diff --git a/src/posix/nanoev_posix.ml b/src/posix/nanoev_posix.ml
index 8601421..ccc6806 100644
--- a/src/posix/nanoev_posix.ml
+++ b/src/posix/nanoev_posix.ml
@@ -3,6 +3,8 @@ open struct
 
   let ( let@ ) = ( @@ )
   let now_ns : unit -> int64 = Mtime_clock.now_ns
+  let[@inline] ns_of_s (t : float) : int64 = Int64.of_float (t *. 1e9)
+  let[@inline] ns_to_s (t : int64) : float = Int64.to_float t /. 1e9
 end
 
 module Fd_tbl = Hashtbl.Make (struct
@@ -16,9 +18,7 @@ end)
 
 module P = Iomux.Poll
 module Flags = P.Flags
-open Iomux.Util
 
-(* TODO: remove
 module Sync_queue = struct
   type 'a t = {
     q: 'a Queue.t;
@@ -31,68 +31,133 @@ module Sync_queue = struct
     Mutex.lock self.mutex;
     Queue.push x self.q;
     Mutex.unlock self.mutex
+
+  let transfer (self : _ t) q : unit =
+    Mutex.lock self.mutex;
+    Queue.transfer self.q q;
+    Mutex.unlock self.mutex
 end
-*)
 
 (** Callback list *)
 type cbs =
   | Nil
   | Sub : 'a * 'b * (closed:bool -> 'a -> 'b -> unit) * cbs -> cbs
 
-(** Single callback *)
-type cb = Cb : 'a * 'b * (closed:bool -> 'a -> 'b -> unit) -> cb
-
-let[@inline] cb_is_empty = function
-  | Nil -> true
-  | Sub _ -> false
-
 type timer_ev =
   | Timer : {
-      deadline: float;
+      deadline: int64;
       x: 'a;
       y: 'b;
       f: 'a -> 'b -> unit;
     }
       -> timer_ev
 
-type per_fd = {
+type fd_data = {
   fd: Unix.file_descr;
   mutable idx: int;
-      (** Index in the buffer. Can change because we swap FDs sometimes to
-          remove items. *)
+      (** Index in the poll buffer. Mutable because we might change it when we
+          swap FDs to remove items. *)
   mutable r: cbs;
   mutable w: cbs;
 }
+(** Data associated to a given FD *)
 
-let[@inline] per_fd_flags (self : per_fd) : Flags.t =
+let[@inline] fd_flags (self : fd_data) : Flags.t =
   let fl = ref Flags.empty in
-  (if self.r <> Nil then fl := Flags.(!fl + pollin));
-  (if self.w <> Nil then fl := Flags.(!fl + pollout));
+  (if self.r != Nil then fl := Flags.(!fl + pollin));
+  (if self.w != Nil then fl := Flags.(!fl + pollout));
   !fl
 
 type queued_task =
-  | Q_timer of timer_ev
-  | Q_on_readable of Unix.file_descr * cb
-  | Q_on_writable of Unix.file_descr * cb
+  | Q_run_after of timer_ev
+  | Q_on_readable :
+      Unix.file_descr * 'a * 'b * (closed:bool -> 'a -> 'b -> unit)
+      -> queued_task
+  | Q_on_writable :
+      Unix.file_descr * 'a * 'b * (closed:bool -> 'a -> 'b -> unit)
+      -> queued_task
   | Q_clear
+  | Q_close of Unix.file_descr
 
 type st = {
   timer: timer_ev Heap.t;
-  fds: per_fd Fd_tbl.t;
+  fds: fd_data Fd_tbl.t;
   poll: P.t;
   mutable len: int;  (** length of the active prefix of the [poll] buffer *)
   wakeup_rd: Unix.file_descr;
   wakeup_wr: Unix.file_descr;
   wakeup_triggered: bool Atomic.t;
       (** Make [wakeup_from_outside] idempotent within an iteration of [step] *)
-  in_poll: Thread.t option Atomic.t;
+  in_poll: bool Atomic.t;
       (** Are we currently inside a call to [poll], and in which thread? Useful
           for other threads to know whether to wake us up via the pipe *)
-  queued_tasks: queued_task Queue.t;
+  mutable owner_thread: int;
+      (** Thread allowed to perform operations on this poll instance. Starts at
+          [-1]. *)
+  queued_tasks: queued_task Sync_queue.t;
       (** While in [poll()], changes get queued, so we don't invalidate the poll
           buffer before the syscall returns *)
-  lock: Mutex.t;
 }
+(* TODO: [Thread.t] field to remember the owner thread, and
+     thread-safe queue for externally queued tasks.
+     Only owner thread can call [step]. *)
+
+let[@inline] queue_task_ (self : st) t : unit =
+  Sync_queue.push self.queued_tasks t
+
+(** [true] if called from the owner thread *)
+let[@inline] in_owner_thread (self : st) : bool =
+  self.owner_thread != -1 && self.owner_thread == Thread.(id (self ()))
+
+let[@inline] in_poll (self : st) : bool = Atomic.get self.in_poll
+let[@inline] leq_timer (Timer a) (Timer b) = a.deadline <= b.deadline
+
+let create_st () : st =
+  let wakeup_rd, wakeup_wr = Unix.pipe () in
+  (* reading end must be non blocking so it's not always immediately
+    ready; writing end is blocking to make it simpler to wakeup from other
+    threads *)
+  Unix.set_nonblock wakeup_rd;
+  let self =
+    {
+      timer = Heap.create ~leq:leq_timer ();
+      fds = Fd_tbl.create 16;
+      poll = P.create ();
+      len = 0;
+      wakeup_rd;
+      wakeup_wr;
+      wakeup_triggered = Atomic.make false;
+      in_poll = Atomic.make false;
+      owner_thread = -1;
+      queued_tasks = Sync_queue.create ();
+    }
+  in
+
+  (* always watch for the pipe being readable *)
+  P.set_index self.poll 0 self.wakeup_rd Flags.pollin;
+  self.len <- 1;
+
+  self
+
+let max_fds (self : st) : int = P.maxfds self.poll
+
+let[@inline never] wakeup_real_ (self : st) : unit =
+  let@ _sp =
+    Trace_.with_span ~__FILE__ ~__LINE__ "nanoev.wakeup-from-outside"
+  in
+  let b = Bytes.make 1 '!' in
+  ignore (Unix.write self.wakeup_wr b 0 1 : int)
+
+let[@inline] wakeup_ (self : st) : unit =
+  if not (Atomic.exchange self.wakeup_triggered true) then wakeup_real_ self
+
+let wakeup_from_outside (self : st) : unit =
+  let already_awake =
+    (* to avoid race conditions we only take the shortcut if
+        this is called from the owner thread *)
+    in_owner_thread self && not (Atomic.get self.in_poll)
+  in
+  if not already_awake then wakeup_ self
 
 let rec perform_cbs ~closed = function
   | Nil -> ()
@@ -100,217 +165,209 @@ let rec perform_cbs ~closed = function
     f ~closed x y;
     perform_cbs ~closed tail
 
-let rec perform_cbs_closed ~closed = function
-  | Nil -> ()
-  | Sub (x, y, f, tail) ->
-    f ~closed x y;
-    perform_cbs_closed ~closed tail
+(** Change the event loop right now. This must be called only from the owner
+    thread and outside of [poll]. *)
+module Run_now_ = struct
+  let rec perform_cbs_closed ~closed = function
+    | Nil -> ()
+    | Sub (x, y, f, tail) ->
+      f ~closed x y;
+      perform_cbs_closed ~closed tail
 
-let[@inline] leq_timer (Timer a) (Timer b) = a.deadline <= b.deadline
+  let clear_ (self : st) : unit =
+    Heap.clear self.timer;
+    Fd_tbl.clear self.fds;
+    for i = 0 to P.maxfds self.poll - 1 do
+      P.set_index self.poll i P.invalid_fd Flags.empty
+    done;
+    Atomic.set self.wakeup_triggered false;
+    self.len <- 0;
+    ()
 
-let[@inline] with_lock_ (self : st) f =
-  Mutex.lock self.lock;
-  match f self with
-  | exception e ->
-    Mutex.unlock self.lock;
-    raise e
-  | res ->
-    Mutex.unlock self.lock;
-    res
+  let get_fd_ (self : st) fd : fd_data =
+    (* assert (in_owner_thread self && not (in_poll self)); *)
+    match Fd_tbl.find self.fds fd with
+    | per_fd -> per_fd
+    | exception Not_found ->
+      let idx =
+        if self.len = P.maxfds self.poll then
+          invalid_arg "No available slot in poll";
+        let n = self.len in
+        self.len <- self.len + 1;
+        n
+      in
+      let per_fd = { idx; fd; r = Nil; w = Nil } in
+      Fd_tbl.add self.fds fd per_fd;
+      per_fd
 
-let create_st () : st =
-  let wakeup_rd, wakeup_wr = Unix.pipe () in
-  Unix.set_nonblock wakeup_rd;
-  {
-    timer = Heap.create ~leq:leq_timer ();
-    fds = Fd_tbl.create 16;
-    poll = P.create ();
-    len = 0;
-    wakeup_rd;
-    wakeup_wr;
-    wakeup_triggered = Atomic.make false;
-    in_poll = Atomic.make None;
-    queued_tasks = Queue.create ();
-    lock = Mutex.create ();
-  }
+  let remove_fd_ (self : st) (fd_data : fd_data) : unit =
+    Fd_tbl.remove self.fds fd_data.fd;
+    P.set_index self.poll fd_data.idx P.invalid_fd Flags.empty;
 
-let max_fds (self : st) : int = P.maxfds self.poll
+    (* assert (in_owner_thread self && not (in_poll self)); *)
+    if fd_data.idx > 0 && fd_data.idx + 1 < self.len then (
+      (* not the last element nor the first (pipe_rd), move the last element
+        here to keep the buffer non sparse *)
+      let last_fd = P.get_fd self.poll (self.len - 1) in
+      assert (last_fd <> fd_data.fd);
+      match Fd_tbl.find_opt self.fds last_fd with
+      | None -> assert false
+      | Some last_fd_data ->
+        (* move the last FD to [idx] *)
+        last_fd_data.idx <- fd_data.idx;
+        P.set_index self.poll fd_data.idx last_fd (fd_flags last_fd_data)
+    );
+
+    self.len <- self.len - 1;
+    ()
+
+  let close_ (self : st) fd : unit =
+    let@ _sp = Trace_.with_span ~__FILE__ ~__LINE__ "nanoev.close" in
+    let r, w =
+      match Fd_tbl.find self.fds fd with
+      | fd_data ->
+        remove_fd_ self fd_data;
+        fd_data.r, fd_data.w
+      | exception Not_found -> Nil, Nil
+    in
+    perform_cbs_closed ~closed:true r;
+    perform_cbs_closed ~closed:true w;
+    ()
+
+  let on_readable_ self fd x y f : unit =
+    let@ _sp = Trace_.with_span ~__FILE__ ~__LINE__ "nanoev.on-readable" in
+    let fd_data = get_fd_ self fd in
+    fd_data.r <- Sub (x, y, f, fd_data.r);
+    P.set_index self.poll fd_data.idx fd (fd_flags fd_data)
+
+  let on_writable_ self fd x y f : unit =
+    let@ _sp = Trace_.with_span ~__FILE__ ~__LINE__ "nanoev.on-writable" in
+    let fd_data = get_fd_ self fd in
+    fd_data.w <- Sub (x, y, f, fd_data.w);
+    P.set_index self.poll fd_data.idx fd (fd_flags fd_data)
+
+  let run_after_s_ self ev : unit = Heap.insert self.timer ev
+
+  let perform_task_ self (t : queued_task) : unit =
+    match t with
+    | Q_run_after t -> run_after_s_ self t
+    | Q_on_readable (fd, x, y, f) -> on_readable_ self fd x y f
+    | Q_on_writable (fd, x, y, f) -> on_writable_ self fd x y f
+    | Q_clear -> clear_ self
+    | Q_close fd -> close_ self fd
+end
 
 let clear (self : st) =
-  let@ self = with_lock_ self in
-  Heap.clear self.timer;
-  Fd_tbl.clear self.fds;
-  for i = 0 to P.maxfds self.poll - 1 do
-    P.set_index self.poll i P.invalid_fd Flags.empty
-  done;
-  self.len <- 0;
-  ()
-
-let wakeup_from_outside (self : st) : unit =
-  if not (Atomic.exchange self.wakeup_triggered true) then
-    let@ _sp =
-      Trace_.with_span ~__FILE__ ~__LINE__ "nanoev.wakeup-from-outside"
-    in
-    let b = Bytes.make 1 '!' in
-    ignore (Unix.write self.wakeup_wr b 0 1 : int)
-
-let get_fd_ (self : st) fd : per_fd =
-  match Fd_tbl.find self.fds fd with
-  | per_fd -> per_fd
-  | exception Not_found ->
-    let idx =
-      if self.len = P.maxfds self.poll then
-        invalid_arg "No available slot in poll";
-      let n = self.len in
-      self.len <- self.len + 1;
-      n
-    in
-    let per_fd = { idx; fd; r = Nil; w = Nil } in
-    Fd_tbl.add self.fds fd per_fd;
-    per_fd
-
-let close_ (self : st) fd : unit =
-  let@ _sp = Trace_.with_span ~__FILE__ ~__LINE__ "nanoev.close" in
-  let r, w =
-    let@ self = with_lock_ self in
-    match Fd_tbl.find self.fds fd with
-    | per_fd ->
-      Fd_tbl.remove self.fds fd;
-
-      (* not the last element, move the last element here *)
-      if per_fd.idx + 1 < self.len then (
-        let last_fd = P.get_fd self.poll (self.len - 1) in
-        match Fd_tbl.find_opt self.fds last_fd with
-        | None -> ()
-        | Some last_per_fd ->
-          last_per_fd.idx <- per_fd.idx;
-          P.set_index self.poll per_fd.idx last_fd (per_fd_flags last_per_fd)
-      );
-      self.len <- self.len - 1;
-      per_fd.r, per_fd.w
-    | exception Not_found ->
-      invalid_arg "File descriptor is not known to Nanoev"
-  in
-
-  (* call callbacks outside of the lock's critical section *)
-  perform_cbs_closed ~closed:true r;
-  perform_cbs_closed ~closed:true w;
-  ()
-
-let on_readable self fd x y f : unit =
-  let@ _sp = Trace_.with_span ~__FILE__ ~__LINE__ "nanoev.on-readable" in
-  let@ self = with_lock_ self in
-  let per_fd = get_fd_ self fd in
-  per_fd.r <- Sub (x, y, f, per_fd.r);
-  (* FIXME: P.set_index *)
-  self.sub_up_to_date <- false;
-  if Atomic.get self.in_select then wakeup_from_outside self
-
-let on_writable self fd x y f : unit =
-  let@ _sp = Trace_.with_span ~__FILE__ ~__LINE__ "nanoev.on-writable" in
-  let@ self = with_lock_ self in
-  let per_fd = get_fd_ self fd in
-  per_fd.w <- Sub (x, y, f, per_fd.w);
-  (* FIXME: P.set_index *)
-  self.sub_up_to_date <- false;
-  if Atomic.get self.in_select then wakeup_from_outside self
-
-let run_after_s self time x y f : unit =
-  let@ _sp = Trace_.with_span ~__FILE__ ~__LINE__ "nanoev.run-after-s" in
-  let@ self = with_lock_ self in
-  let deadline = now_ () +. time in
-  Heap.insert self.timer (Timer { deadline; x; y; f });
-  if Atomic.get self.in_select then wakeup_from_outside self
-
-let recompute_if_needed (self : st) =
-  if not self.sub_up_to_date then (
-    let@ _sp = Trace_.with_span ~__FILE__ ~__LINE__ "recompute-if-needed" in
-    self.sub_up_to_date <- true;
-    self.sub_r <- [];
-    self.sub_w <- [];
-    Hashtbl.iter
-      (fun fd per_fd ->
-        if cb_is_empty per_fd.r && cb_is_empty per_fd.w then
-          Hashtbl.remove self.fds fd;
-        if not (cb_is_empty per_fd.r) then self.sub_r <- fd :: self.sub_r;
-        if not (cb_is_empty per_fd.w) then self.sub_w <- fd :: self.sub_w)
-      self.fds
+  if in_owner_thread self && not (in_poll self) then
+    Run_now_.clear_ self
+  else (
+    queue_task_ self @@ Q_clear;
+    wakeup_from_outside self
   )
 
-let next_deadline_ (self : st) : float option =
+let close (self : st) fd : unit =
+  if in_owner_thread self && not (in_poll self) then
+    Run_now_.close_ self fd
+  else (
+    queue_task_ self @@ Q_close fd;
+    wakeup_from_outside self
+  )
+
+let on_readable self fd x y f : unit =
+  if in_owner_thread self && not (in_poll self) then
+    Run_now_.on_readable_ self fd x y f
+  else (
+    queue_task_ self @@ Q_on_readable (fd, x, y, f);
+    wakeup_from_outside self
+  )
+
+let on_writable self fd x y f : unit =
+  if in_owner_thread self && not (in_poll self) then
+    Run_now_.on_writable_ self fd x y f
+  else (
+    queue_task_ self @@ Q_on_writable (fd, x, y, f);
+    wakeup_from_outside self
+  )
+
+let run_after_s self (time : float) x y f : unit =
+  let deadline = Int64.add (now_ns ()) (ns_of_s time) in
+  let ev = Timer { deadline; x; y; f } in
+  if in_owner_thread self && not (in_poll self) then
+    Run_now_.run_after_s_ self ev
+  else (
+    queue_task_ self @@ Q_run_after ev;
+    wakeup_from_outside self
+  )
+
+let next_deadline_ (self : st) : int64 option =
   match Heap.peek_min_exn self.timer with
   | exception Heap.Empty -> None
   | Timer t -> Some t.deadline
 
 let step (self : st) : unit =
-  let@ _sp = Trace_.with_span ~__FILE__ ~__LINE__ "nanoev.unix.step" in
-  (* gather the subscriptions and timeout *)
-  let timeout, sub_r, sub_w =
-    let@ self = with_lock_ self in
-    recompute_if_needed self;
-    let timeout =
-      match next_deadline_ self with
-      | None -> 30.
-      | Some d -> max 0. (d -. now_ ())
-    in
-    timeout, self.sub_r, self.sub_w
+  let@ _sp = Trace_.with_span ~__FILE__ ~__LINE__ "nanoev.posix.step" in
+
+  self.owner_thread <- Thread.(id (self ()));
+  let timeout_ns : int64 =
+    match next_deadline_ self with
+    | None -> 30_000_000_000L
+    | Some d -> Int64.max 0L (Int64.sub d (now_ns ()))
   in
 
-  (* enter [select] *)
-  Atomic.set self.in_select true;
-  let r_reads, r_writes, _ =
+  (* process all queued tasks.
+
+    NOTE: race condition: if another thread queues tasks after we do
+    the transfer, it will call [wakeup_from_outside] and make the pipe_rd FD
+    readable. So as soon as we call [poll], it will return and we will find
+    the queued tasks waiting for us. *)
+  let local_q = Queue.create () in
+  Sync_queue.transfer self.queued_tasks local_q;
+  while not (Queue.is_empty local_q) do
+    let t = Queue.pop local_q in
+    Run_now_.perform_task_ self t
+  done;
+
+  Atomic.set self.in_poll true;
+
+  (* enter [poll] *)
+  let num_ready_fds =
     let@ _sp =
-      Trace_.with_span ~__FILE__ ~__LINE__ "select" ~data:(fun () ->
-          [
-            "timeout", `Float timeout;
-            "reads", `Int (List.length sub_r);
-            "writes", `Int (List.length sub_w);
-          ])
+      Trace_.with_span ~__FILE__ ~__LINE__ "poll" ~data:(fun () ->
+          [ "timeout", `Float (ns_to_s timeout_ns); "len", `Int self.len ])
     in
-    Unix.select (self.wakeup_rd :: sub_r) sub_w [] timeout
+    P.ppoll_or_poll self.poll self.len (Nanoseconds timeout_ns)
   in
-  Atomic.set self.in_select false;
 
-  (* drain pipe *)
+  Atomic.set self.in_poll false;
+
+  (* drain notification pipe *)
   if Atomic.exchange self.wakeup_triggered false then (
-    let b1 = Bytes.create 1 in
-    while try Unix.read self.wakeup_rd b1 0 1 > 0 with _ -> false do
+    let b1 = Bytes.create 8 in
+    while try Unix.read self.wakeup_rd b1 0 8 > 0 with _ -> false do
       ()
     done
   );
 
-  (* gather the [per_fd] that are ready *)
-  let ready_r = ref [] in
-  let ready_w = ref [] in
-
-  (* gather the [per_fd] that have updates *)
-  (let@ self = with_lock_ self in
-   if r_reads != [] || r_writes != [] then self.sub_up_to_date <- false;
-
-   List.iter
-     (fun fd ->
-       if fd != self.wakeup_rd then (
-         let per_fd = Hashtbl.find self.fds fd in
-         ready_r := per_fd :: !ready_r
-       ))
-     r_reads;
-   List.iter
-     (fun fd ->
-       let per_fd = Hashtbl.find self.fds fd in
-       ready_w := per_fd :: !ready_w)
-     r_writes);
-
   (* call callbacks *)
-  List.iter
-    (fun fd ->
-      perform_cbs ~closed:false fd.r;
-      fd.r <- Nil)
-    !ready_r;
-  List.iter
-    (fun fd ->
-      perform_cbs ~closed:false fd.w;
-      fd.w <- Nil)
-    !ready_w;
+  P.iter_ready self.poll num_ready_fds (fun _idx fd flags ->
+      if fd <> self.wakeup_rd then (
+        let fd_data =
+          try Fd_tbl.find self.fds fd with Not_found -> assert false
+        in
+
+        if Flags.mem Flags.pollin flags then (
+          let r = fd_data.r in
+          fd_data.r <- Nil;
+          perform_cbs ~closed:false r
+        );
+        if Flags.mem Flags.pollout flags then (
+          let w = fd_data.w in
+          fd_data.w <- Nil;
+          perform_cbs ~closed:false w
+        );
+
+        if Flags.empty = fd_flags fd_data then Run_now_.remove_fd_ self fd_data
+      ));
 
   ()
 
diff --git a/src/posix/nanoev_posix.mli b/src/posix/nanoev_posix.mli
index 63ee287..5d23b61 100644
--- a/src/posix/nanoev_posix.mli
+++ b/src/posix/nanoev_posix.mli
@@ -5,4 +5,6 @@ include module type of struct
 end
 
 val create : unit -> t
-val create_with : Iomux.Poll.t -> t
+(** Create a new nanoev loop using [Iomux] (poll/ppoll).
+
+    {b NOTE}: this is NOT thread-safe *)