diff --git a/bench_primes.sh b/bench_primes.sh
new file mode 100755
index 00000000..52b618c5
--- /dev/null
+++ b/bench_primes.sh
@@ -0,0 +1,3 @@
+#!/bin/sh
+OPTS="--profile=release --display=quiet"
+exec dune exec $OPTS -- benchs/primes.exe $@
diff --git a/benchs/dune b/benchs/dune
index 14393230..6d2ec5ff 100644
--- a/benchs/dune
+++ b/benchs/dune
@@ -1,5 +1,5 @@
 (executables
- (names fib_rec pi)
+ (names fib_rec pi primes)
  (preprocess
   (action
    (run %{project_root}/src/cpp/cpp.exe %{input-file})))
diff --git a/benchs/primes.ml b/benchs/primes.ml
new file mode 100644
index 00000000..1a8557d9
--- /dev/null
+++ b/benchs/primes.ml
@@ -0,0 +1,60 @@
+let ( let@ ) = ( @@ )
+let spf = Printf.sprintf
+
+let generate' chan =
+  for i = 2 to Int.max_int do
+    Moonpool.Chan.push chan i
+  done
+
+let filter' in_chan out_chan prime =
+  let rec loop () =
+    let n = Moonpool.Chan.pop in_chan in
+    if n mod prime <> 0 then Moonpool.Chan.push out_chan n;
+    loop ()
+  in
+  loop ()
+
+let main ~chan_size ~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 ~max_size:chan_size () in
+  Moonpool.run_async runner
+    (let chan = !primes in
+     fun () -> generate' chan);
+
+  for _i = 1 to n do
+    let prime = Moonpool.Chan.pop !primes in
+    on_prime prime;
+    let filtered_chan = Moonpool.Chan.create ~max_size:chan_size () 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 chan_size = ref 0 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";
+      "--chan-size", Arg.Set_int chan_size, " channel size";
+    ]
+    |> 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 ~chan_size:!chan_size ~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/src/core/chan.ml b/src/core/chan.ml
index 5ce82376..be4ac3b9 100644
--- a/src/core/chan.ml
+++ b/src/core/chan.ml
@@ -1,193 +1,124 @@
-module A = Atomic_
-
-type 'a or_error = 'a Fut.or_error
-type 'a waiter = 'a Fut.promise
-
-let[@inline] list_is_empty_ = function
-  | [] -> true
-  | _ :: _ -> false
-
-(** Simple functional queue *)
-module Q : sig
-  type 'a t
-
-  val return : 'a -> 'a t
-  val is_empty : _ t -> bool
-
-  exception Empty
-
-  val pop_exn : 'a t -> 'a * 'a t
-  val push : 'a t -> 'a -> 'a t
-  val iter : ('a -> unit) -> 'a t -> unit
-end = struct
-  type 'a t = {
-    hd: 'a list;
-    tl: 'a list;
-  }
-  (** Queue containing elements of type 'a.
-
-      invariant: if hd=[], then tl=[] *)
-
-  let[@inline] return x : _ t = { hd = [ x ]; tl = [] }
-
-  let[@inline] make_ hd tl =
-    match hd with
-    | [] -> { hd = List.rev tl; tl = [] }
-    | _ :: _ -> { hd; tl }
-
-  let[@inline] is_empty self = list_is_empty_ self.hd
-  let[@inline] push self x : _ t = make_ self.hd (x :: self.tl)
-
-  let iter f (self : _ t) : unit =
-    List.iter f self.hd;
-    List.iter f self.tl
-
-  exception Empty
-
-  let pop_exn self =
-    match self.hd with
-    | [] ->
-      assert (list_is_empty_ self.tl);
-      raise Empty
-    | x :: hd' ->
-      let self' = make_ hd' self.tl in
-      x, self'
-end
-
 exception Closed
 
-type 'a state =
-  | Empty
-  | St_closed
-  | Elems of 'a Q.t
-  | Waiters of 'a waiter Q.t
-
-type 'a t = { st: 'a state A.t } [@@unboxed]
-
-let create () : _ t = { st = A.make Empty }
-
-(** Produce a state from a queue of waiters *)
-let[@inline] mk_st_waiters_ ws : _ state =
-  if Q.is_empty ws then
-    Empty
-  else
-    Waiters ws
-
-(** Produce a state from a queue of elements *)
-let[@inline] mk_st_elems_ q : _ state =
-  if Q.is_empty q then
-    Empty
-  else
-    Elems q
+type 'a t = {
+  q: 'a Queue.t;
+  mutex: Mutex.t;  (** protects critical section *)
+  mutable closed: bool;
+  max_size: int;
+  push_waiters: Trigger.t Queue.t;
+  pop_waiters: Trigger.t Queue.t;
+}
+
+let create ~max_size () : _ t =
+  if max_size < 0 then invalid_arg "Chan: max_size < 0";
+  {
+    max_size;
+    mutex = Mutex.create ();
+    closed = false;
+    q = Queue.create ();
+    push_waiters = Queue.create ();
+    pop_waiters = Queue.create ();
+  }
 
-let push (self : _ t) x : unit =
-  while
-    let old_st = A.get self.st in
-    match old_st with
-    | St_closed -> raise Closed
-    | Empty -> not (A.compare_and_set self.st old_st (Elems (Q.return x)))
-    | Waiters ws ->
-      (* awake first waiter and give it [x] *)
-      let w, ws' = Q.pop_exn ws in
-      let new_st = mk_st_waiters_ ws' in
-      if A.compare_and_set self.st old_st new_st then (
-        Fut.fulfill w (Ok x);
-        false
-      ) else
-        true
-    | Elems q -> not (A.compare_and_set self.st old_st (Elems (Q.push q x)))
-  do
-    Domain_.relax ()
-  done
+let try_push (self : _ t) x : bool =
+  let res = ref false in
+  if Mutex.try_lock self.mutex then (
+    if self.closed then (
+      Mutex.unlock self.mutex;
+      raise Closed
+    );
+
+    match Queue.length self.q with
+    | 0 ->
+      let to_awake = Queue.create () in
+      Queue.push x self.q;
+      Queue.transfer self.pop_waiters to_awake;
+      res := true;
+      Mutex.unlock self.mutex;
+      (* wake up pop triggers if needed. Be careful to do that
+         outside the critical section*)
+      Queue.iter Trigger.signal to_awake
+    | n when n < self.max_size ->
+      Queue.push x self.q;
+      Mutex.unlock self.mutex
+    | _ -> Mutex.unlock self.mutex
+  );
+  !res
 
 let try_pop (type elt) self : elt option =
-  let module M = struct
-    exception Found of elt
-  end in
-  try
-    (* a bit of spinning *)
-    for _retry = 1 to 10 do
-      let old_st = A.get self.st in
-      match old_st with
-      | Elems q ->
-        let x, q' = Q.pop_exn q in
-        let new_st = mk_st_elems_ q' in
-        if A.compare_and_set self.st old_st new_st then
-          raise_notrace (M.Found x)
-        else
-          Domain_.relax ()
-      | _ -> raise_notrace Exit
-    done;
-    None
-  with
-  | M.Found x -> Some x
-  | Exit -> None
-
-let pop (type elt) (self : _ t) : elt Fut.t =
-  let module M = struct
-    exception Ret of elt
-    exception Fut of elt Fut.t
-  end in
-  try
-    while
-      let old_st = A.get self.st in
-      (match old_st with
-      | St_closed ->
-        let bt = Printexc.get_callstack 10 in
-        raise_notrace (M.Fut (Fut.fail Closed bt))
-      | Elems q ->
-        let x, q' = Q.pop_exn q in
-        let new_st = mk_st_elems_ q' in
-        if A.compare_and_set self.st old_st new_st then raise_notrace (M.Ret x)
-      | Empty ->
-        let fut, promise = Fut.make () in
-        let new_st = Waiters (Q.return promise) in
-        if A.compare_and_set self.st old_st new_st then
-          raise_notrace (M.Fut fut)
-      | Waiters ws ->
-        let fut, promise = Fut.make () in
-        (* add new promise at the end of the queue of waiters *)
-        let new_st = Waiters (Q.push ws promise) in
-        if A.compare_and_set self.st old_st new_st then
-          raise_notrace (M.Fut fut));
-      true
-    do
-      Domain_.relax ()
-    done;
-    (* never reached *)
-    assert false
-  with
-  | M.Ret x -> Fut.return x
-  | M.Fut f -> f
-
-let pop_block_exn (self : 'a t) : 'a =
-  match try_pop self with
-  | Some x -> x
-  | None -> Fut.wait_block_exn @@ pop self
+  let res = ref None in
+  if Mutex.try_lock self.mutex then (
+    (match Queue.pop self.q with
+    | exception Queue.Empty ->
+      if self.closed then (
+        Mutex.unlock self.mutex;
+        raise Closed
+      )
+    | x -> res := Some x);
+    Mutex.unlock self.mutex
+  );
+  !res
 
 let close (self : _ t) : unit =
-  while
-    let old_st = A.get self.st in
-    match old_st with
-    | St_closed -> false (* exit *)
-    | Elems _ | Empty -> not (A.compare_and_set self.st old_st St_closed)
-    | Waiters ws ->
-      if A.compare_and_set self.st old_st St_closed then (
-        (* fail all waiters with [Closed]. *)
-        let bt = Printexc.get_callstack 10 in
-        Q.iter (fun w -> Fut.fulfill_idempotent w (Error (Closed, bt))) ws;
-        false
-      ) else
-        true
-  do
-    Domain_.relax ()
-  done
+  let q = Queue.create () in
+  Mutex.lock self.mutex;
+  if not self.closed then (
+    self.closed <- true;
+    Queue.transfer self.pop_waiters q;
+    Queue.transfer self.push_waiters q
+  );
+  Mutex.unlock self.mutex;
+  Queue.iter Trigger.signal q
 
 [@@@ifge 5.0]
 
-let pop_await self =
-  match try_pop self with
-  | Some x -> x
-  | None -> Fut.await @@ pop self
+let rec push (self : _ t) x : unit =
+  Mutex.lock self.mutex;
+
+  if self.closed then (
+    Mutex.unlock self.mutex;
+    raise Closed
+  );
+
+  match Queue.length self.q with
+  | 0 ->
+    Queue.push x self.q;
+    let to_wakeup = Queue.create () in
+    Queue.transfer self.pop_waiters to_wakeup;
+    Mutex.unlock self.mutex;
+    Queue.iter Trigger.signal to_wakeup
+  | n when n < self.max_size ->
+    Queue.push x self.q;
+    Mutex.unlock self.mutex
+  | _ ->
+    let tr = Trigger.create () in
+    Queue.push tr self.push_waiters;
+    Mutex.unlock self.mutex;
+    Trigger.await_exn tr;
+    push self x
+
+let rec pop (self : 'a t) : 'a =
+  Mutex.lock self.mutex;
+  match Queue.pop self.q with
+  | x ->
+    if Queue.is_empty self.q then (
+      let to_wakeup = Queue.create () in
+      Queue.transfer self.push_waiters to_wakeup;
+      Mutex.unlock self.mutex;
+      Queue.iter Trigger.signal to_wakeup
+    ) else
+      Mutex.unlock self.mutex;
+    x
+  | exception Queue.Empty ->
+    if self.closed then (
+      Mutex.unlock self.mutex;
+      raise Closed
+    );
+
+    let tr = Trigger.create () in
+    Queue.push tr self.pop_waiters;
+    Mutex.unlock self.mutex;
+    Trigger.await_exn tr;
+    pop self
 
 [@@@endif]
diff --git a/src/core/chan.mli b/src/core/chan.mli
index 083cf8d5..7ec1163d 100644
--- a/src/core/chan.mli
+++ b/src/core/chan.mli
@@ -1,42 +1,29 @@
 (** Channels.
 
-   Channels are pipelines of values where threads can push into
-   one end, and pull from the other end.
+    The channels have bounded size. Push/pop return futures or can use effects
+    to provide an [await]-friendly version.
 
-   Unlike {!Moonpool.Blocking_queue}, channels are designed so
-   that pushing never blocks, and pop'ing values returns a future.
-
-   @since 0.3
+    The channels became bounded since @NEXT_RELEASE .
 *)
 
-type 'a or_error = 'a Fut.or_error
-
 type 'a t
 (** Channel carrying values of type ['a]. *)
 
-val create : unit -> 'a t
+val create : max_size:int -> unit -> 'a t
 (** Create a channel. *)
 
 exception Closed
 
-val push : 'a t -> 'a -> unit
-(** [push chan x] pushes [x] into [chan]. This does not block.
+val try_push : 'a t -> 'a -> bool
+(** [try_push chan x] pushes [x] into [chan]. This does not block.
+    Returns [true] if it succeeded in pushing.
     @raise Closed if the channel is closed. *)
 
-val pop : 'a t -> 'a Fut.t
-(** Pop an element. This returns a future that will be
-    fulfilled when an element is available.
-    @raise Closed if the channel is closed, or fails the future
-      if the channel is closed before an element is available for it. *)
-
 val try_pop : 'a t -> 'a option
 (** [try_pop chan] pops and return an element if one is available
-    immediately. Otherwise it returns [None]. *)
-
-val pop_block_exn : 'a t -> 'a
-(** Like [pop], but blocks if an element is not available immediately.
-    The precautions around blocking from inside a thread pool
-    are the same as explained in {!Fut.wait_block}. *)
+    immediately. Otherwise it returns [None].
+    @raise Closed if the channel is closed and empty.
+    *)
 
 val close : _ t -> unit
 (** Close the channel. Further push and pop calls will fail.
@@ -44,9 +31,23 @@ val close : _ t -> unit
 
 [@@@ifge 5.0]
 
-val pop_await : 'a t -> 'a
-(** Like {!pop} but suspends the current thread until an element is
-    available. See {!Fut.await} for more details.
-    @since 0.3 *)
+val push : 'a t -> 'a -> unit
+(** Push the value into the channel, suspending the current task
+    if the channel is currently full.
+    @raise Closed if the channel is closed
+    @since NEXT_RELEASE *)
+
+val pop : 'a t -> 'a
+(** Pop an element. This might suspend the current task if the
+    channel is currently empty.
+    @raise Closed if the channel is empty and closed.
+    @since NEXT_RELEASE *)
+
+(*
+val pop_block_exn : 'a t -> 'a
+(** Like [pop], but blocks if an element is not available immediately.
+    The precautions around blocking from inside a thread pool
+    are the same as explained in {!Fut.wait_block}. *)
+*)
 
 [@@@endif]
diff --git a/src/sync/moonpool_sync.ml b/src/sync/moonpool_sync.ml
index 99065305..f2c29ba7 100644
--- a/src/sync/moonpool_sync.ml
+++ b/src/sync/moonpool_sync.ml
@@ -1,4 +1,5 @@
 module Mutex = Picos_std_sync.Mutex
+module Chan = Chan
 module Condition = Picos_std_sync.Condition
 module Lock = Lock
 module Event = Event
diff --git a/test/dune b/test/dune
index c10d08f4..af881591 100644
--- a/test/dune
+++ b/test/dune
@@ -7,7 +7,6 @@
   t_futs1
   t_tree_futs
   t_props
-  t_chan_train
   t_resource
   t_unfair
   t_ws_deque
diff --git a/test/effect-based/dune b/test/effect-based/dune
index bf1feb81..faa9254d 100644
--- a/test/effect-based/dune
+++ b/test/effect-based/dune
@@ -3,6 +3,7 @@
   t_fib1
   t_futs1
   t_many
+  t_chan_train
   t_fib_fork_join
   t_fib_fork_join_all
   t_sort
diff --git a/test/t_chan_train.ml b/test/effect-based/t_chan_train.ml
similarity index 88%
rename from test/t_chan_train.ml
rename to test/effect-based/t_chan_train.ml
index 20645a73..e972dabc 100644
--- a/test/t_chan_train.ml
+++ b/test/effect-based/t_chan_train.ml
@@ -3,17 +3,15 @@ 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
 
 let mk_chan (ic : event Chan.t) : event Chan.t =
-  let out = Chan.create () in
+  let out = Chan.create ~max_size:16 () in
 
   let rec loop () =
-    let* ev = Chan.pop ic in
+    let ev = Chan.pop ic in
     Chan.push out ev;
     match ev with
     | E_close -> Fut.return ()
@@ -44,7 +42,7 @@ let run () =
   (* start trains *)
   let trains =
     List.init n_trains (fun _ ->
-        let c = Chan.create () in
+        let c = Chan.create ~max_size:16 () in
         let out = mk_train len_train c in
         c, out)
   in
@@ -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);