diff --git a/src/d_pool_.ml b/src/d_pool_.ml
index 076d61ba..2dedc339 100644
--- a/src/d_pool_.ml
+++ b/src/d_pool_.ml
@@ -13,14 +13,6 @@ type worker_state = {
   th_count: int Atomic_.t;  (** Number of threads on this *)
 }
 
-let work_ (st : worker_state) : unit =
-  Dla_.setup_domain ();
-  while Atomic_.get st.th_count > 0 do
-    match Bb_queue.pop st.q with
-    | Run f -> (try f () with _ -> ())
-    | Decr -> Atomic_.decr st.th_count
-  done
-
 (** Array of (optional) workers.
 
     Workers are started/stop on demand. *)
@@ -30,6 +22,16 @@ let domains_ : worker_state option Lock.t array =
   let n = max 1 (Domain_.recommended_number () - 1) in
   Array.init n (fun _ -> Lock.create None)
 
+let work_ idx (st : worker_state) : unit =
+  Dla_.setup_domain ();
+  while Atomic_.get st.th_count > 0 do
+    match Bb_queue.pop st.q with
+    | Run f -> (try f () with _ -> ())
+    | Decr ->
+      if Atomic_.fetch_and_add st.th_count (-1) = 1 then
+        Lock.set domains_.(idx) None
+  done
+
 let[@inline] n_domains () : int = Array.length domains_
 
 let run_on (i : int) (f : unit -> unit) : unit =
@@ -42,7 +44,7 @@ let run_on (i : int) (f : unit -> unit) : unit =
       st
     | None ->
       let st = { th_count = Atomic_.make 1; q = Bb_queue.create () } in
-      let _domain : domain = Domain_.spawn (fun () -> work_ st) in
+      let _domain : domain = Domain_.spawn (fun () -> work_ i st) in
       Bb_queue.push st.q (Run f);
       Some st)