diff --git a/bench_primes.sh b/bench_primes.sh
old mode 100644
new mode 100755
diff --git a/benchs/primes.ml b/benchs/primes.ml
index 570b850f..e3e60d06 100644
--- a/benchs/primes.ml
+++ b/benchs/primes.ml
@@ -1,32 +1,58 @@
 let ( let@ ) = ( @@ )
+let spf = Printf.sprintf
 
 let generate' chan =
   for i = 2 to Int.max_int do
-    Moonpool.Chan.push chan i;
-    Thread.yield ()
+    Moonpool.Chan.push chan i
   done
 
 let filter' in_chan out_chan prime =
   let rec loop () =
-    let n = Moonpool.Chan.pop_await in_chan in
+    let n = Moonpool.Chan.pop in_chan in
     if n mod prime <> 0 then Moonpool.Chan.push out_chan n;
     loop ()
   in
   loop ()
 
-let () =
+let main ~n ~on_prime () : unit =
   let@ runner = Moonpool.Ws_pool.with_ () in
   let@ () = Moonpool.Ws_pool.run_wait_block runner in
-  let primes = ref @@ Moonpool.Chan.create () in
+  let primes = ref @@ Moonpool.Chan.create ~max_size:32 () in
   Moonpool.run_async runner
     (let chan = !primes in
      fun () -> generate' chan);
-  for _i = 1 to 10 do
-    let prime = Moonpool.Chan.pop_await !primes in
-    Format.printf "%d\n@?" prime;
-    let filtered_chan = Moonpool.Chan.create () in
+
+  for _i = 1 to n do
+    let prime = Moonpool.Chan.pop !primes in
+    on_prime prime;
+    let filtered_chan = Moonpool.Chan.create ~max_size:32 () in
     Moonpool.run_async runner
       (let in_chan = !primes in
        fun () -> filter' in_chan filtered_chan prime);
     primes := filtered_chan
   done
+
+let () =
+  let n = ref 10_000 in
+  let time = ref true in
+  let opts =
+    [
+      "-n", Arg.Set_int n, " number of iterations";
+      "--no-time", Arg.Clear time, " do not compute time";
+    ]
+    |> Arg.align
+  in
+  Arg.parse opts ignore "";
+  Printf.printf "computing %d primes\n%!" !n;
+
+  let t_start = Unix.gettimeofday () in
+
+  let n_primes = Atomic.make 0 in
+  main ~n:!n ~on_prime:(fun _ -> Atomic.incr n_primes) ();
+
+  let elapsed : float = Unix.gettimeofday () -. t_start in
+  Printf.printf "computed %d primes%s\n%!" (Atomic.get n_primes)
+    (if !time then
+       spf " in %.4fs" elapsed
+     else
+       "")
diff --git a/test/effect-based/t_chan_train.ml b/test/effect-based/t_chan_train.ml
index 09b51987..e972dabc 100644
--- a/test/effect-based/t_chan_train.ml
+++ b/test/effect-based/t_chan_train.ml
@@ -3,8 +3,6 @@ open Moonpool
 (* large pool, some of our tasks below are long lived *)
 let pool = Ws_pool.create ~num_threads:30 ()
 
-open Fut.Infix
-
 type event =
   | E_int of int
   | E_close
@@ -66,7 +64,9 @@ let run () =
         let sum = ref 0 in
         try
           while true do
-            match Chan.pop_block_exn oc with
+            match
+              Fut.spawn ~on:pool (fun () -> Chan.pop oc) |> Fut.wait_block_exn
+            with
             | E_close -> raise Exit
             | E_int x -> sum := !sum + x
           done;
diff --git a/test/fiber/t_fib1.ml b/test/fiber/t_fib1.ml
index 7ceedbf4..77360b2b 100644
--- a/test/fiber/t_fib1.ml
+++ b/test/fiber/t_fib1.ml
@@ -52,14 +52,14 @@ let () =
   let clock = ref TS.init in
   let fib =
     F.spawn_top ~on:runner @@ fun () ->
-    let chan_progress = Chan.create () in
-    let chans = Array.init 5 (fun _ -> Chan.create ()) in
+    let chan_progress = Chan.create ~max_size:4 () in
+    let chans = Array.init 5 (fun _ -> Chan.create ~max_size:4 ()) in
 
     let subs =
       List.init 5 (fun i ->
           F.spawn ~protect:false @@ fun _n ->
           Thread.delay (float i *. 0.01);
-          Chan.pop_await chans.(i);
+          Chan.pop chans.(i);
           Chan.push chan_progress i;
           F.check_if_cancelled ();
           i)
@@ -70,7 +70,7 @@ let () =
     F.spawn_ignore (fun () ->
         for i = 0 to 4 do
           Chan.push chans.(i) ();
-          let i' = Chan.pop_await chan_progress in
+          let i' = Chan.pop chan_progress in
           assert (i = i')
         done);
 
@@ -110,8 +110,8 @@ let () =
           @@ Exn_bt.show ebt)
     in
 
-    let chans_unblock = Array.init 10 (fun _i -> Chan.create ()) in
-    let chan_progress = Chan.create () in
+    let chans_unblock = Array.init 10 (fun _i -> Chan.create ~max_size:4 ()) in
+    let chan_progress = Chan.create ~max_size:4 () in
 
     logf (TS.tick_get clock) "start fibers";
     let subs =
@@ -126,7 +126,7 @@ let () =
           Thread.delay 0.002;
 
           (* sync for determinism *)
-          Chan.pop_await chans_unblock.(i);
+          Chan.pop chans_unblock.(i);
           Chan.push chan_progress i;
 
           if i = 7 then (
@@ -150,7 +150,7 @@ let () =
     F.spawn_ignore (fun () ->
         for j = 0 to 9 do
           Chan.push chans_unblock.(j) ();
-          let j' = Chan.pop_await chan_progress in
+          let j' = Chan.pop chan_progress in
           assert (j = j')
         done);