue') = Task_Queue.enqueue name grp deps pri job queue;

          val future = Future {promised = false, task = task, result = result};

        in (future, queue') end;

    in

      SYNCHRONIZED "enqueue" (fn () =>

        let

          val (futures, queue') = fold_map enqueue es (! queue);

          val _ = queue := queue';

          val minimal = forall (not o Task_Queue.known_task queue') deps;

          val _ = if minimal then signal work_available else ();

          val _ = scheduler_check ();

        in futures end)

    end;

fun fork e =

  (singleton o forks) {name = "fork", group = NONE, deps = [], pri = 0, interrupts = true} e;

(* join *)

fun get_result x =

  (case peek x of

    NONE => Exn.Exn (Fail "Unfinished future")

  | SOME res =>

      if Exn.is_interrupt_exn res then

        (case Task_Queue.group_status (Task_Queue.group_of_task (task_of x)) of

          [] => res

        | exns => Exn.Exn (Par_Exn.make exns))

      else res);

local

fun join_next deps = (*requires SYNCHRONIZED*)

  if null deps then NONE

  else

    (case Unsynchronized.change_result queue (Task_Queue.dequeue_deps (Thread.self ()) deps) of

      (NONE, []) => NONE

    | (NONE, deps') =>

        (worker_waiting deps' (fn () => worker_wait true work_finished); join_next deps')

    | (SOME work, deps') => SOME (work, deps'));

fun execute_work NONE = ()

  | execute_work (SOME (work, deps')) =

      (worker_joining (fn () => worker_exec work); join_work deps')

and join_work deps =

  Multithreading.with_attributes Multithreading.no_interrupts

    (fn _ => execute_work (SYNCHRONIZED "join" (fn () => join_next deps)));

in

fun join_results xs =

  let

    val _ =

      if forall is_finished xs then ()

      else if Multithreading.self_critical () then

        raise Fail "Cannot join future values within critical section"

      else if is_some (worker_task ()) then join_work (map task_of xs)

      else List.app (ignore o Single_Assignment.await o result_of) xs;

  in map get_result xs end;

end;

fun join_result x = singleton join_results x;

fun joins xs = Par_Exn.release_all (join_results xs);

fun join x = Exn.release (join_result x);

fun join_tasks tasks =

  if null tasks then ()

  else

    (singleton o forks)

      {name = "join_tasks", group = SOME (new_group NONE),

        deps = tasks, pri = 0, interrupts = false} I

    |> join;

(* fast-path operations -- bypass task queue if possible *)

fun value_result (res: 'a Exn.result) =

  let

    val task = Task_Queue.dummy_task;

    val group = Task_Queue.group_of_task task;

    val result = Single_Assignment.var "value" : 'a result;

    val _ = assign_result group result (identify_result (Position.thread_data ()) res);

  in Future {promised = false, task = task, result = result} end;

fun value x = value_result (Exn.Res x);

fun cond_forks args es =

  if Multithreading.enabled () then forks args es

  else map (fn e => value_result (Exn.interruptible_capture e ())) es;

fun map_future f x =

  if is_finished x then value_result (Exn.interruptible_capture (f o join) x)

  else

    let

      val task = task_of x;

      val group = Task_Queue.group_of_task task;

      val (result, job) = future_job group true (fn () => f (join x));

      val extended = SYNCHRONIZED "extend" (fn () =>

        (case Task_Queue.extend task job (! queue) of

          SOME queue' => (queue := queue'; true)

        | NONE => false));

    in

      if extended then Future {promised = false, task = task, result = result}

      else

        (singleton o cond_forks)

          {name = "map_future", group = SOME group, deps = [task],

            pri = Task_Queue.pri_of_task task, interrupts = true}

          (fn () => f (join x))

    end;

(* promised futures -- fulfilled by external means *)

fun promise_group group abort : 'a future =

  let

    val result = Single_Assignment.var "promise" : 'a result;

    fun assign () = assign_result group result Exn.interrupt_exn

      handle Fail _ => true

        | exn =>

            if Exn.is_interrupt exn

            then raise Fail "Concurrent attempt to fulfill promise"

            else reraise exn;

    fun job () =

      Multithreading.with_attributes Multithreading.no_interrupts

        (fn _ => Exn.release (Exn.capture assign () before abort ()));

    val task = SYNCHRONIZED "enqueue_passive" (fn () =>

      Unsynchronized.change_result queue (Task_Queue.enqueue_passive group job));

  in Future {promised = true, task = task, result = result} end;

fun promise abort = promise_group (worker_subgroup ()) abort;

fun fulfill_result (Future {promised, task, result}) res =

  if not promised then raise Fail "Not a promised future"

  else

    let

      val group = Task_Queue.group_of_task task;

      val pos = Position.thread_data ();

      fun job ok =

        assign_result group result (if ok then identify_result pos res else Exn.interrupt_exn);

      val _ =

        Multithreading.with_attributes Multithreading.no_interrupts (fn _ =>

          let

            val passive_job =

              SYNCHRONIZED "fulfill_result" (fn () =>

                Unsynchronized.change_result queue

                  (Task_Queue.dequeue_passive (Thread.self ()) task));

          in

            (case passive_job of

              SOME true => worker_exec (task, [job])

            | SOME false => ()

            | NONE => ignore (job (not (Task_Queue.is_canceled group))))

          end);

      val _ =

        if is_some (Single_Assignment.peek result) then ()

        else worker_waiting [task] (fn () => ignore (Single_Assignment.await result));

    in () end;

fun fulfill x res = fulfill_result x (Exn.Res res);

(* group snapshot *)

fun group_snapshot group =

  SYNCHRONIZED "group_snapshot" (fn () =>

    Task_Queue.group_tasks (! queue) group);

(* terminate *)

fun terminate group =

  SYNCHRONIZED "terminate" (fn () =>

    let val _ = cancel_group_unsynchronized group;

    in Task_Queue.group_tasks (! queue) group end)

  |> join_tasks;

(* shutdown *)

fun shutdown () =

  if not Multithreading.available then ()

  else if is_some (worker_task ()) then

    raise Fail "Cannot shutdown while running as worker thread"

  else

    SYNCHRONIZED "shutdown" (fn () =>

      while scheduler_active () do

       (Multithreading.tracing 1 (fn () => "SHUTDOWN: wait");

        wait scheduler_event));

(*final declarations of this structure!*)

val map = map_future;

end;

type 'a future = 'a Future.future;