more reactive scheduler: reduced loop timeout, propagate broadcast interrupt via TaskQueue.cancel_all;
(* Title: Pure/Concurrent/future.ML
Author: Makarius
Future values.
Notes:
* Futures are similar to delayed evaluation, i.e. delay/force is
generalized to fork/join (and variants). The idea is to model
parallel value-oriented computations, but *not* communicating
processes.
* Futures are grouped; failure of one group member causes the whole
group to be interrupted eventually.
* 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 is 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.
*)
signature FUTURE =
sig
val enabled: unit -> bool
type task = Task_Queue.task
type group = Task_Queue.group
val thread_data: unit -> (string * task) option
type 'a future
val task_of: 'a future -> task
val group_of: 'a future -> group
val peek: 'a future -> 'a Exn.result option
val is_finished: 'a future -> bool
val value: 'a -> 'a future
val fork: (unit -> 'a) -> 'a future
val fork_group: group -> (unit -> 'a) -> 'a future
val fork_deps: 'b future list -> (unit -> 'a) -> 'a future
val fork_pri: int -> (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 map: ('a -> 'b) -> 'a future -> 'b future
val interrupt_task: string -> unit
val cancel: 'a future -> unit
val shutdown: unit -> unit
end;
structure Future: FUTURE =
struct
(** future values **)
fun enabled () =
Multithreading.enabled () andalso
not (Multithreading.self_critical ());
(* identifiers *)
type task = Task_Queue.task;
type group = Task_Queue.group;
local val tag = Universal.tag () : (string * task) option Universal.tag in
fun thread_data () = the_default NONE (Thread.getLocal tag);
fun setmp_thread_data data f x = Library.setmp_thread_data tag (thread_data ()) (SOME data) f x;
end;
(* datatype future *)
datatype 'a future = Future of
{task: task,
group: group,
result: 'a Exn.result option ref};
fun task_of (Future {task, ...}) = task;
fun group_of (Future {group, ...}) = group;
fun peek (Future {result, ...}) = ! result;
fun is_finished x = is_some (peek x);
fun value x = Future
{task = Task_Queue.new_task 0,
group = Task_Queue.new_group (),
result = ref (SOME (Exn.Result x))};
(** scheduling **)
(* global state *)
val queue = ref Task_Queue.empty;
val next = ref 0;
val workers = ref ([]: (Thread.thread * bool) list);
val scheduler = ref (NONE: Thread.thread option);
val excessive = ref 0;
val canceled = ref ([]: Task_Queue.group list);
val do_shutdown = ref false;
(* synchronization *)
local
val lock = Mutex.mutex ();
val cond = ConditionVar.conditionVar ();
in
fun SYNCHRONIZED name = SimpleThread.synchronized name lock;
fun wait () = (*requires SYNCHRONIZED*)
ConditionVar.wait (cond, lock);
fun wait_timeout timeout = (*requires SYNCHRONIZED*)
ignore (ConditionVar.waitUntil (cond, lock, Time.+ (Time.now (), timeout)));
fun notify_all () = (*requires SYNCHRONIZED*)
ConditionVar.broadcast cond;
end;
(* worker activity *)
fun trace_active () =
let
val ws = ! workers;
val m = string_of_int (length ws);
val n = string_of_int (length (filter #2 ws));
in Multithreading.tracing 1 (fn () => "SCHEDULE: " ^ m ^ " workers, " ^ n ^ " active") end;
fun change_active active = (*requires SYNCHRONIZED*)
change workers (AList.update Thread.equal (Thread.self (), active));
(* execute *)
fun do_cancel group = (*requires SYNCHRONIZED*)
change canceled (insert Task_Queue.eq_group group);
fun execute name (task, group, run) =
let
val _ = trace_active ();
val ok = setmp_thread_data (name, task) run ();
val _ = SYNCHRONIZED "execute" (fn () =>
(change queue (Task_Queue.finish task);
if ok then ()
else if Task_Queue.cancel (! queue) group then ()
else do_cancel group;
notify_all ()));
in () end;
(* worker threads *)
fun worker_wait () = (*requires SYNCHRONIZED*)
(change_active false; wait (); change_active true);
fun worker_next () = (*requires SYNCHRONIZED*)
if ! excessive > 0 then
(dec excessive;
change workers (filter_out (fn (thread, _) => Thread.equal (thread, Thread.self ())));
notify_all ();
NONE)
else
(case change_result queue Task_Queue.dequeue of
NONE => (worker_wait (); worker_next ())
| some => some);
fun worker_loop name =
(case SYNCHRONIZED name worker_next of
NONE => ()
| SOME work => (execute name work; worker_loop name));
fun worker_start name = (*requires SYNCHRONIZED*)
change workers (cons (SimpleThread.fork false (fn () => worker_loop name), true));
(* scheduler *)
fun scheduler_next () = (*requires SYNCHRONIZED*)
let
(*worker threads*)
val _ =
(case List.partition (Thread.isActive o #1) (! workers) of
(_, []) => ()
| (active, inactive) =>
(workers := active; Multithreading.tracing 0 (fn () =>
"SCHEDULE: disposed " ^ string_of_int (length inactive) ^ " dead worker threads")));
val _ = trace_active ();
val m = if ! do_shutdown then 0 else Multithreading.max_threads_value ();
val l = length (! workers);
val _ = excessive := l - m;
val _ =
if m > l then funpow (m - l) (fn () => worker_start ("worker " ^ string_of_int (inc next))) ()
else ();
(*canceled groups*)
val _ = change canceled (filter_out (Task_Queue.cancel (! queue)));
(*shutdown*)
val continue = not (! do_shutdown andalso null (! workers));
val _ = if continue then () else scheduler := NONE;
val _ = notify_all ();
val _ = interruptible (fn () => wait_timeout (Time.fromSeconds 1)) ()
handle Exn.Interrupt => List.app do_cancel (Task_Queue.cancel_all (! queue));
in continue end;
fun scheduler_loop () =
while SYNCHRONIZED "scheduler" scheduler_next do ();
fun scheduler_active () = (*requires SYNCHRONIZED*)
(case ! scheduler of NONE => false | SOME thread => Thread.isActive thread);
fun scheduler_check name = SYNCHRONIZED name (fn () =>
if not (scheduler_active ()) then
(do_shutdown := false; scheduler := SOME (SimpleThread.fork false scheduler_loop))
else if ! do_shutdown then error "Scheduler shutdown in progress"
else ());
(* future values: fork independent computation *)
fun future opt_group deps pri (e: unit -> 'a) =
let
val _ = scheduler_check "future check";
val group = (case opt_group of SOME group => group | NONE => Task_Queue.new_group ());
val result = ref (NONE: 'a Exn.result option);
val run = Multithreading.with_attributes (Thread.getAttributes ())
(fn _ => fn ok =>
let
val res = if ok then Exn.capture e () else Exn.Exn Exn.Interrupt;
val _ = result := SOME res;
val res_ok =
(case res of
Exn.Result _ => true
| Exn.Exn Exn.Interrupt => (Task_Queue.invalidate_group group; true)
| _ => false);
in res_ok end);
val task = SYNCHRONIZED "future" (fn () =>
change_result queue (Task_Queue.enqueue group deps pri run) before notify_all ());
in Future {task = task, group = group, result = result} end;
fun fork e = future NONE [] 0 e;
fun fork_group group e = future (SOME group) [] 0 e;
fun fork_deps deps e = future NONE (map task_of deps) 0 e;
fun fork_pri pri e = future NONE [] pri e;
(* join: retrieve results *)
fun join_results [] = []
| join_results xs = uninterruptible (fn _ => fn () =>
let
val _ = scheduler_check "join check";
val _ = Multithreading.self_critical () andalso
exists (not o is_finished) xs andalso
error "Cannot join future values within critical section";
fun join_loop _ [] = ()
| join_loop name tasks =
(case SYNCHRONIZED name (fn () =>
change_result queue (Task_Queue.dequeue_towards tasks)) of
NONE => ()
| SOME (work, tasks') => (execute name work; join_loop name tasks'));
val _ =
(case thread_data () of
NONE =>
(*alien thread -- refrain from contending for resources*)
while exists (not o is_finished) xs
do SYNCHRONIZED "join_thread" (fn () => wait ())
| SOME (name, task) =>
(*proper task -- actively work towards results*)
let
val unfinished = xs |> map_filter
(fn Future {task, result = ref NONE, ...} => SOME task | _ => NONE);
val _ = SYNCHRONIZED "join" (fn () =>
(change queue (Task_Queue.depend unfinished task); notify_all ()));
val _ = join_loop ("join_loop: " ^ name) unfinished;
val _ =
while exists (not o is_finished) xs
do SYNCHRONIZED "join_task" (fn () => worker_wait ());
in () end);
in xs |> map (fn Future {result = ref (SOME res), ...} => res) end) ();
fun join_result x = singleton join_results x;
fun join x = Exn.release (join_result x);
fun map f x =
let val task = task_of x
in future NONE [task] (Task_Queue.pri_of_task task) (fn () => f (join x)) end;
(* misc operations *)
(*interrupt: permissive signal, may get ignored*)
fun interrupt_task id = SYNCHRONIZED "interrupt"
(fn () => Task_Queue.interrupt_external (! queue) id);
(*cancel: present and future group members will be interrupted eventually*)
fun cancel x =
(scheduler_check "cancel check";
SYNCHRONIZED "cancel" (fn () => (do_cancel (group_of x); notify_all ())));
(*global join and shutdown*)
fun shutdown () =
if Multithreading.available then
(scheduler_check "shutdown check";
SYNCHRONIZED "shutdown" (fn () =>
(while not (scheduler_active ()) do wait ();
while not (Task_Queue.is_empty (! queue)) do wait ();
do_shutdown := true;
notify_all ();
while not (null (! workers)) do wait ();
while scheduler_active () do wait ();
OS.Process.sleep (Time.fromMilliseconds 300))))
else ();
end;
type 'a future = 'a Future.future;