(*  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 the immediate peers to be

     interrupted eventually (i.e. none by default).  Interrupted tasks

     that lack regular result information, will pick up parallel

     exceptions from the cumulative group context (as Par_Exn).

   * 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

   val worker_task: unit -> Task_Queue.task option

   val worker_group: unit -> Task_Queue.group option

   val worker_subgroup: unit -> Task_Queue.group

   type 'a future

   val task_of: 'a future -> Task_Queue.task

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

   val is_finished: 'a future -> bool

   val get_finished: 'a future -> 'a

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

   val cancel_group: Task_Queue.group -> unit

   val cancel: 'a future -> unit

   type fork_params =

    {name: string, group: Task_Queue.group option, deps: Task_Queue.task list,

     pri: int, interrupts: bool}

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

   val fork_pri: int -> (unit -> 'a) -> 'a future

   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 join: 'a future -> 'a

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

   val value: 'a -> 'a future

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

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

   val promise_group: Task_Queue.group -> 'a future

   val promise: unit -> 'a future

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

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

   val shutdown: unit -> unit

   val status: (unit -> 'a) -> 'a

 end;

 structure Future: FUTURE =

 struct

 (** future values **)

 (* identifiers *)

 local

   val tag = Universal.tag () : Task_Queue.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 () = Task_Queue.new_group (worker_group ());

 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_Queue.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);

 fun get_finished x =

   (case peek x of

     SOME res => Exn.release res

   | NONE => raise Fail "Unfinished future evaluation");

 (** 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;

 fun broadcast_work () = (*requires SYNCHRONIZED*)

  (ConditionVar.broadcast work_available;

   ConditionVar.broadcast work_finished);

 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 ([]: Task_Queue.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;

 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;

 (* cancellation primitives *)

 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 ();

 fun cancel_now group = (*requires SYNCHRONIZED*)

   Task_Queue.cancel (! queue) group;

 fun cancel_later group = (*requires SYNCHRONIZED*)

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

   broadcast scheduler_event);

 (* 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 _ = Multithreading.tracing 2 (fn () =>

       let

         val s = Task_Queue.str_of_task_groups task;

         fun micros time = string_of_int (Time.toNanoseconds time div 1000);

         val (run, wait, deps) = Task_Queue.timing_of_task task;

       in "TASK " ^ s ^ " " ^ micros run ^ " " ^ micros wait ^ " (" ^ commas deps ^ ")" end);

     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 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*)

   let

     val state =

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

         SOME state => state

       | NONE => raise Fail "Unregistered worker thread");

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

     val _ = wait cond;

     val _ = state := Working;

   in () end;

 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 *)

 val status_ticks = Unsynchronized.ref 0;

 val last_round = Unsynchronized.ref Time.zeroTime;

 val next_round = seconds 0.05;

 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 ();

     (* queue and worker status *)

     val _ =

       if tick then Unsynchronized.change status_ticks (fn i => (i + 1) mod 10) else ();

     val _ =

       if tick andalso ! status_ticks = 0 then

         Multithreading.tracing 1 (fn () =>

           let

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

             val total = length (! workers);

             val active = count_workers Working;

             val waiting = count_workers Waiting;

           in

             "SCHEDULE " ^ Time.toString now ^ ": " ^

               string_of_int ready ^ " ready, " ^

               string_of_int pending ^ " pending, " ^

               string_of_int running ^ " running, " ^

               string_of_int passive ^ " passive; " ^

               string_of_int total ^ " workers, " ^

               string_of_int active ^ " active, " ^

               string_of_int waiting ^ " waiting "

           end)

       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 () =>

             "SCHEDULE: 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 if m = 9999 then 1

       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 cancel_now);

         broadcast_work ());

     (* 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 scheduler := NONE;

     val _ = broadcast scheduler_event;

   in continue end

   handle exn =>

     if Exn.is_interrupt exn then

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

       List.app cancel_later (Task_Queue.cancel_all (! queue));

       broadcast_work (); 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 **)

 (* cancellation *)

 (*cancel: present and future group members will be interrupted eventually*)

 fun cancel_group group = SYNCHRONIZED "cancel" (fn () =>

  (if cancel_now group then () else cancel_later group;

   signal work_available; scheduler_check ()));

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

 (* future jobs *)

 fun assign_result group result raw_res =

   let

     val res =

       (case raw_res of

         Exn.Exn exn => Exn.Exn (#2 (Par_Exn.serial exn))

       | _ => raw_res);

     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;

 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 res end;

   in (result, job) end;

 (* fork *)

 type fork_params =

  {name: string, group: Task_Queue.group option, deps: Task_Queue.task list,

   pri: int, interrupts: bool};

 fun forks ({name, group, deps, pri, interrupts}: fork_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_pri pri e =

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

 fun fork e = fork_pri 0 e;

 (* join *)

 local

 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);

 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 join x = Exn.release (join_result x);

 (* fast-path versions -- bypassing task queue *)

 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 res;

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

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

 fun map_future f x =

   let

     val task = task_of x;

     val group = Task_Queue.new_group (SOME (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

         (forks {name = "Future.map", group = SOME group, deps = [task],

           pri = Task_Queue.pri_of_task task, interrupts = true})

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

   end;

 fun cond_forks args es =

   if Multithreading.enabled () then forks args es

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

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

 fun promise_group group : 'a future =

   let

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

     fun abort () = 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;

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

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

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

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

 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;

       fun job ok = assign_result group result (if ok then res else Exn.interrupt_exn);

       val _ =

         Multithreading.with_attributes Multithreading.no_interrupts (fn _ =>

           let

             val still_passive =

               SYNCHRONIZED "fulfill_result" (fn () =>

                 Unsynchronized.change_result queue

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

           in if still_passive then worker_exec (task, [job]) else () 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);

 (* shutdown *)

 fun shutdown () =

   if Multithreading.available then

     SYNCHRONIZED "shutdown" (fn () =>

      while scheduler_active () do

       (wait scheduler_event; broadcast_work ()))

   else ();

 (* status markup *)

 fun status e =

   let

     val task_props =

       (case worker_task () of

         NONE => I

       | SOME task => Markup.properties [(Markup.taskN, Task_Queue.str_of_task task)]);

     val _ = Output.status (Markup.markup_only (task_props Markup.forked));

     val x = e ();  (*sic -- report "joined" only for success*)

     val _ = Output.status (Markup.markup_only (task_props Markup.joined));

   in x end;

 (*final declarations of this structure!*)

 val map = map_future;

 end;

 type 'a future = 'a Future.future;