diff --git a/README.md b/README.md
index ab451e08..b06a1975 100644
--- a/README.md
+++ b/README.md
@@ -33,7 +33,7 @@ the workers of `pool`, as soon as one is available. No result is returned by `ru
 
 ```ocaml
 # #require "threads";;
-# let pool = Moonpool.Fifo_pool.create ~min:4 ();;
+# let pool = Moonpool.Fifo_pool.create ~num_threads:4 ();;
 val pool : Moonpool.Runner.t = <abstr>
 
 # begin
diff --git a/benchs/fib_rec.ml b/benchs/fib_rec.ml
index d3df44df..571b8495 100644
--- a/benchs/fib_rec.ml
+++ b/benchs/fib_rec.ml
@@ -57,10 +57,14 @@ let () = assert (List.init 10 fib_direct = [ 1; 1; 2; 3; 5; 8; 13; 21; 34; 55 ])
 
 let create_pool ~psize ~kind () =
   match kind with
-  | "fifo" -> Fifo_pool.create ~min:psize ()
-  | "pool" -> Ws_pool.create ~min:psize ()
+  | "fifo" -> Fifo_pool.create ?num_threads:psize ()
+  | "pool" -> Ws_pool.create ?num_threads:psize ()
   | _ -> assert false
 
+let str_of_int_opt = function
+  | None -> "None"
+  | Some i -> Printf.sprintf "Some %d" i
+
 let run ~psize ~n ~seq ~dl ~fj ~await ~niter ~kind () : unit =
   let pool = lazy (create_pool ~kind ~psize ()) in
   let dl_pool =
@@ -80,14 +84,16 @@ let run ~psize ~n ~seq ~dl ~fj ~await ~niter ~kind () : unit =
         Domainslib.Task.run pool (fun () ->
             Domainslib.Task.await pool @@ fib_dl ~pool n)
       ) else if fj then (
-        Printf.printf "compute fib %d using fork-join with pool size=%d\n%!" n
-          psize;
+        Printf.printf "compute fib %d using fork-join with pool size=%s\n%!" n
+          (str_of_int_opt psize);
         fib_fj ~on:(Lazy.force pool) n |> Fut.wait_block_exn
       ) else if await then (
-        Printf.printf "compute fib %d using await with pool size=%d\n%!" n psize;
+        Printf.printf "compute fib %d using await with pool size=%s\n%!" n
+          (str_of_int_opt psize);
         fib_await ~on:(Lazy.force pool) n |> Fut.wait_block_exn
       ) else (
-        Printf.printf "compute fib %d with pool size=%d\n%!" n psize;
+        Printf.printf "compute fib %d with pool size=%s\n%!" n
+          (str_of_int_opt psize);
         fib ~on:(Lazy.force pool) n |> Fut.wait_block_exn
       )
     in
@@ -103,7 +109,7 @@ let run ~psize ~n ~seq ~dl ~fj ~await ~niter ~kind () : unit =
 
 let () =
   let n = ref 40 in
-  let psize = ref 16 in
+  let psize = ref None in
   let seq = ref false in
   let niter = ref 3 in
   let kind = ref "pool" in
@@ -112,7 +118,7 @@ let () =
   let fj = ref false in
   let opts =
     [
-      "-psize", Arg.Set_int psize, " pool size";
+      "-psize", Arg.Int (fun i -> psize := Some i), " pool size";
       "-n", Arg.Set_int n, " fib <n>";
       "-seq", Arg.Set seq, " sequential";
       "-dl", Arg.Set dl, " domainslib";
diff --git a/benchs/pi.ml b/benchs/pi.ml
index 65304a80..c8ef57b5 100644
--- a/benchs/pi.ml
+++ b/benchs/pi.ml
@@ -21,14 +21,14 @@ let with_pool ~kind f =
   match kind with
   | "pool" ->
     if !j = 0 then
-      Ws_pool.with_ ~per_domain:1 f
+      Ws_pool.with_ f
     else
-      Ws_pool.with_ ~min:!j f
+      Ws_pool.with_ ~num_threads:!j f
   | "fifo" ->
     if !j = 0 then
-      Fifo_pool.with_ ~per_domain:1 f
+      Fifo_pool.with_ f
     else
-      Fifo_pool.with_ ~min:!j f
+      Fifo_pool.with_ ~num_threads:!j f
   | _ -> assert false
 
 (** Run in parallel using {!Fut.for_} *)
diff --git a/test/effect-based/t_fib1.ml b/test/effect-based/t_fib1.ml
index ca3f2861..a7c8ebee 100644
--- a/test/effect-based/t_fib1.ml
+++ b/test/effect-based/t_fib1.ml
@@ -26,13 +26,13 @@ let fib ~on x : int Fut.t =
 let () = assert (List.init 10 fib_direct = [ 1; 1; 2; 3; 5; 8; 13; 21; 34; 55 ])
 
 let fib_40 : int =
-  let pool = Ws_pool.create ~min:8 () in
+  let pool = Ws_pool.create ~num_threads:8 () in
   fib ~on:pool 40 |> Fut.wait_block_exn
 
 let () = Printf.printf "fib 40 = %d\n%!" fib_40
 
 let run_test () =
-  let pool = Ws_pool.create ~min:8 () in
+  let pool = Ws_pool.create ~num_threads:8 () in
 
   assert (
     List.init 10 (fib ~on:pool)
diff --git a/test/effect-based/t_fib_fork_join.ml b/test/effect-based/t_fib_fork_join.ml
index bdf60337..4e6639b2 100644
--- a/test/effect-based/t_fib_fork_join.ml
+++ b/test/effect-based/t_fib_fork_join.ml
@@ -27,13 +27,13 @@ let fib ~on x : int Fut.t =
 let () = assert (List.init 10 fib_direct = [ 1; 1; 2; 3; 5; 8; 13; 21; 34; 55 ])
 
 let fib_40 : int =
-  let pool = Ws_pool.create ~min:8 () in
+  let pool = Ws_pool.create ~num_threads:8 () in
   fib ~on:pool 40 |> Fut.wait_block_exn
 
 let () = Printf.printf "fib 40 = %d\n%!" fib_40
 
 let run_test () =
-  let pool = Ws_pool.create ~min:8 () in
+  let pool = Ws_pool.create ~num_threads:8 () in
 
   assert (
     List.init 10 (fib ~on:pool)
diff --git a/test/effect-based/t_fib_fork_join_all.ml b/test/effect-based/t_fib_fork_join_all.ml
index ed82902e..3caee9b9 100644
--- a/test/effect-based/t_fib_fork_join_all.ml
+++ b/test/effect-based/t_fib_fork_join_all.ml
@@ -22,13 +22,13 @@ let rec fib x : int =
   )
 
 let fib_40 : int =
-  let@ pool = Ws_pool.with_ ~min:8 () in
+  let@ pool = Ws_pool.with_ ~num_threads:8 () in
   Fut.spawn ~on:pool (fun () -> fib 40) |> Fut.wait_block_exn
 
 let () = Printf.printf "fib 40 = %d\n%!" fib_40
 
 let run_test () =
-  let@ pool = Ws_pool.with_ ~min:8 () in
+  let@ pool = Ws_pool.with_ ~num_threads:8 () in
 
   let fut =
     Fut.spawn ~on:pool (fun () ->
diff --git a/test/effect-based/t_fork_join.ml b/test/effect-based/t_fork_join.ml
index 5b467187..5c7134ca 100644
--- a/test/effect-based/t_fork_join.ml
+++ b/test/effect-based/t_fork_join.ml
@@ -5,7 +5,7 @@ let ( let@ ) = ( @@ )
 
 open! Moonpool
 
-let pool = Ws_pool.create ~min:4 ()
+let pool = Ws_pool.create ~num_threads:4 ()
 
 let () =
   let x =
@@ -270,7 +270,7 @@ end
 let t_eval =
   let arb = Q.set_stats [ "size", Evaluator.size ] Evaluator.arb in
   Q.Test.make ~name:"same eval" arb (fun e ->
-      let@ pool = Ws_pool.with_ ~min:4 () in
+      let@ pool = Ws_pool.with_ ~num_threads:4 () in
       (* Printf.eprintf "eval %s\n%!" (Evaluator.show e); *)
       let x = Evaluator.eval_seq e in
       let y = Evaluator.eval_fork_join ~pool e in
@@ -288,7 +288,7 @@ let t_for_nested ~min ~chunk_size () =
       let ref_l2 = List.map (List.map neg) ref_l1 in
 
       let l1, l2 =
-        let@ pool = Ws_pool.with_ ~min () in
+        let@ pool = Ws_pool.with_ ~num_threads:min () in
         let@ () = Ws_pool.run_wait_block pool in
         let l1 =
           Fork_join.map_list ~chunk_size (Fork_join.map_list ~chunk_size neg) l
@@ -310,7 +310,7 @@ let t_map ~chunk_size () =
   Q.Test.make ~name:"map1"
     Q.(small_list small_int |> Q.set_stats [ "len", List.length ])
     (fun l ->
-      let@ pool = Ws_pool.with_ ~min:4 () in
+      let@ pool = Ws_pool.with_ ~num_threads:4 () in
       let@ () = Ws_pool.run_wait_block pool in
 
       let a1 =
diff --git a/test/effect-based/t_fork_join_heavy.ml b/test/effect-based/t_fork_join_heavy.ml
index ad9f7044..a981bee1 100644
--- a/test/effect-based/t_fork_join_heavy.ml
+++ b/test/effect-based/t_fork_join_heavy.ml
@@ -27,7 +27,7 @@ let run ~min () =
     let@ _sp = Trace.with_span ~__FILE__ ~__LINE__ "step" in
 
     let l1, l2 =
-      let@ pool = Ws_pool.with_ ~min () in
+      let@ pool = Ws_pool.with_ ~num_threads:min () in
       let@ () = Ws_pool.run_wait_block pool in
 
       let l1, l2 =
diff --git a/test/effect-based/t_futs1.ml b/test/effect-based/t_futs1.ml
index 182ca9d5..4df18226 100644
--- a/test/effect-based/t_futs1.ml
+++ b/test/effect-based/t_futs1.ml
@@ -2,7 +2,7 @@
 
 open! Moonpool
 
-let pool = Ws_pool.create ~min:4 ()
+let pool = Ws_pool.create ~num_threads:4 ()
 
 let () =
   let fut = Array.init 10 (fun i -> Fut.spawn ~on:pool (fun () -> i)) in
diff --git a/test/effect-based/t_many.ml b/test/effect-based/t_many.ml
index 4362932c..6a2b5918 100644
--- a/test/effect-based/t_many.ml
+++ b/test/effect-based/t_many.ml
@@ -30,19 +30,19 @@ let run ~pool () =
 
 let () =
   (print_endline "with fifo";
-   let@ pool = Fifo_pool.with_ ~min:4 () in
+   let@ pool = Fifo_pool.with_ ~num_threads:4 () in
    run ~pool ());
 
   (print_endline "with WS(1)";
-   let@ pool = Ws_pool.with_ ~min:1 () in
+   let@ pool = Ws_pool.with_ ~num_threads:1 () in
    run ~pool ());
 
   (print_endline "with WS(2)";
-   let@ pool = Ws_pool.with_ ~min:2 () in
+   let@ pool = Ws_pool.with_ ~num_threads:2 () in
    run ~pool ());
 
   (print_endline "with WS(4)";
-   let@ pool = Ws_pool.with_ ~min:4 () in
+   let@ pool = Ws_pool.with_ ~num_threads:4 () in
    run ~pool ());
 
   ()
diff --git a/test/effect-based/t_sort.ml b/test/effect-based/t_sort.ml
index 8d3fe17c..8ccc372f 100644
--- a/test/effect-based/t_sort.ml
+++ b/test/effect-based/t_sort.ml
@@ -59,7 +59,7 @@ let rec quicksort arr i len : unit =
       (fun () -> quicksort arr !low (len - (!low - i)))
   )
 
-let pool = Moonpool.Ws_pool.create ~min:8 ()
+let pool = Moonpool.Ws_pool.create ~num_threads:8 ()
 
 let () =
   let arr = Array.init 400_000 (fun _ -> Random.int 300_000) in
diff --git a/test/t_bench1.ml b/test/t_bench1.ml
index 95cd87a5..cd1a8bfd 100644
--- a/test/t_bench1.ml
+++ b/test/t_bench1.ml
@@ -8,7 +8,7 @@ let rec fib x =
 
 let run ~psize ~n ~j () : _ Fut.t =
   Printf.printf "pool size=%d, n=%d, j=%d\n%!" psize n j;
-  let pool = Ws_pool.create ~min:psize ~per_domain:0 () in
+  let pool = Ws_pool.create ~num_threads:psize () in
 
   (* TODO: a ppx for tracy so we can use instrumentation *)
   let loop () =
diff --git a/test/t_chan_train.ml b/test/t_chan_train.ml
index bb3e24f7..132d5540 100644
--- a/test/t_chan_train.ml
+++ b/test/t_chan_train.ml
@@ -1,7 +1,7 @@
 open Moonpool
 
 (* large pool, some of our tasks below are long lived *)
-let pool = Ws_pool.create ~min:30 ()
+let pool = Ws_pool.create ~num_threads:30 ()
 
 open (val Fut.infix pool)
 
diff --git a/test/t_fib.ml b/test/t_fib.ml
index 3a98e395..3fc53bf9 100644
--- a/test/t_fib.ml
+++ b/test/t_fib.ml
@@ -4,8 +4,8 @@ let ( let@ ) = ( @@ )
 
 let with_pool ~kind () f =
   match kind with
-  | `Fifo_pool -> Fifo_pool.with_ ~min:4 () f
-  | `Ws_pool -> Ws_pool.with_ ~min:4 () f
+  | `Fifo_pool -> Fifo_pool.with_ ~num_threads:4 () f
+  | `Ws_pool -> Ws_pool.with_ ~num_threads:4 () f
 
 let rec fib x =
   if x <= 1 then
diff --git a/test/t_fib_rec.ml b/test/t_fib_rec.ml
index 286e6aac..94e206b7 100644
--- a/test/t_fib_rec.ml
+++ b/test/t_fib_rec.ml
@@ -25,7 +25,7 @@ let () = assert (List.init 10 fib_direct = [ 1; 1; 2; 3; 5; 8; 13; 21; 34; 55 ])
 let fib_40 : int lazy_t =
   lazy
     (let@ _sp = Trace.with_span ~__FILE__ ~__LINE__ "fib40" in
-     let pool = Fifo_pool.create ~min:8 () in
+     let pool = Fifo_pool.create ~num_threads:8 () in
      let r = fib ~on:pool 40 |> Fut.wait_block_exn in
      Ws_pool.shutdown pool;
      r)
@@ -49,12 +49,12 @@ let run_test ~pool () =
 
 let run_test_size ~size () =
   Printf.printf "test pool(%d)\n%!" size;
-  let@ pool = Ws_pool.with_ ~min:size () in
+  let@ pool = Ws_pool.with_ ~num_threads:size () in
   run_test ~pool ()
 
 let run_test_fifo ~size () =
   Printf.printf "test fifo(%d)\n%!" size;
-  let@ pool = Fifo_pool.with_ ~min:size () in
+  let@ pool = Fifo_pool.with_ ~num_threads:size () in
   run_test ~pool ()
 
 let setup_counter () =
diff --git a/test/t_futs1.ml b/test/t_futs1.ml
index ee2d96a6..03a1ac13 100644
--- a/test/t_futs1.ml
+++ b/test/t_futs1.ml
@@ -1,7 +1,7 @@
 open! Moonpool
 
-let pool = Ws_pool.create ~min:4 ()
-let pool2 = Ws_pool.create ~min:2 ()
+let pool = Ws_pool.create ~num_threads:4 ()
+let pool2 = Ws_pool.create ~num_threads:2 ()
 
 let () =
   let fut = Fut.return 1 in
diff --git a/test/t_props.ml b/test/t_props.ml
index 9fa64fbe..698650fd 100644
--- a/test/t_props.ml
+++ b/test/t_props.ml
@@ -7,8 +7,8 @@ let add_test t = tests := t :: !tests
 
 let with_pool ~kind () f =
   match kind with
-  | `Fifo_pool -> Fifo_pool.with_ ~min:4 ~per_domain:1 () f
-  | `Ws_pool -> Ws_pool.with_ ~min:4 ~per_domain:1 () f
+  | `Fifo_pool -> Fifo_pool.with_ () f
+  | `Ws_pool -> Ws_pool.with_ () f
 
 let () =
   add_test @@ fun ~kind ->
diff --git a/test/t_resource.ml b/test/t_resource.ml
index c990f708..4c20e9fb 100644
--- a/test/t_resource.ml
+++ b/test/t_resource.ml
@@ -4,8 +4,8 @@ let ( let@ ) = ( @@ )
 
 let with_pool ~kind () f =
   match kind with
-  | `Fifo_pool -> Fifo_pool.with_ ~min:4 ~per_domain:1 () f
-  | `Ws_pool -> Ws_pool.with_ ~min:4 ~per_domain:1 () f
+  | `Fifo_pool -> Fifo_pool.with_ () f
+  | `Ws_pool -> Ws_pool.with_ () f
 
 (* test proper resource handling *)
 let run ~kind () =
diff --git a/test/t_tree_futs.ml b/test/t_tree_futs.ml
index 3507be0a..0bc96a03 100644
--- a/test/t_tree_futs.ml
+++ b/test/t_tree_futs.ml
@@ -4,8 +4,8 @@ let ( let@ ) = ( @@ )
 
 let with_pool ~kind ~j () f =
   match kind with
-  | `Fifo_pool -> Fifo_pool.with_ ~min:j () f
-  | `Ws_pool -> Ws_pool.with_ ~min:j () f
+  | `Fifo_pool -> Fifo_pool.with_ ~num_threads:j () f
+  | `Ws_pool -> Ws_pool.with_ ~num_threads:j () f
 
 type 'a tree =
   | Leaf of 'a
diff --git a/test/t_unfair.ml b/test/t_unfair.ml
index f535a450..cee4373e 100644
--- a/test/t_unfair.ml
+++ b/test/t_unfair.ml
@@ -20,8 +20,8 @@ let run ~kind () =
     in
 
     match kind with
-    | `Simple -> Fifo_pool.create ~min:3 ~on_init_thread ~around_task ()
-    | `Ws_pool -> Ws_pool.create ~min:3 ~on_init_thread ~around_task ()
+    | `Simple -> Fifo_pool.create ~num_threads:3 ~on_init_thread ~around_task ()
+    | `Ws_pool -> Ws_pool.create ~num_threads:3 ~on_init_thread ~around_task ()
   in
 
   (* make all threads busy *)