src/Pure/Concurrent/future.ML
author wenzelm
Thu Oct 09 21:34:11 2008 +0200 (2008-10-09 ago)
changeset 28558 2a6297b4273c
parent 28548 003f52c2bb8f
child 28575 ed869f019642
permissions -rw-r--r--
replaced str_of by general peek;
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(*  Title:      Pure/Concurrent/future.ML
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    ID:         $Id$
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    Author:     Makarius
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Future values.
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Notes:
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  * Futures are similar to delayed evaluation, i.e. delay/force is
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    generalized to fork/join (and variants).  The idea is to model
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    parallel value-oriented computations, but *not* communicating
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    processes.
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  * Futures are grouped; failure of one group member causes the whole
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    group to be interrupted eventually.
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  * Forked futures are evaluated spontaneously by a farm of worker
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    threads in the background; join resynchronizes the computation and
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    delivers results (values or exceptions).
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  * The pool of worker threads is limited, usually in correlation with
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    the number of physical cores on the machine.  Note that allocation
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    of runtime resources is distorted either if workers yield CPU time
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    (e.g. via system sleep or wait operations), or if non-worker
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    threads contend for significant runtime resources independently.
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*)
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signature FUTURE =
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sig
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  type task = TaskQueue.task
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  type group = TaskQueue.group
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  val thread_data: unit -> (string * task * group) option
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  type 'a T
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  val task_of: 'a T -> task
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  val group_of: 'a T -> group
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  val peek: 'a T -> 'a Exn.result option
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  val is_finished: 'a T -> bool
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  val future: group option -> task list -> bool -> (unit -> 'a) -> 'a T
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  val fork: (unit -> 'a) -> 'a T
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  val fork_background: (unit -> 'a) -> 'a T
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  val join_results: 'a T list -> 'a Exn.result list
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  val join: 'a T -> 'a
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  val focus: task list -> unit
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  val interrupt_task: string -> unit
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  val cancel: 'a T -> unit
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  val shutdown: unit -> unit
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end;
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structure Future: FUTURE =
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struct
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(** future values **)
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(* identifiers *)
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type task = TaskQueue.task;
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type group = TaskQueue.group;
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local val tag = Universal.tag () : (string * task * group) option Universal.tag in
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  fun thread_data () = the_default NONE (Thread.getLocal tag);
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  fun setmp_thread_data data f x = Library.setmp_thread_data tag (thread_data ()) (SOME data) f x;
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end;
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(* datatype future *)
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datatype 'a T = Future of
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 {task: task,
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  group: group,
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  result: 'a Exn.result option ref};
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fun task_of (Future {task, ...}) = task;
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fun group_of (Future {group, ...}) = group;
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fun peek (Future {result, ...}) = ! result;
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fun is_finished x = is_some (peek x);
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(** scheduling **)
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(* global state *)
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val queue = ref TaskQueue.empty;
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val next = ref 0;
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val workers = ref ([]: (Thread.thread * bool) list);
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val scheduler = ref (NONE: Thread.thread option);
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val excessive = ref 0;
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val canceled = ref ([]: TaskQueue.group list);
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val do_shutdown = ref false;
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(* synchronization *)
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local
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  val lock = Mutex.mutex ();
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  val cond = ConditionVar.conditionVar ();
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in
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fun SYNCHRONIZED name e = Exn.release (uninterruptible (fn restore_attributes => fn () =>
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  let
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    val _ =
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      if Mutex.trylock lock then ()
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      else
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       (Multithreading.tracing 3 (fn () => name ^ ": locking ...");
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        Mutex.lock lock;
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        Multithreading.tracing 3 (fn () => name ^ ": ... locked"));
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    val result = Exn.capture (restore_attributes e) ();
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    val _ = Mutex.unlock lock;
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  in result end) ());
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fun wait name = (*requires SYNCHRONIZED*)
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 (Multithreading.tracing 3 (fn () => name ^ ": wait ...");
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  ConditionVar.wait (cond, lock);
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  Multithreading.tracing 3 (fn () => name ^ ": ... continue"));
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fun wait_timeout name timeout = (*requires SYNCHRONIZED*)
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 (Multithreading.tracing 3 (fn () => name ^ ": wait ...");
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  ConditionVar.waitUntil (cond, lock, Time.+ (Time.now (), timeout));
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  Multithreading.tracing 3 (fn () => name ^ ": ... continue"));
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fun notify_all () = (*requires SYNCHRONIZED*)
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  ConditionVar.broadcast cond;
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end;
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(* worker activity *)
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fun trace_active () =
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  let
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    val ws = ! workers;
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    val m = string_of_int (length ws);
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    val n = string_of_int (length (filter #2 ws));
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  in Multithreading.tracing 1 (fn () => "SCHEDULE: " ^ m ^ " workers, " ^ n ^ " active") end;
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fun change_active active = (*requires SYNCHRONIZED*)
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  change workers (AList.update Thread.equal (Thread.self (), active));
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(* execute *)
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fun execute name (task, group, run) =
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  let
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    val _ = trace_active ();
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    val ok = setmp_thread_data (name, task, group) run ();
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    val _ = SYNCHRONIZED "execute" (fn () =>
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     (change queue (TaskQueue.finish task);
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      if ok then ()
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      else if TaskQueue.cancel (! queue) group then ()
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      else change canceled (cons group);
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      notify_all ()));
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  in () end;
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(* worker threads *)
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fun worker_wait name = (*requires SYNCHRONIZED*)
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  (change_active false; wait name; change_active true);
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fun worker_next name = (*requires SYNCHRONIZED*)
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  if ! excessive > 0 then
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    (dec excessive;
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     change workers (filter_out (fn (thread, _) => Thread.equal (thread, Thread.self ())));
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     notify_all ();
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     NONE)
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  else
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    (case change_result queue TaskQueue.dequeue of
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      NONE => (worker_wait name; worker_next name)
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    | some => some);
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fun worker_loop name =
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  (case SYNCHRONIZED name (fn () => worker_next name) of
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    NONE => Multithreading.tracing 3 (fn () => name ^ ": exit")
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  | SOME work => (execute name work; worker_loop name));
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fun worker_start name = (*requires SYNCHRONIZED*)
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  change workers (cons (SimpleThread.fork false (fn () => worker_loop name), true));
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(* scheduler *)
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fun scheduler_next () = (*requires SYNCHRONIZED*)
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  let
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    (*worker threads*)
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    val _ =
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      (case List.partition (Thread.isActive o #1) (! workers) of
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        (_, []) => ()
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      | (active, inactive) =>
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          (workers := active; Multithreading.tracing 0 (fn () =>
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            "SCHEDULE: disposed " ^ string_of_int (length inactive) ^ " dead worker threads")));
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    val _ = trace_active ();
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    val m = if ! do_shutdown then 0 else Multithreading.max_threads_value ();
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    val l = length (! workers);
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    val _ = excessive := l - m;
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    val _ =
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      if m > l then funpow (m - l) (fn () => worker_start ("worker " ^ string_of_int (inc next))) ()
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      else ();
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    (*canceled groups*)
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    val _ =  change canceled (filter_out (TaskQueue.cancel (! queue)));
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    (*shutdown*)
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    val continue = not (! do_shutdown andalso null (! workers));
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    val _ = if continue then () else scheduler := NONE;
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    val _ = notify_all ();
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    val _ = wait_timeout "scheduler" (Time.fromSeconds 3);
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  in continue end;
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fun scheduler_loop () =
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 (while SYNCHRONIZED "scheduler" scheduler_next do ();
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  Multithreading.tracing 3 (fn () => "scheduler: exit"));
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fun scheduler_active () = (*requires SYNCHRONIZED*)
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  (case ! scheduler of NONE => false | SOME thread => Thread.isActive thread);
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fun scheduler_check name = SYNCHRONIZED name (fn () =>
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  if not (scheduler_active ()) then
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    (Multithreading.tracing 3 (fn () => "scheduler: fork");
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     do_shutdown := false; scheduler := SOME (SimpleThread.fork false scheduler_loop))
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  else if ! do_shutdown then error "Scheduler shutdown in progress"
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  else ());
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(* future values: fork independent computation *)
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fun future opt_group deps pri (e: unit -> 'a) =
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  let
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    val _ = scheduler_check "future check";
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    val group = (case opt_group of SOME group => group | NONE => TaskQueue.new_group ());
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    val result = ref (NONE: 'a Exn.result option);
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    val run = Multithreading.with_attributes (Thread.getAttributes ())
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      (fn _ => fn ok =>
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        let
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          val res = if ok then Exn.capture e () else Exn.Exn Exn.Interrupt;
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          val _ = result := SOME res;
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          val res_ok =
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            (case res of
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              Exn.Result _ => true
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            | Exn.Exn Exn.Interrupt => (TaskQueue.invalidate_group group; true)
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            | _ => false);
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        in res_ok end);
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    val task = SYNCHRONIZED "future" (fn () =>
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      change_result queue (TaskQueue.enqueue group deps pri run) before notify_all ());
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  in Future {task = task, group = group, result = result} end;
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fun fork_common pri = future (Option.map #3 (thread_data ())) [] pri;
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fun fork e = fork_common true e;
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fun fork_background e = fork_common false e;
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(* join: retrieve results *)
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fun join_results [] = []
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  | join_results xs = uninterruptible (fn _ => fn () =>
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      let
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        val _ = scheduler_check "join check";
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        val _ = Multithreading.self_critical () andalso
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          error "Cannot join future values within critical section";
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        fun join_loop _ [] = ()
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          | join_loop name tasks =
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              (case SYNCHRONIZED name (fn () =>
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                  change_result queue (TaskQueue.dequeue_towards tasks)) of
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                NONE => ()
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              | SOME (work, tasks') => (execute name work; join_loop name tasks'));
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        val _ =
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          (case thread_data () of
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            NONE =>
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              (*alien thread -- refrain from contending for resources*)
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              while exists (not o is_finished) xs
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              do SYNCHRONIZED "join_thread" (fn () => wait "join_thread")
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          | SOME (name, task, _) =>
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              (*proper task -- actively work towards results*)
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              let
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                val unfinished = xs |> map_filter
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                  (fn Future {task, result = ref NONE, ...} => SOME task | _ => NONE);
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                val _ = SYNCHRONIZED "join" (fn () =>
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                  (change queue (TaskQueue.depend unfinished task); notify_all ()));
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                val _ = join_loop ("join_loop: " ^ name) unfinished;
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                val _ =
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                  while exists (not o is_finished) xs
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                  do SYNCHRONIZED "join_task" (fn () => worker_wait "join_task");
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              in () end);
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      in xs |> map (fn Future {result = ref (SOME res), ...} => res) end) ();
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fun join x = Exn.release (singleton join_results x);
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(* misc operations *)
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(*focus: collection of high-priority task*)
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fun focus tasks = SYNCHRONIZED "focus" (fn () =>
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  change queue (TaskQueue.focus tasks));
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(*interrupt: permissive signal, may get ignored*)
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fun interrupt_task id = SYNCHRONIZED "interrupt"
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  (fn () => TaskQueue.interrupt_external (! queue) id);
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(*cancel: present and future group members will be interrupted eventually*)
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fun cancel x =
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 (scheduler_check "cancel check";
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  SYNCHRONIZED "cancel" (fn () => (change canceled (cons (group_of x)); notify_all ())));
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(*global join and shutdown*)
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fun shutdown () =
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  if Multithreading.available then
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   (scheduler_check "shutdown check";
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    SYNCHRONIZED "shutdown" (fn () =>
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     (while not (scheduler_active ()) do wait "shutdown: scheduler inactive";
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      while not (TaskQueue.is_empty (! queue)) do wait "shutdown: join";
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      do_shutdown := true;
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      notify_all ();
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      while not (null (! workers)) do wait "shutdown: workers";
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      while scheduler_active () do wait "shutdown: scheduler still active";
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      OS.Process.sleep (Time.fromMilliseconds 300))))
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  else ();
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end;