diff --git a/src/bench/whybench.ml b/src/bench/whybench.ml
index c530782c1b4f2368f848dcf7c39ec79fcfb29086..0c720dcba11f43e5b56747253d0f19cb3c4fdcb5 100644
--- a/src/bench/whybench.ml
+++ b/src/bench/whybench.ml
@@ -203,7 +203,7 @@ let () =
 
   let main = get_main config in
   Whyconf.load_plugins main;
-
+  Scheduler.maximum_running_proofs := Whyconf.running_provers_max main;
   (** listings*)
 
   let opt_list = ref false in
@@ -373,18 +373,18 @@ let count_result =
     Mnm.add res.B.tool tr m in
   List.fold_left fold m
 
-let () =
-  (** WHY some outputs are mixed, altought there is a mutex? *)
-  let m = Mutex.create () in
-  Scheduler.async := (fun f v ->
-    let f v = Mutex.lock m; f v; Mutex.unlock m in
-    ignore (Thread.create f v))
+let () = Scheduler.async := (fun f v -> ignore (Thread.create f v))
+
 
 let () =
+  let m = Mutex.create () in
   let callback tool prob task i res =
-    eprintf "%s %a %i with %s : %a@."
+    Mutex.lock m;
+    printf "%s %a %i with %s : %a@."
       prob Pretty.print_pr (task_goal task) i tool
-      Scheduler.print_pas res in
+      Scheduler.print_pas res;
+    Mutex.unlock m
+  in
   let l = B.all_list ~callback !tools !probs in
   (* let print_result fmt res = *)
   (*   fprintf fmt "%s %a %i with %s : %a@." *)
diff --git a/src/ide/scheduler.ml b/src/ide/scheduler.ml
index 040717cae2ebf94ba1134dfb342500bc2630b7ef..0ff9aef06bbf3c94f085b0e303b3bdfd47ed220f 100644
--- a/src/ide/scheduler.ml
+++ b/src/ide/scheduler.ml
@@ -8,6 +8,8 @@ let coef_buf = 2
 
 let async = ref (fun f () -> f ())
 
+let debug = Debug.register_flag "scheduler"
+
 type proof_attempt_status =
   | Scheduled (** external proof attempt is scheduled *)
   | Running (** external proof attempt is in progress *)
@@ -158,6 +160,9 @@ let event_handler () =
           Queue.pop prover_attempts_queue
         in
         incr scheduled_proofs;
+        Debug.dprintf debug
+          "scheduled_proofs = %i; maximum_running_proofs = %i@."
+          !scheduled_proofs !maximum_running_proofs;
         Mutex.unlock queue_lock;
         (* build the prover task from goal in [a] *)
         !async (fun () -> callback Scheduled) ();