Avoid creating lots of or unnecessary systhreads on OCaml 4

bench/bench_bounded_q.ml

@@ -3,6 +3,8 @@ open Picos_structured
 open Picos_sync
 module Queue = Stdlib.Queue
 
+let is_ocaml4 = String.starts_with ~prefix:"4." Sys.ocaml_version
+
 module Bounded_q : sig
   type 'a t
 
@@ -119,7 +121,7 @@ let run_one ~budgetf ~n_adders ~n_takers ?(n_msgs = 10 * Util.iter_factor) () =
   let wrap _ () = Scheduler.run in
   let work domain_index () =
     Flock.join_after @@ fun () ->
-    Flock.fork yielder;
+    if not is_ocaml4 then Flock.fork yielder;
     if domain_index < n_adders then
       let rec work () =
         let n = Countdown.alloc n_msgs_to_add ~domain_index ~batch:100 in

bench/bench_spawn.ml

@@ -1,6 +1,8 @@
 open Multicore_bench
 open Picos
 
+let is_ocaml4 = String.starts_with ~prefix:"4." Sys.ocaml_version
+
 let factor =
   Util.iter_factor
   * if String.starts_with ~prefix:"4." Sys.ocaml_version then 1 else 10
@@ -31,4 +33,5 @@ let run_one ~budgetf () =
     ~work ()
   |> Times.to_thruput_metrics ~n:n_spawns ~singular:"spawn" ~config
 
-let run_suite ~budgetf = if Sys.int_size <= 32 then [] else run_one ~budgetf ()
+let run_suite ~budgetf =
+  if Sys.int_size <= 32 || is_ocaml4 then [] else run_one ~budgetf ()