src/Pure/Concurrent/future.ML

allow worker guest threads, which participate actively in future joins, but are outside thread accounting;

(*  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 worker_subgroup: unit -> group
  val worker_guest: 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 forked_proofs: int 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 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 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 worker_subgroup () = new_group (worker_group ());

fun worker_guest name group f x =
  if is_some (worker_task ()) then
    raise Fail "Already running as worker thread"
  else 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;
val forked_proofs = Unsynchronized.ref 0;

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_proof", Markup.print_int (! forked_proofs)),
        ("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
          NONE => res
        | SOME exn => Exn.Exn exn)
      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
        error "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);


(* 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);


(* terminate *)

fun terminate group =
  let
    val tasks =
      SYNCHRONIZED "terminate" (fn () =>
        let val _ = cancel_group_unsynchronized group;
        in Task_Queue.group_tasks (! queue) group end);
  in
    if null tasks then ()
    else
      (singleton o forks)
        {name = "terminate", group = SOME (new_group NONE),
          deps = tasks, pri = 0, interrupts = false} I
      |> join
  end;


(* 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;