breaking: change interface for number of threads

now the user can specify `num_threads`; if not provided a sensible
default is picked.

src/fifo_pool.ml

@@ -56,13 +56,12 @@ type ('a, 'b) create_args =
   ?on_exit_thread:(dom_id:int -> t_id:int -> unit -> unit) ->
   ?on_exn:(exn -> Printexc.raw_backtrace -> unit) ->
   ?around_task:(t -> 'b) * (t -> 'b -> unit) ->
-  ?min:int ->
-  ?per_domain:int ->
+  ?num_threads:int ->
   'a
 
 let create ?(on_init_thread = default_thread_init_exit_)
     ?(on_exit_thread = default_thread_init_exit_) ?(on_exn = fun _ _ -> ())
-    ?around_task ?min:(min_threads = 1) ?(per_domain = 0) () : t =
+    ?around_task ?num_threads () : t =
   (* wrapper *)
   let around_task =
     match around_task with
@@ -70,11 +69,10 @@ let create ?(on_init_thread = default_thread_init_exit_)
     | None -> AT_pair (ignore, fun _ _ -> ())
   in
 
-  (* number of threads to run *)
-  let min_threads = max 1 min_threads in
   let num_domains = D_pool_.n_domains () in
-  assert (num_domains >= 1);
-  let num_threads = max min_threads (num_domains * per_domain) in
+
+  (* number of threads to run *)
+  let num_threads = Util_pool_.num_threads ?num_threads () in
 
   (* make sure we don't bias towards the first domain(s) in {!D_pool_} *)
   let offset = Random.int num_domains in
@@ -141,11 +139,10 @@ let create ?(on_init_thread = default_thread_init_exit_)
 
   runner
 
-let with_ ?on_init_thread ?on_exit_thread ?on_exn ?around_task ?min ?per_domain
-    () f =
+let with_ ?on_init_thread ?on_exit_thread ?on_exn ?around_task ?num_threads () f
+    =
   let pool =
-    create ?on_init_thread ?on_exit_thread ?on_exn ?around_task ?min ?per_domain
-      ()
+    create ?on_init_thread ?on_exit_thread ?on_exn ?around_task ?num_threads ()
   in
   let@ () = Fun.protect ~finally:(fun () -> shutdown pool) in
   f pool

src/fifo_pool.mli

@@ -21,8 +21,7 @@ type ('a, 'b) create_args =
   ?on_exit_thread:(dom_id:int -> t_id:int -> unit -> unit) ->
   ?on_exn:(exn -> Printexc.raw_backtrace -> unit) ->
   ?around_task:(t -> 'b) * (t -> 'b -> unit) ->
-  ?min:int ->
-  ?per_domain:int ->
+  ?num_threads:int ->
   'a
 (** Arguments used in {!create}. See {!create} for explanations. *)
 
@@ -30,8 +29,9 @@ val create : (unit -> t, _) create_args
 (** [create ()] makes a new thread pool.
      @param on_init_thread called at the beginning of each new thread in the pool.
      @param min minimum size of the pool. See {!Pool.create_args}.
-     @param per_domain is the number of threads allocated per domain in the fixed
-       domain pool. See {!Pool.create_args}.
+      The default is [Domain.recommended_domain_count()], ie one worker per
+      CPU core.
+      On OCaml 4 the default is [4] (since there is only one domain).
      @param on_exit_thread called at the end of each worker thread in the pool.
      @param around_task a pair of [before, after] functions
      ran around each task. See {!Pool.create_args}.

src/util_pool_.ml

@@ -0,0 +1,11 @@
+let num_threads ?num_threads () : int =
+  let n_domains = D_pool_.n_domains () in
+
+  (* number of threads to run *)
+  let num_threads =
+    match num_threads with
+    | Some j -> max 1 j
+    | None -> n_domains
+  in
+
+  num_threads

src/util_pool_.mli

@@ -0,0 +1,5 @@
+(** Utils for pools *)
+
+val num_threads : ?num_threads:int -> unit -> int
+(** Number of threads a pool should have.
+    @param num_threads user-specified number of threads *)

src/ws_pool.ml

@@ -198,14 +198,13 @@ type ('a, 'b) create_args =
   ?on_exit_thread:(dom_id:int -> t_id:int -> unit -> unit) ->
   ?on_exn:(exn -> Printexc.raw_backtrace -> unit) ->
   ?around_task:(t -> 'b) * (t -> 'b -> unit) ->
-  ?min:int ->
-  ?per_domain:int ->
+  ?num_threads:int ->
   'a
 (** Arguments used in {!create}. See {!create} for explanations. *)
 
 let create ?(on_init_thread = default_thread_init_exit_)
     ?(on_exit_thread = default_thread_init_exit_) ?(on_exn = fun _ _ -> ())
-    ?around_task ?min:(min_threads = 1) ?(per_domain = 0) () : t =
+    ?around_task ?num_threads () : t =
   (* wrapper *)
   let around_task =
     match around_task with
@@ -213,11 +212,8 @@ let create ?(on_init_thread = default_thread_init_exit_)
     | None -> AT_pair (ignore, fun _ _ -> ())
   in
 
-  (* number of threads to run *)
-  let min_threads = max 1 min_threads in
   let num_domains = D_pool_.n_domains () in
-  assert (num_domains >= 1);
-  let num_threads = max min_threads (num_domains * per_domain) in
+  let num_threads = Util_pool_.num_threads ?num_threads () in
 
   (* make sure we don't bias towards the first domain(s) in {!D_pool_} *)
   let offset = Random.int num_domains in
@@ -301,11 +297,10 @@ let create ?(on_init_thread = default_thread_init_exit_)
 
   runner
 
-let with_ ?on_init_thread ?on_exit_thread ?on_exn ?around_task ?min ?per_domain
-    () f =
+let with_ ?on_init_thread ?on_exit_thread ?on_exn ?around_task ?num_threads () f
+    =
   let pool =
-    create ?on_init_thread ?on_exit_thread ?on_exn ?around_task ?min ?per_domain
-      ()
+    create ?on_init_thread ?on_exit_thread ?on_exn ?around_task ?num_threads ()
   in
   let@ () = Fun.protect ~finally:(fun () -> shutdown pool) in
   f pool

src/ws_pool.mli

@@ -26,8 +26,7 @@ type ('a, 'b) create_args =
   ?on_exit_thread:(dom_id:int -> t_id:int -> unit -> unit) ->
   ?on_exn:(exn -> Printexc.raw_backtrace -> unit) ->
   ?around_task:(t -> 'b) * (t -> 'b -> unit) ->
-  ?min:int ->
-  ?per_domain:int ->
+  ?num_threads:int ->
   'a
 (** Arguments used in {!create}. See {!create} for explanations. *)
 
@@ -35,14 +34,11 @@ val create : (unit -> t, _) create_args
 (** [create ()] makes a new thread pool.
      @param on_init_thread called at the beginning of each new thread
        in the pool.
-     @param min minimum size of the pool. It will be at least [1] internally,
-      so [0] or negative values make no sense.
-     @param per_domain is the number of threads allocated per domain in the fixed
-       domain pool. The default value is [0], but setting, say, [~per_domain:2]
-       means that if there are [8] domains (which might be the case on an 8-core machine)
-       then the minimum size of the pool is [16].
-       If both [min] and [per_domain] are specified, the maximum of both
-       [min] and [per_domain * num_of_domains] is used.
+     @param num_threads size of the pool, ie. number of worker threads.
+      It will be at least [1] internally, so [0] or negative values make no sense.
+      The default is [Domain.recommended_domain_count()], ie one worker
+      thread per CPU core.
+      On OCaml 4 the default is [4] (since there is only one domain).
      @param on_exit_thread called at the end of each thread in the pool
      @param around_task a pair of [before, after], where [before pool] is called
       before a task is processed,