diff --git a/src/io/IO_unix.ml b/src/io/IO_unix.ml.tmp
similarity index 100%
rename from src/io/IO_unix.ml
rename to src/io/IO_unix.ml.tmp
diff --git a/src/io/IO_unix.mli b/src/io/IO_unix.mli.tmp
similarity index 100%
rename from src/io/IO_unix.mli
rename to src/io/IO_unix.mli.tmp
diff --git a/src/io/cancel_handle.ml b/src/io/cancel_handle.ml
new file mode 100644
index 00000000..89d653ca
--- /dev/null
+++ b/src/io/cancel_handle.ml
@@ -0,0 +1,8 @@
+(** A handle to cancel something *)
+type t =
+  | Cancel1 : ('a -> unit) * 'a -> t
+  | Cancel2 : ('a -> 'b -> unit) * 'a * 'b -> t
+
+let cancel = function
+  | Cancel1 (f, x) -> f x
+  | Cancel2 (f, x, y) -> f x y
diff --git a/src/io/common_.ml b/src/io/common_.ml
index 6cfcb1a8..1bbb377b 100644
--- a/src/io/common_.ml
+++ b/src/io/common_.ml
@@ -1,6 +1,6 @@
+module A = Moonpool.Atomic
 module FLS = Moonpool_fib.Fls
+module Fiber = Moonpool_fib.Fiber
 
 let ( let@ ) = ( @@ )
 let spf = Printf.sprintf
-
-type cancel_handle = { cancel: unit -> unit } [@@unboxed]
diff --git a/src/io/ev_loop.ml b/src/io/ev_loop.ml
new file mode 100644
index 00000000..3c085a7d
--- /dev/null
+++ b/src/io/ev_loop.ml
@@ -0,0 +1,358 @@
+open Common_
+module B_queue = Moonpool.Blocking_queue
+
+type io_mode =
+  | Read
+  | Write
+
+module IO_wait = struct
+  type t = { f: unit -> unit } [@@unboxed]
+  (** A single event, waiting on a unix FD *)
+
+  let[@inline] make f : t = { f }
+end
+
+module Per_fd = struct
+  type t = {
+    fd: Unix.file_descr;
+    mutable reads: IO_wait.t Handle.Map.t;
+    mutable writes: IO_wait.t Handle.Map.t;
+  }
+
+  let[@inline] no_reads self = Handle.Map.is_empty self.reads
+  let[@inline] no_writes self = Handle.Map.is_empty self.writes
+  let[@inline] is_empty self = no_reads self && no_writes self
+
+  let cancel_read (self : t) h : bool =
+    if Handle.Map.mem h self.reads then (
+      self.reads <- Handle.Map.remove h self.reads;
+      true
+    ) else
+      false
+
+  let cancel_write (self : t) h : bool =
+    if Handle.Map.mem h self.writes then (
+      self.reads <- Handle.Map.remove h self.writes;
+      true
+    ) else
+      false
+end
+
+module IO_tbl = struct
+  type t = {
+    mutable n_read: int;
+    mutable n_write: int;
+    tbl: (Unix.file_descr, Per_fd.t) Hashtbl.t;
+  }
+
+  let create () : t = { tbl = Hashtbl.create 32; n_read = 0; n_write = 0 }
+
+  let get_or_create (self : t) fd : Per_fd.t =
+    try Hashtbl.find self.tbl fd
+    with Not_found ->
+      let per_fd =
+        { Per_fd.fd; reads = Handle.Map.empty; writes = Handle.Map.empty }
+      in
+      Hashtbl.add self.tbl fd per_fd;
+      per_fd
+
+  let update_subs poll (per_fd : Per_fd.t) =
+    let ev =
+      match Per_fd.no_reads per_fd, Per_fd.no_writes per_fd with
+      | true, true -> Poll.Event.none
+      | true, false -> Poll.Event.write
+      | false, true -> Poll.Event.read
+      | false, false -> Poll.Event.read_write
+    in
+    Poll.set poll per_fd.fd ev
+
+  let add_io_wait (self : t) poll fd mode (h : Handle.t) (ev : IO_wait.t) =
+    let per_fd = get_or_create self fd in
+    (match mode with
+    | Read ->
+      self.n_read <- 1 + self.n_read;
+      per_fd.reads <- Handle.Map.add h ev per_fd.reads
+    | Write ->
+      self.n_write <- 1 + self.n_write;
+      per_fd.writes <- Handle.Map.add h ev per_fd.writes);
+    update_subs poll per_fd
+
+  let cancel (self : t) (fd : Unix.file_descr) (h : Handle.t) : unit =
+    match Hashtbl.find_opt self.tbl fd, Handle.handle_type h with
+    | None, _ -> ()
+    | Some per_fd, H_read ->
+      if Per_fd.cancel_read per_fd h then self.n_read <- self.n_read - 1
+    | Some per_fd, H_write ->
+      if Per_fd.cancel_write per_fd h then self.n_write <- self.n_write - 1
+    | Some _, H_timer -> assert false
+
+  let[@inline] trigger_waiter (_h : Handle.t) (io : IO_wait.t) = io.f ()
+
+  let handle_ready_read (self : t) ~ignore_read fd (per_fd : Per_fd.t) =
+    if fd <> ignore_read then (
+      Handle.Map.iter trigger_waiter per_fd.reads;
+      self.n_read <- self.n_read - Handle.Map.cardinal per_fd.reads;
+      per_fd.reads <- Handle.Map.empty
+    )
+
+  let handle_ready_write (self : t) (per_fd : Per_fd.t) =
+    Handle.Map.iter trigger_waiter per_fd.writes;
+    self.n_write <- self.n_write - Handle.Map.cardinal per_fd.writes;
+    per_fd.writes <- Handle.Map.empty
+
+  let update_all (self : t) ~ignore_read (poll : Poll.t) : unit =
+    let handle_ready fd (ev : Poll.Event.t) =
+      match Hashtbl.find self.tbl fd with
+      | exception Not_found -> ()
+      | per_fd ->
+        if ev.readable then handle_ready_read self ~ignore_read fd per_fd;
+        if ev.writable then handle_ready_write self per_fd;
+        update_subs poll per_fd;
+        if Per_fd.is_empty per_fd then Hashtbl.remove self.tbl fd
+    in
+    Poll.iter_ready poll ~f:handle_ready
+end
+
+(** Messages from other threads *)
+module Incoming_msg = struct
+  type t =
+    | On_ready of Unix.file_descr * io_mode * Handle.t * (unit -> unit)
+    | Run_after of float * Handle.t * (unit -> unit)
+    | Run_every of float * Handle.t * (Cancel_handle.t -> unit)
+    | Cancel_io of Unix.file_descr * Handle.t
+    | Cancel_timer of Handle.t
+end
+
+module Main_loop = struct
+  (** Process timers that have expired, and return the duration until the next timer *)
+  let process_expired_timers_ (t : Timer.t) : float option =
+    let rec loop () =
+      match Timer.next t with
+      | Timer.Empty -> None
+      | Timer.Run f ->
+        f ();
+        loop ()
+      | Timer.Wait f ->
+        if f > 0. then
+          Some f
+        else
+          None
+    in
+    loop ()
+
+  type state = {
+    timer: Timer.t;
+    io_tbl: IO_tbl.t;
+    incoming: Incoming_msg.t B_queue.t;
+    poll: Poll.t;
+    in_poll: bool A.t;
+    buf4: bytes;
+    _magic_pipe_read: Unix.file_descr;
+    _magic_pipe_write: Unix.file_descr;
+  }
+
+  let create () : state =
+    let _magic_pipe_read, _magic_pipe_write = Unix.pipe ~cloexec:true () in
+    Unix.set_nonblock _magic_pipe_read;
+    Unix.set_nonblock _magic_pipe_write;
+    let poll = Poll.create () in
+    {
+      timer = Timer.create ();
+      io_tbl = IO_tbl.create ();
+      incoming = B_queue.create ();
+      in_poll = A.make false;
+      poll;
+      buf4 = Bytes.create 4;
+      _magic_pipe_read;
+      _magic_pipe_write;
+    }
+
+  let push (self : state) (msg : Incoming_msg.t) =
+    B_queue.push self.incoming msg;
+    if A.exchange self.in_poll false then (
+      (* maybe wake up the loop *)
+      let b = Bytes.make 1 '\x01' in
+      ignore (Unix.write self._magic_pipe_write b 0 1 : int)
+    )
+
+  let cancel_timer_ (self : state) (h : Handle.t) =
+    push self (Incoming_msg.Cancel_timer h)
+
+  (** Make sure the pipe is empty *)
+  let drain_pipe_ (self : state) =
+    while
+      match Unix.read self._magic_pipe_read self.buf4 0 4 with
+      | 0 -> false
+      | _n -> true
+      | exception Unix.Unix_error ((Unix.EAGAIN | Unix.EWOULDBLOCK), _, _) ->
+        false
+    do
+      ()
+    done
+
+  let process_msg (self : state) (msg : Incoming_msg.t) =
+    match msg with
+    | On_ready (fd, mode, h, f) ->
+      IO_tbl.add_io_wait self.io_tbl self.poll fd mode h (IO_wait.make f)
+    | Run_after (delay, h, f) -> Timer.run_after_s self.timer delay h f
+    | Run_every (delay, h, f) ->
+      let cancel = Cancel_handle.Cancel2 (cancel_timer_, self, h) in
+      Timer.run_every_s self.timer delay h (fun () -> f cancel)
+    | Cancel_io (fd, h) -> IO_tbl.cancel self.io_tbl fd h
+    | Cancel_timer h -> Timer.cancel self.timer h
+
+  let loop (self : state) : unit =
+    let local = Queue.create () in
+    while true do
+      (* process incoming messages *)
+      B_queue.transfer self.incoming local;
+      while not (Queue.is_empty local) do
+        let msg = Queue.pop local in
+        process_msg self msg
+      done;
+
+      (* check IOs *)
+      IO_tbl.update_all self.io_tbl ~ignore_read:self._magic_pipe_read self.poll;
+
+      (* update timers, get next timeout *)
+      let timer = process_expired_timers_ self.timer in
+      let timeout =
+        match timer with
+        | None -> Poll.Timeout.Never
+        | Some t_s ->
+          let t_ns = Int64.of_float @@ ceil @@ (t_s *. 1e9) in
+          Poll.Timeout.After t_ns
+      in
+
+      (* wait for something to happen *)
+      A.set self.in_poll true;
+      ignore (Poll.wait self.poll timeout : [ `Ok | `Timeout ]);
+      A.set self.in_poll false;
+
+      drain_pipe_ self;
+
+      IO_tbl.update_all self.io_tbl ~ignore_read:self._magic_pipe_read self.poll
+    done
+end
+
+type t = Main_loop.state
+
+let cur_ : t option A.t = A.make None
+
+open struct
+  (* used only for init *)
+  let m = Mutex.create ()
+
+  let[@inline never] create_and_set_ () : t =
+    Mutex.lock m;
+    match A.get cur_ with
+    | Some t ->
+      Mutex.unlock m;
+      t
+    | None ->
+      let st = Main_loop.create () in
+      (* start background thread *)
+      let _t : Thread.t =
+        Moonpool.start_thread_on_some_domain Main_loop.loop st
+      in
+      A.set cur_ (Some st);
+      Mutex.unlock m;
+      st
+end
+
+let[@inline] get_or_create () : t =
+  match A.get cur_ with
+  | Some t -> t
+  | None -> create_and_set_ ()
+
+let on_readable (self : t) fd f : Cancel_handle.t =
+  let h = Handle.fresh H_read in
+  Main_loop.push self (Incoming_msg.On_ready (fd, Read, h, f));
+  Cancel_handle.Cancel2 (Main_loop.push, self, Incoming_msg.Cancel_io (fd, h))
+
+let on_writable (self : t) fd f : Cancel_handle.t =
+  let h = Handle.fresh H_write in
+  Main_loop.push self (Incoming_msg.On_ready (fd, Write, h, f));
+  Cancel_handle.Cancel2 (Main_loop.push, self, Incoming_msg.Cancel_io (fd, h))
+
+let run_after_s (self : t) delay f : Cancel_handle.t =
+  let h = Handle.fresh H_timer in
+  Main_loop.push self (Incoming_msg.Run_after (delay, h, f));
+  Cancel_handle.Cancel2 (Main_loop.push, self, Incoming_msg.Cancel_timer h)
+
+let run_every_s (self : t) delay f : Cancel_handle.t =
+  let h = Handle.fresh H_timer in
+  Main_loop.push self (Incoming_msg.Run_every (delay, h, f));
+  Cancel_handle.Cancel2 (Main_loop.push, self, Incoming_msg.Cancel_timer h)
+
+(*
+class unix_ev_loop =
+  let _timer = Timer.create () in
+  let _io_wait : IO_tbl.t = IO_tbl.create () in
+  let _in_blocking_section = ref false in
+
+  let _magic_pipe_read, _magic_pipe_write = Unix.pipe ~cloexec:true () in
+  let () =
+    Unix.set_nonblock _magic_pipe_read;
+    Unix.set_nonblock _magic_pipe_write
+  in
+
+  let[@inline] has_pending_tasks () =
+    _io_wait.n_read > 0 || _io_wait.n_write > 0 || Timer.has_tasks _timer
+  in
+
+  object
+    (* val read_ : (cancel_handle -> unit) Int_tbl.t = Int_tbl.create 32 *)
+    method one_step ~block () : unit =
+      let delay = process_expired_timers_ _timer in
+
+      let delay =
+        if block then
+          Option.value delay ~default:10.
+        else
+          (* do not wait *)
+          0.
+      in
+
+      let reads, writes = IO_tbl.prepare_select _io_wait in
+      if has_pending_tasks () then (
+        _in_blocking_section := true;
+        let reads, writes, _ =
+          Unix.select (_magic_pipe_read :: reads) writes [] delay
+        in
+        _in_blocking_section := false;
+        IO_tbl.handle_ready ~ignore_read:_magic_pipe_read _io_wait reads writes
+      );
+      ()
+
+    method on_readable
+        : Unix.file_descr -> (cancel_handle -> unit) -> cancel_handle =
+      fun fd f : cancel_handle ->
+        let ev = IO_wait.make f in
+        IO_tbl.add_io_wait _io_wait fd Read ev;
+        ev.as_cancel_handle
+
+    method on_writable
+        : Unix.file_descr -> (cancel_handle -> unit) -> cancel_handle =
+      fun fd f : cancel_handle ->
+        let ev = IO_wait.make f in
+        IO_tbl.add_io_wait _io_wait fd Write ev;
+        ev.as_cancel_handle
+
+    method on_timer
+        : float -> repeat:bool -> (cancel_handle -> unit) -> cancel_handle =
+      fun delay ~repeat f ->
+        if repeat then
+          Timer.run_every _timer delay f
+        else
+          Timer.run_after _timer delay f
+
+    method interrupt_if_in_blocking_section =
+      if !_in_blocking_section then (
+        let b = Bytes.create 1 in
+        ignore (Unix.write _magic_pipe_write b 0 1 : int)
+      )
+
+    method has_pending_tasks : bool = has_pending_tasks ()
+  end
+  *)
diff --git a/src/io/ev_loop.mli b/src/io/ev_loop.mli
new file mode 100644
index 00000000..047bade6
--- /dev/null
+++ b/src/io/ev_loop.mli
@@ -0,0 +1,13 @@
+open Common_
+
+type io_mode =
+  | Read
+  | Write
+
+type t
+
+val get_or_create : unit -> t
+val on_readable : t -> Unix.file_descr -> (unit -> unit) -> Cancel_handle.t
+val on_writable : t -> Unix.file_descr -> (unit -> unit) -> Cancel_handle.t
+val run_after_s : t -> float -> (unit -> unit) -> Cancel_handle.t
+val run_every_s : t -> float -> (Cancel_handle.t -> unit) -> Cancel_handle.t
diff --git a/src/io/handle.ml b/src/io/handle.ml
new file mode 100644
index 00000000..7b4f2f98
--- /dev/null
+++ b/src/io/handle.ml
@@ -0,0 +1,48 @@
+open Common_
+
+type handle_type =
+  | H_read
+  | H_write
+  | H_timer
+
+type t = int
+
+let n_bits_shard = 3
+let n_shards_ = 8
+let shard_mask_ = (1 lsl n_bits_shard) - 1
+let () = assert (1 lsl n_bits_shard = n_shards_)
+
+(* array of counters, sharded on thread ID to reduce contention.
+   TODO: avoid false sharing *)
+let generators_ : int A.t array = Array.init n_shards_ (fun _ -> A.make 0)
+
+let int_of_handle_type = function
+  | H_read -> 0
+  | H_write -> 1
+  | H_timer -> 2
+
+let[@inline] handle_type (self : t) : handle_type =
+  match self land 0b11 with
+  | 0 -> H_read
+  | 1 -> H_write
+  | 2 -> H_timer
+  | _ -> assert false
+
+let fresh (ty : handle_type) : t =
+  let shard = (Thread.id @@ Thread.self ()) land shard_mask_ in
+  let n = A.fetch_and_add (Array.unsafe_get generators_ shard) 1 in
+  let n =
+    (n lsl (2 + n_bits_shard)) lor (shard lsl 2) lor int_of_handle_type ty
+  in
+  n
+
+module As_key = struct
+  type t = int
+
+  let equal : t -> t -> bool = ( = )
+  let hash = Hashtbl.hash
+  let compare : t -> t -> int = Stdlib.compare
+end
+
+module Map = Map.Make (As_key)
+module Tbl = Hashtbl.Make (As_key)
diff --git a/src/io/handle.mli b/src/io/handle.mli
new file mode 100644
index 00000000..ff580d05
--- /dev/null
+++ b/src/io/handle.mli
@@ -0,0 +1,21 @@
+(** Handles.
+
+    Each subscription has a unique handle that is used
+    to cancel it and refer to it. *)
+
+type handle_type =
+  | H_read
+  | H_write
+  | H_timer
+
+type t = private int
+(** A handle. Its internal structure is unspecified. *)
+
+val fresh : handle_type -> t
+(** Get a fresh handle *)
+
+val handle_type : t -> handle_type
+(** Recover the type of the handle *)
+
+module Map : Map.S with type key = t
+module Tbl : Hashtbl.S with type key = t
diff --git a/src/io/heap_.ml b/src/io/heap_.ml
index 752585ec..12fa0d9e 100644
--- a/src/io/heap_.ml
+++ b/src/io/heap_.ml
@@ -26,24 +26,9 @@ module type S = sig
   val find_min : t -> elt option
   (** [find_min h] find the minimal element of the heap [h]. *)
 
-  val find_min_exn : t -> elt
-  (** [find_min_exn h] is like {!find_min} but can fail.
-      @raise Empty if the heap is empty. *)
-
-  val take : t -> (t * elt) option
-  (** [take h] extracts and returns the minimum element, and the new heap (without
-      this element), or [None] if the heap [h] is empty. *)
-
   val take_exn : t -> t * elt
   (** [take_exn h] is like {!take}, but can fail.
       @raise Empty if the heap is empty. *)
-
-  val delete_one : (elt -> elt -> bool) -> elt -> t -> t
-  (** [delete_one eq x h] uses [eq] to find one occurrence of a value [x]
-      if it exist in the heap [h], and delete it.
-      If [h] do not contain [x] then it return [h]. *)
-
-  val size : t -> int
 end
 
 module Make (E : PARTIAL_ORD) : S with type elt = E.t = struct
@@ -88,46 +73,11 @@ module Make (E : PARTIAL_ORD) : S with type elt = E.t = struct
 
   let insert x h = merge (N (1, x, E, E)) h
 
-  let find_min_exn = function
-    | E -> raise Empty
-    | N (_, x, _, _) -> x
-
   let find_min = function
     | E -> None
     | N (_, x, _, _) -> Some x
 
-  let take = function
-    | E -> None
-    | N (_, x, l, r) -> Some (merge l r, x)
-
   let take_exn = function
     | E -> raise Empty
     | N (_, x, l, r) -> merge l r, x
-
-  let delete_one eq x h =
-    let rec aux = function
-      | E -> false, E
-      | N (_, y, l, r) as h ->
-        if eq x y then
-          true, merge l r
-        else if E.leq y x then (
-          let found_left, l1 = aux l in
-          let found, r1 =
-            if found_left then
-              true, r
-            else
-              aux r
-          in
-          if found then
-            true, _make_node y l1 r1
-          else
-            false, h
-        ) else
-          false, h
-    in
-    snd (aux h)
-
-  let rec size = function
-    | E -> 0
-    | N (_, _, l, r) -> 1 + size l + size r
 end
diff --git a/src/io/heap_.mli b/src/io/heap_.mli
index 3d60f5c9..5393395f 100644
--- a/src/io/heap_.mli
+++ b/src/io/heap_.mli
@@ -30,24 +30,9 @@ module type S = sig
   val find_min : t -> elt option
   (** [find_min h] find the minimal element of the heap [h]. *)
 
-  val find_min_exn : t -> elt
-  (** [find_min_exn h] is like {!find_min} but can fail.
-      @raise Empty if the heap is empty. *)
-
-  val take : t -> (t * elt) option
-  (** [take h] extracts and returns the minimum element, and the new heap (without
-      this element), or [None] if the heap [h] is empty. *)
-
   val take_exn : t -> t * elt
   (** [take_exn h] is like {!take}, but can fail.
       @raise Empty if the heap is empty. *)
-
-  val delete_one : (elt -> elt -> bool) -> elt -> t -> t
-  (** [delete_one eq x h] uses [eq] to find one occurrence of a value [x]
-      if it exist in the heap [h], and delete it.
-      If [h] do not contain [x] then it return [h]. *)
-
-  val size : t -> int
 end
 
 module Make (E : PARTIAL_ORD) : S with type elt = E.t
diff --git a/src/io/moonpool_io.ml b/src/io/moonpool_io.ml
index ead33a7b..d6958c13 100644
--- a/src/io/moonpool_io.ml
+++ b/src/io/moonpool_io.ml
@@ -1,10 +1,5 @@
 open Common_
-include Fuseau
-module IO_unix = IO_unix
 module Timer = Timer
-module Net = Net
-
-let main f =
-  let loop = new U_loop.unix_ev_loop in
-  let@ () = U_loop.with_cur loop in
-  Fuseau.main ~loop:(loop :> Event_loop.t) f
+module Ev_loop = Ev_loop
+(* TODO: module IO_unix = IO_unix *)
+(* TODO: module Net = Net *)
diff --git a/src/io/moonpool_io.mli b/src/io/moonpool_io.mli
index c9bad41e..1fae812e 100644
--- a/src/io/moonpool_io.mli
+++ b/src/io/moonpool_io.mli
@@ -1,20 +1,4 @@
-(** Simple event loop based on {!Unix.select}.
-
-    This library combines {!Fuseau}'s fibers with a simple
-    event loop for IOs based on {!Unix.select}.
-    It's useful for simple situations or portability.
-    For bigger system it's probably better to use another
-    event loop. *)
-
-include module type of struct
-  include Moonpool_fib
-end
-
-module IO_unix = IO_unix
-module Net = Net
+(* module IO_unix = IO_unix *)
+(* module Net = Net *)
 module Timer = Timer
-
-(* FIXME: lazy background thread for the poll loop? *)
-
-val main : (unit -> 'a) -> 'a
-(** A version of {!Moonpool_fib.main} that uses a Unix-based event loop. *)
+module Ev_loop = Ev_loop
diff --git a/src/io/net.ml b/src/io/net.ml.tmp
similarity index 90%
rename from src/io/net.ml
rename to src/io/net.ml.tmp
index e1817905..8db14797 100644
--- a/src/io/net.ml
+++ b/src/io/net.ml.tmp
@@ -35,16 +35,17 @@ module Sockaddr = struct
 end
 
 module TCP_server = struct
-  type t = { fiber: unit Fiber.t } [@@unboxed]
+  type t = { fut: unit Fut.t } [@@unboxed]
 
   exception Stop
 
-  let stop_ fiber =
-    let ebt = Exn_bt.get Stop in
-    Fuseau.Fiber.Private_.cancel fiber ebt
+  let stop_ fut =
+    let ebt = Exn_bt.get_callstack 20 Stop in
+    let promise = Fut.Private_.unsafe_promise_of_fut fut in
+    Fut.fulfill_idempotent promise (Error ebt)
 
-  let stop self = stop_ self.fiber
-  let join self = Fuseau.await self.fiber
+  let stop self = stop_ self.fut
+  let join self = Fut.await self.fut
 
   let with_serve' (addr : Sockaddr.t) handle_client (f : t -> 'a) : 'a =
     let sock = Unix.socket Unix.PF_INET Unix.SOCK_STREAM 0 in
@@ -54,8 +55,8 @@ module TCP_server = struct
     Unix.setsockopt sock Unix.SO_REUSEADDR true;
     Unix.listen sock 32;
 
-    let fiber = Fuseau.Fiber.Private_.create () in
-    let self = { fiber } in
+    let fut, _ = Fut.make () in
+    let self = { fut } in
 
     let loop_client client_sock client_addr : unit =
       Unix.set_nonblock client_sock;
@@ -69,11 +70,11 @@ module TCP_server = struct
     in
 
     let loop () =
-      while not (Fiber.is_done fiber) do
+      while not (Fut.is_done fut) do
         match Unix.accept sock with
         | client_sock, client_addr ->
           ignore
-            (Fuseau.spawn ~propagate_cancel_to_parent:false (fun () ->
+            (Fiber.spawn ~protect:true (fun () ->
                  loop_client client_sock client_addr)
               : _ Fiber.t)
         | exception Unix.Unix_error ((Unix.EAGAIN | Unix.EWOULDBLOCK), _, _) ->
diff --git a/src/io/net.mli b/src/io/net.mli.tmp
similarity index 100%
rename from src/io/net.mli
rename to src/io/net.mli.tmp
diff --git a/src/io/timer.ml b/src/io/timer.ml
index 4346bccc..c7ddddc2 100644
--- a/src/io/timer.ml
+++ b/src/io/timer.ml
@@ -1,18 +1,22 @@
-open Common_
 module Time = Time_
 
 type instant_s = float
 type duration_s = float
 
+type tick_res =
+  | Wait of float
+  | Run of (unit -> unit)
+  | Empty
+
 type kind =
   | Once
   | Every of duration_s
 
 type task = {
+  handle: Handle.t;
   mutable deadline: instant_s;
   mutable active: bool;
-  f: cancel_handle -> unit;
-  as_cancel_handle: cancel_handle;
+  mutable f: unit -> unit;
   kind: kind;
 }
 
@@ -22,52 +26,41 @@ module Task_heap = Heap_.Make (struct
   let[@inline] leq t1 t2 = t1.deadline <= t2.deadline
 end)
 
-type t = { mutable tasks: Task_heap.t }
+type t = {
+  mutable tasks: Task_heap.t;
+  by_handle: task Handle.Tbl.t;
+}
 
 (** accepted time diff for actions. *)
 let epsilon_s = 0.000_001
 
-type tick_res =
-  | Wait of float
-  | Run of (cancel_handle -> unit) * cancel_handle
-  | Empty
-
 let[@inline] has_tasks self = not (Task_heap.is_empty self.tasks)
-let[@inline] num_tasks self : int = Task_heap.size self.tasks
 
-let[@inline] pop_task_ self : unit =
-  let tasks, _t = Task_heap.take_exn self.tasks in
+let pop_task_ self : unit =
+  let tasks, t = Task_heap.take_exn self.tasks in
+  Handle.Tbl.remove self.by_handle t.handle;
   self.tasks <- tasks
 
-let run_after self delay f : cancel_handle =
-  let now = Time.time_s () in
-  let deadline = now +. delay in
-  let rec task =
-    {
-      deadline;
-      f;
-      kind = Once;
-      active = true;
-      as_cancel_handle = { cancel = (fun () -> task.active <- false) };
-    }
-  in
-  self.tasks <- Task_heap.insert task self.tasks;
-  task.as_cancel_handle
+let cancel (self : t) (h : Handle.t) =
+  match Handle.Tbl.find_opt self.by_handle h with
+  | None -> ()
+  | Some t ->
+    t.active <- false;
+    t.f <- ignore (* free memory captured by [f] *)
 
-let run_every self delay f : cancel_handle =
+let run_after_s self delay handle f : unit =
   let now = Time.time_s () in
   let deadline = now +. delay in
-  let rec task =
-    {
-      deadline;
-      f;
-      kind = Every delay;
-      active = true;
-      as_cancel_handle = { cancel = (fun () -> task.active <- false) };
-    }
-  in
+  let task = { handle; deadline; f; kind = Once; active = true } in
   self.tasks <- Task_heap.insert task self.tasks;
-  task.as_cancel_handle
+  Handle.Tbl.add self.by_handle handle task
+
+let run_every_s self delay handle f : unit =
+  let now = Time.time_s () in
+  let deadline = now +. delay in
+  let task = { handle; deadline; f; kind = Every delay; active = true } in
+  self.tasks <- Task_heap.insert task self.tasks;
+  Handle.Tbl.add self.by_handle handle task
 
 let rec next (self : t) : tick_res =
   match Task_heap.find_min self.tasks with
@@ -85,12 +78,13 @@ let rec next (self : t) : tick_res =
       (match task.kind with
       | Once -> ()
       | Every dur ->
-        (* schedule the next iteration *)
-        task.deadline <- now +. dur;
+        (* schedule the next iteration. If we're past the deadline,
+           use the deadline as the starting point for the next iteration. *)
+        task.deadline <- min now task.deadline +. dur;
         self.tasks <- Task_heap.insert task self.tasks);
 
-      Run (task.f, task.as_cancel_handle)
+      Run task.f
     ) else
       Wait remaining_time_s
 
-let create () = { tasks = Task_heap.empty }
+let create () = { tasks = Task_heap.empty; by_handle = Handle.Tbl.create 32 }
diff --git a/src/io/timer.mli b/src/io/timer.mli
index 42f6c0ed..e56e4499 100644
--- a/src/io/timer.mli
+++ b/src/io/timer.mli
@@ -1,17 +1,18 @@
-open Common_
+(** A simple timer implementation *)
 
 type t
+(** A timer. Not thread-safe. *)
 
 val create : unit -> t
 (** A new timer. *)
 
 type tick_res =
   | Wait of float
-  | Run of (cancel_handle -> unit) * cancel_handle
+  | Run of (unit -> unit)
   | Empty
 
 val next : t -> tick_res
-val run_after : t -> float -> (cancel_handle -> unit) -> cancel_handle
-val run_every : t -> float -> (cancel_handle -> unit) -> cancel_handle
+val run_after_s : t -> float -> Handle.t -> (unit -> unit) -> unit
+val run_every_s : t -> float -> Handle.t -> (unit -> unit) -> unit
+val cancel : t -> Handle.t -> unit
 val has_tasks : t -> bool
-val num_tasks : t -> int
diff --git a/src/io/u_loop.ml b/src/io/u_loop.ml
deleted file mode 100644
index 04fcc1d8..00000000
--- a/src/io/u_loop.ml
+++ /dev/null
@@ -1,202 +0,0 @@
-open Common_
-
-type io_mode =
-  | Read
-  | Write
-
-module IO_wait = struct
-  type t = {
-    mutable active: bool;
-    f: cancel_handle -> unit;
-    as_cancel_handle: cancel_handle;
-  }
-  (** A single event, waiting on a unix FD *)
-
-  let make f : t =
-    let rec self =
-      {
-        active = true;
-        f;
-        as_cancel_handle = { cancel = (fun () -> self.active <- false) };
-      }
-    in
-    self
-end
-
-module Per_fd = struct
-  type t = {
-    fd: Unix.file_descr;
-    mutable reads: IO_wait.t list;
-    mutable writes: IO_wait.t list;
-  }
-
-  let[@inline] is_empty self = self.reads = [] && self.writes = []
-end
-
-module IO_tbl = struct
-  type t = {
-    mutable n_read: int;
-    mutable n_write: int;
-    tbl: (Unix.file_descr, Per_fd.t) Hashtbl.t;
-  }
-
-  let create () : t = { tbl = Hashtbl.create 32; n_read = 0; n_write = 0 }
-
-  let get_or_create (self : t) fd : Per_fd.t =
-    try Hashtbl.find self.tbl fd
-    with Not_found ->
-      let per_fd = { Per_fd.fd; reads = []; writes = [] } in
-      Hashtbl.add self.tbl fd per_fd;
-      per_fd
-
-  let add_io_wait (self : t) fd mode (ev : IO_wait.t) =
-    let per_fd = get_or_create self fd in
-    match mode with
-    | Read ->
-      self.n_read <- 1 + self.n_read;
-      per_fd.reads <- ev :: per_fd.reads
-    | Write ->
-      self.n_write <- 1 + self.n_write;
-      per_fd.writes <- ev :: per_fd.writes
-
-  let prepare_select (self : t) =
-    let reads = ref [] in
-    let writes = ref [] in
-    Hashtbl.iter
-      (fun _ (per_fd : Per_fd.t) ->
-        if Per_fd.is_empty per_fd then
-          Hashtbl.remove self.tbl per_fd.fd
-        else (
-          if per_fd.reads <> [] then reads := per_fd.fd :: !reads;
-          if per_fd.writes <> [] then writes := per_fd.fd :: !writes
-        ))
-      self.tbl;
-    !reads, !writes
-
-  let trigger_waiter (io : IO_wait.t) =
-    if io.active then io.f io.as_cancel_handle
-
-  let handle_ready ~ignore_read (self : t) (reads : Unix.file_descr list)
-      (writes : Unix.file_descr list) : unit =
-    List.iter
-      (fun fd ->
-        if fd <> ignore_read then (
-          let per_fd = Hashtbl.find self.tbl fd in
-          List.iter trigger_waiter per_fd.reads;
-          self.n_read <- self.n_read - List.length per_fd.reads;
-          per_fd.reads <- []
-        ))
-      reads;
-
-    List.iter
-      (fun fd ->
-        let per_fd = Hashtbl.find self.tbl fd in
-        List.iter trigger_waiter per_fd.writes;
-        self.n_write <- self.n_write - List.length per_fd.writes;
-        per_fd.writes <- [])
-      writes;
-    ()
-end
-
-let run_timer_ (t : Timer.t) =
-  let rec loop () =
-    match Timer.next t with
-    | Timer.Empty -> None
-    | Timer.Run (f, ev_h) ->
-      f ev_h;
-      loop ()
-    | Timer.Wait f ->
-      if f > 0. then
-        Some f
-      else
-        None
-  in
-  loop ()
-
-class unix_ev_loop =
-  let _timer = Timer.create () in
-  let _io_wait : IO_tbl.t = IO_tbl.create () in
-  let _in_blocking_section = ref false in
-
-  let _magic_pipe_read, _magic_pipe_write = Unix.pipe ~cloexec:true () in
-  let () =
-    Unix.set_nonblock _magic_pipe_read;
-    Unix.set_nonblock _magic_pipe_write
-  in
-
-  let[@inline] has_pending_tasks () =
-    _io_wait.n_read > 0 || _io_wait.n_write > 0 || Timer.has_tasks _timer
-  in
-
-  object
-    (* val read_ : (cancel_handle -> unit) Int_tbl.t = Int_tbl.create 32 *)
-    method one_step ~block () : unit =
-      let delay = run_timer_ _timer in
-
-      let delay =
-        if block then
-          Option.value delay ~default:10.
-        else
-          (* do not wait *)
-          0.
-      in
-
-      let reads, writes = IO_tbl.prepare_select _io_wait in
-      if has_pending_tasks () then (
-        _in_blocking_section := true;
-        let reads, writes, _ =
-          Unix.select (_magic_pipe_read :: reads) writes [] delay
-        in
-        _in_blocking_section := false;
-        IO_tbl.handle_ready ~ignore_read:_magic_pipe_read _io_wait reads writes
-      );
-      ()
-
-    method on_readable
-        : Unix.file_descr -> (cancel_handle -> unit) -> cancel_handle =
-      fun fd f : cancel_handle ->
-        let ev = IO_wait.make f in
-        IO_tbl.add_io_wait _io_wait fd Read ev;
-        ev.as_cancel_handle
-
-    method on_writable
-        : Unix.file_descr -> (cancel_handle -> unit) -> cancel_handle =
-      fun fd f : cancel_handle ->
-        let ev = IO_wait.make f in
-        IO_tbl.add_io_wait _io_wait fd Write ev;
-        ev.as_cancel_handle
-
-    method on_timer
-        : float -> repeat:bool -> (cancel_handle -> unit) -> cancel_handle =
-      fun delay ~repeat f ->
-        if repeat then
-          Timer.run_every _timer delay f
-        else
-          Timer.run_after _timer delay f
-
-    method interrupt_if_in_blocking_section =
-      if !_in_blocking_section then (
-        let b = Bytes.create 1 in
-        ignore (Unix.write _magic_pipe_write b 0 1 : int)
-      )
-
-    method has_pending_tasks : bool = has_pending_tasks ()
-  end
-
-open struct
-  let k_ev_loop : unix_ev_loop option ref TLS.key =
-    TLS.new_key (fun () -> ref None)
-end
-
-(** Access the event loop from within it *)
-let[@inline] cur () =
-  match !(TLS.get k_ev_loop) with
-  | None -> failwith "must be called from inside Fuseau_unix"
-  | Some ev -> ev
-
-let with_cur (ev : unix_ev_loop) f =
-  let r = TLS.get k_ev_loop in
-  let old = !r in
-  r := Some ev;
-  let finally () = r := old in
-  Fun.protect ~finally f