diff --git a/src/core/chan.ml b/src/core/chan.ml
index 5ce82376..099e7074 100644
--- a/src/core/chan.ml
+++ b/src/core/chan.ml
@@ -1,7 +1,8 @@
 module A = Atomic_
 
 type 'a or_error = 'a Fut.or_error
-type 'a waiter = 'a Fut.promise
+type 'a pop_waiter = 'a Fut.promise
+type 'a push_waiter = 'a * unit Fut.promise
 
 let[@inline] list_is_empty_ = function
   | [] -> true
@@ -11,6 +12,7 @@ let[@inline] list_is_empty_ = function
 module Q : sig
   type 'a t
 
+  val empty : 'a t
   val return : 'a -> 'a t
   val is_empty : _ t -> bool
 
@@ -28,6 +30,7 @@ end = struct
 
       invariant: if hd=[], then tl=[] *)
 
+  let empty = { hd = []; tl = [] }
   let[@inline] return x : _ t = { hd = [ x ]; tl = [] }
 
   let[@inline] make_ hd tl =
@@ -56,138 +59,177 @@ end
 
 exception Closed
 
-type 'a state =
-  | Empty
-  | St_closed
-  | Elems of 'a Q.t
-  | Waiters of 'a waiter Q.t
+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;
+}
 
-type 'a t = { st: 'a state A.t } [@@unboxed]
+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 create () : _ t = { st = A.make Empty }
+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
+    );
 
-(** 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
-
-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
+    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]
+
+(* TODO: remove
+   (** A waiter queue, somewhat similar to a condition. *)
+   module Waiters_ = struct
+     type t = { waiters: Trigger.t Q.t A.t } [@@unboxed]
+
+     let create () : t = { waiters = A.make Q.empty }
+
+     let add_waiter self (tr : Trigger.t) : unit =
+       while
+         let q = A.get self.waiters in
+         not (A.compare_and_set self.waiters q (Q.push q tr))
+       do
+         Domain_.relax ()
+       done
+
+     let wait_await (self : t) : unit =
+       let tr = Trigger.create () in
+       add_waiter self tr;
+       Trigger.await_exn tr
+
+     exception Empty = Q.Empty
+
+     let rec pop_waiter (self : t) : Trigger.t =
+       let q = A.get self.waiters in
+       let x, q' = Q.pop_exn q in
+       if A.compare_and_set self.waiters q q' then
+         x
+       else (
+         Domain_.relax ();
+         pop_waiter self
+       )
+
+     let rec pop_all (self : t) : Trigger.t Q.t =
+       let q = A.get self.waiters in
+       if A.compare_and_set self.waiters q Q.empty then
+         q
+       else (
+         Domain_.relax ();
+         pop_all self
+       )
+
+     let signal (self : t) : unit =
+       match pop_waiter self with
+       | exception Empty -> ()
+       | tr -> Trigger.signal tr
+
+     let broadcast (self : t) : unit =
+       let waiters = pop_all self in
+       Q.iter Trigger.signal waiters
+   end
+*)
diff --git a/src/core/chan.mli b/src/core/chan.mli
index 083cf8d5..142f43ce 100644
--- a/src/core/chan.mli
+++ b/src/core/chan.mli
@@ -1,12 +1,9 @@
 (** 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
@@ -14,39 +11,43 @@ 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}. *)
-
 val close : _ t -> unit
 (** Close the channel. Further push and pop calls will fail.
     This is idempotent. *)
 
 [@@@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