diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index fb8daa9..0a73e77 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -56,6 +56,6 @@ jobs:
           dune-cache: true
           allow-prerelease-opam: true
 
-      - run: opam install ocamlformat.0.26.2
+      - run: opam install ocamlformat.0.27.0
       - run: opam exec -- make format-check
 
diff --git a/.ocamlformat b/.ocamlformat
index 7818345..f33f722 100644
--- a/.ocamlformat
+++ b/.ocamlformat
@@ -1,4 +1,4 @@
-version = 0.26.2
+version = 0.27.0
 profile=conventional
 margin=80
 if-then-else=k-r
diff --git a/dune-project b/dune-project
index e0cd8bb..b9b1aca 100644
--- a/dune-project
+++ b/dune-project
@@ -20,21 +20,64 @@
  (synopsis "Tiny event loop abstraction")
  (depends ocaml dune base-unix)
  (depopts
-  (trace (>= 0.7))
-  (picos
-   (and (>= 0.5) (< 0.7))))
+  (trace
+   (>= 0.7)))
  (tags
   (unix select async)))
 
+(package
+ (name nanoev-picos)
+ (synopsis "Use nanoev from picos")
+ (depends
+  ocaml
+  dune
+  base-unix
+  (nanoev
+   (= :version))
+  (iostream
+   (>= 0.3))
+  (picos
+   (and
+    (>= 0.5)
+    (< 0.7)))
+  (picos_std
+   (and
+    (>= 0.5)
+    (< 0.7))))
+ (tags
+  (unix select async)))
+
+(package
+ (name nanoev-posix)
+ (synopsis "Use mtime+iomux (posix compliant) as a backend for nanoev")
+ (depends
+  ocaml
+  dune
+  base-unix
+  iomux
+  (nanoev (= :version))
+  (nanoev-picos (= :version))
+  (mtime
+   (>= 2.0))
+  (moonpool :with-test)
+  (trace :with-test)
+  (trace-tef :with-test))
+ (tags
+  (unix select async iomux nanoev)))
+
 (package
  (name nanoev_tiny_httpd)
  (synopsis "Use nanoev as a basis for tiny_httpd")
  (depends
   ocaml
   dune
-  nanoev
+  (nanoev (= :version))
+  (nanoev-picos (= :version))
   picos
-  (tiny_httpd (>= 0.17)))
- (tags (nanoev http)))
+  picos_std
+  (tiny_httpd
+   (>= 0.17)))
+ (tags
+  (nanoev http)))
 
 ; See the complete stanza docs at https://dune.readthedocs.io/en/stable/reference/dune-project/index.html
diff --git a/examples/echo/dune b/examples/echo/dune
index 7fa6bea..99959c7 100644
--- a/examples/echo/dune
+++ b/examples/echo/dune
@@ -1,4 +1,4 @@
 (executable
  (name echo)
- (libraries nanoev nanoev.unix moonpool moonpool.fib trace trace-tef
-   nanoev_tiny_httpd))
+ (libraries nanoev nanoev.unix nanoev-posix moonpool moonpool.fib trace
+   trace-tef nanoev_tiny_httpd))
diff --git a/examples/echo/echo.ml b/examples/echo/echo.ml
index a5543b7..0519801 100644
--- a/examples/echo/echo.ml
+++ b/examples/echo/echo.ml
@@ -79,33 +79,71 @@ let setup_logging () =
   Logs.set_reporter @@ Logs.format_reporter ();
   Logs.set_level ~all:true (Some Logs.Debug)
 
+let emit_metrics_ pool server () =
+  while true do
+    Trace.counter_int ~level:Info "pool.tasks" (Moonpool.Runner.num_tasks pool);
+    Trace.counter_int ~level:Info "http.active-conns"
+      (Server.active_connections server);
+    Thread.delay 0.3
+  done
+
 let () =
+  Trace.set_current_level Info;
   let@ () = Trace_tef.with_setup () in
   Trace.set_thread_name "main";
 
   let port_ = ref 8080 in
   let max_conn = ref 1024 in
   let j = ref 8 in
+  let backend = ref `Posix in
+  let buf_size = ref 4096 in
+  let max_buf_pool_size = ref None in
+
+  let set_backend = function
+    | "posix" | "poll" | "default" -> backend := `Posix
+    | "unix" | "select" -> backend := `Unix
+    | s -> failwith @@ Printf.sprintf "unknown backend %S" s
+  in
   Arg.parse
     (Arg.align
        [
          "--port", Arg.Set_int port_, " set port";
          "-p", Arg.Set_int port_, " set port";
          "-j", Arg.Set_int j, " number of threads";
+         ( "--max-buf-pool-size",
+           Arg.Int (fun i -> max_buf_pool_size := Some i),
+           " max buffer pool size" );
+         "--buf-size", Arg.Set_int buf_size, " buffer size";
          "--debug", Arg.Unit setup_logging, " enable debug";
          "--max-conns", Arg.Set_int max_conn, " maximum concurrent connections";
+         ( "--backend",
+           Arg.Symbol
+             ([ "posix"; "default"; "unix"; "select"; "poll" ], set_backend),
+           " event loop backend" );
        ])
     (fun _ -> raise (Arg.Bad ""))
     "echo [option]*";
 
-  let@ pool = Moonpool.Ws_pool.with_ ~num_threads:!j () in
-  let@ _runner = Moonpool_fib.main in
+  let@ pool =
+   fun yield ->
+    if !j > 1 then
+      let@ pool = Moonpool.Ws_pool.with_ ~num_threads:!j () in
+      let@ _runner = Moonpool_fib.main in
+      yield pool
+    else
+      Moonpool_fib.main yield
+  in
 
-  let ev = Nanoev_unix.create () in
-  Nanoev_picos.setup_bg_thread ev;
+  let ev =
+    match !backend with
+    | `Posix -> Nanoev_posix.create ()
+    | `Unix -> Nanoev_unix.create ()
+  in
+  let@ () = Nanoev_picos.Background_thread.with_setup ev in
 
   let server =
     Nanoev_tiny_httpd.create ~new_thread:(Moonpool.run_async pool) ~port:!port_
+      ?max_buf_pool_size:!max_buf_pool_size ~buf_size:!buf_size
       ~max_connections:!max_conn ()
   in
 
@@ -273,8 +311,14 @@ let () =
       let s = to_string_top h in
       Response.make_string ~headers:[ "content-type", "text/html" ] @@ Ok s);
 
-  Printf.printf "listening on http://%s:%d\n%!" (Server.addr server)
-    (Server.port server);
+  Printf.printf
+    "listening on http://%s:%d with %d threads, %d max connections, %d max fds\n\
+     %!"
+    (Server.addr server) (Server.port server) !j !max_conn (Nanoev.max_fds ev);
+
+  if Trace.enabled () then
+    ignore (Thread.create (emit_metrics_ pool server) () : Thread.t);
+
   match Server.run server with
   | Ok () -> ()
   | Error e -> raise e
diff --git a/nanoev-picos.opam b/nanoev-picos.opam
new file mode 100644
index 0000000..44e1230
--- /dev/null
+++ b/nanoev-picos.opam
@@ -0,0 +1,34 @@
+# This file is generated by dune, edit dune-project instead
+opam-version: "2.0"
+synopsis: "Use nanoev from picos"
+maintainer: ["Simon Cruanes"]
+authors: ["Simon Cruanes"]
+license: "MIT"
+tags: ["unix" "select" "async"]
+homepage: "https://github.com/c-cube/nanoev"
+bug-reports: "https://github.com/c-cube/nanoev/issues"
+depends: [
+  "ocaml"
+  "dune" {>= "2.7"}
+  "base-unix"
+  "nanoev" {= version}
+  "iostream" {>= "0.3"}
+  "picos" {>= "0.5" & < "0.7"}
+  "picos_std" {>= "0.5" & < "0.7"}
+  "odoc" {with-doc}
+]
+build: [
+  ["dune" "subst"] {dev}
+  [
+    "dune"
+    "build"
+    "-p"
+    name
+    "-j"
+    jobs
+    "@install"
+    "@runtest" {with-test}
+    "@doc" {with-doc}
+  ]
+]
+dev-repo: "git+https://github.com/c-cube/nanoev.git"
diff --git a/nanoev-posix.opam b/nanoev-posix.opam
new file mode 100644
index 0000000..b9df267
--- /dev/null
+++ b/nanoev-posix.opam
@@ -0,0 +1,37 @@
+# This file is generated by dune, edit dune-project instead
+opam-version: "2.0"
+synopsis: "Use mtime+iomux (posix compliant) as a backend for nanoev"
+maintainer: ["Simon Cruanes"]
+authors: ["Simon Cruanes"]
+license: "MIT"
+tags: ["unix" "select" "async" "iomux" "nanoev"]
+homepage: "https://github.com/c-cube/nanoev"
+bug-reports: "https://github.com/c-cube/nanoev/issues"
+depends: [
+  "ocaml"
+  "dune" {>= "2.7"}
+  "base-unix"
+  "iomux"
+  "nanoev" {= version}
+  "nanoev-picos" {= version}
+  "mtime" {>= "2.0"}
+  "moonpool" {with-test}
+  "trace" {with-test}
+  "trace-tef" {with-test}
+  "odoc" {with-doc}
+]
+build: [
+  ["dune" "subst"] {dev}
+  [
+    "dune"
+    "build"
+    "-p"
+    name
+    "-j"
+    jobs
+    "@install"
+    "@runtest" {with-test}
+    "@doc" {with-doc}
+  ]
+]
+dev-repo: "git+https://github.com/c-cube/nanoev.git"
diff --git a/nanoev.opam b/nanoev.opam
index 9fa7ba9..8268f40 100644
--- a/nanoev.opam
+++ b/nanoev.opam
@@ -15,7 +15,6 @@ depends: [
 ]
 depopts: [
   "trace" {>= "0.7"}
-  "picos" {>= "0.5" & < "0.7"}
 ]
 build: [
   ["dune" "subst"] {dev}
diff --git a/nanoev_tiny_httpd.opam b/nanoev_tiny_httpd.opam
index 9504331..08833e7 100644
--- a/nanoev_tiny_httpd.opam
+++ b/nanoev_tiny_httpd.opam
@@ -10,8 +10,10 @@ bug-reports: "https://github.com/c-cube/nanoev/issues"
 depends: [
   "ocaml"
   "dune" {>= "2.7"}
-  "nanoev"
+  "nanoev" {= version}
+  "nanoev-picos" {= version}
   "picos"
+  "picos_std"
   "tiny_httpd" {>= "0.17"}
   "odoc" {with-doc}
 ]
diff --git a/src/core/nanoev.ml b/src/core/nanoev.ml
index c15935e..75d34b7 100644
--- a/src/core/nanoev.ml
+++ b/src/core/nanoev.ml
@@ -7,6 +7,7 @@ module Impl = struct
     clear: 'st -> unit;
     wakeup_from_outside: 'st -> unit;
     close: 'st -> Unix.file_descr -> unit;
+    max_fds: 'st -> int;
     on_readable:
       'a 'b.
       'st ->
@@ -39,6 +40,7 @@ type t = Impl.t
 let[@inline] clear (Ev (ops, st)) = ops.clear st
 let[@inline] wakeup_from_outside (Ev (ops, st)) = ops.wakeup_from_outside st
 let[@inline] close (Ev (ops, st)) fd = ops.close st fd
+let[@inline] max_fds (Ev (ops, st)) = ops.max_fds st
 
 let[@inline] on_readable (Ev (ops, st)) fd x y f : unit =
   ops.on_readable st fd x y f
diff --git a/src/core/nanoev.mli b/src/core/nanoev.mli
index 4792cd6..213b9de 100644
--- a/src/core/nanoev.mli
+++ b/src/core/nanoev.mli
@@ -9,6 +9,7 @@ module Impl : sig
     clear: 'st -> unit;
     wakeup_from_outside: 'st -> unit;
     close: 'st -> Unix.file_descr -> unit;
+    max_fds: 'st -> int;
     on_readable:
       'a 'b.
       'st ->
@@ -43,6 +44,9 @@ val step : t -> unit
 val close : t -> Unix.file_descr -> unit
 (** Close the file descriptor and clean it up *)
 
+val max_fds : t -> int
+(** Maximum number of file descriptors that can be observed at once. *)
+
 val on_readable :
   t -> Unix.file_descr -> 'a -> 'b -> (closed:bool -> 'a -> 'b -> unit) -> unit
 
diff --git a/src/picos/IO_in.ml b/src/picos/IO_in.ml
new file mode 100644
index 0000000..6184eba
--- /dev/null
+++ b/src/picos/IO_in.ml
@@ -0,0 +1,152 @@
+open Common_
+
+class type t = object
+  method input : bytes -> int -> int -> int
+  (** Read into the slice. Returns [0] only if the stream is closed. *)
+
+  method close : unit -> unit
+  (** Close the input. Must be idempotent. *)
+end
+
+let create ?(close = ignore) ~input () : t =
+  object
+    method close = close
+    method input = input
+  end
+
+let empty : t =
+  object
+    method close () = ()
+    method input _ _ _ = 0
+  end
+
+let of_bytes ?(off = 0) ?len (b : bytes) : t =
+  (* i: current position in [b] *)
+  let i = ref off in
+
+  let len =
+    match len with
+    | Some n ->
+      if n > Bytes.length b - off then invalid_arg "Iostream.In.of_bytes";
+      n
+    | None -> Bytes.length b - off
+  in
+  let end_ = off + len in
+
+  object
+    method input b_out i_out len_out =
+      let n = min (end_ - !i) len_out in
+      Bytes.blit b !i b_out i_out n;
+      i := !i + n;
+      n
+
+    method close () = i := end_
+  end
+
+let of_string ?off ?len s : t = of_bytes ?off ?len (Bytes.unsafe_of_string s)
+
+(** Read into the given slice.
+    @return the number of bytes read, [0] means end of input. *)
+let[@inline] input (self : #t) buf i len = self#input buf i len
+
+(** Close the channel. *)
+let[@inline] close self : unit = self#close ()
+
+let rec really_input (self : #t) buf i len =
+  if len > 0 then (
+    let n = input self buf i len in
+    if n = 0 then raise End_of_file;
+    (really_input [@tailrec]) self buf (i + n) (len - n)
+  )
+
+let really_input_string self n : string =
+  let buf = Bytes.create n in
+  really_input self buf 0 n;
+  Bytes.unsafe_to_string buf
+
+let copy_into ?(buf = Bytes.create _default_buf_size) (ic : #t) (oc : IO_out.t)
+    : unit =
+  let continue = ref true in
+  while !continue do
+    let len = input ic buf 0 (Bytes.length buf) in
+    if len = 0 then
+      continue := false
+    else
+      IO_out.output oc buf 0 len
+  done
+
+let concat (l0 : t list) : t =
+  let l = ref l0 in
+  let rec input b i len : int =
+    match !l with
+    | [] -> 0
+    | ic :: tl ->
+      let n = ic#input b i len in
+      if n > 0 then
+        n
+      else (
+        l := tl;
+        input b i len
+      )
+  in
+  let close () = List.iter close l0 in
+  create ~close ~input ()
+
+let input_all ?(buf = Bytes.create 128) (self : #t) : string =
+  let buf = ref buf in
+  let i = ref 0 in
+
+  let[@inline] full_ () = !i = Bytes.length !buf in
+
+  let grow_ () =
+    let old_size = Bytes.length !buf in
+    let new_size = min Sys.max_string_length (old_size + (old_size / 4) + 10) in
+    if old_size = new_size then
+      failwith "input_all: maximum input size exceeded";
+    let new_buf = Bytes.extend !buf 0 (new_size - old_size) in
+    buf := new_buf
+  in
+
+  let rec loop () =
+    if full_ () then grow_ ();
+    let available = Bytes.length !buf - !i in
+    let n = input self !buf !i available in
+    if n > 0 then (
+      i := !i + n;
+      (loop [@tailrec]) ()
+    )
+  in
+  loop ();
+
+  if full_ () then
+    Bytes.unsafe_to_string !buf
+  else
+    Bytes.sub_string !buf 0 !i
+
+let of_unix_fd ?(close_noerr = false) ?(buf = Bytes.create _default_buf_size)
+    (fd : Unix.file_descr) : t =
+  let buf_len = ref 0 in
+  let buf_off = ref 0 in
+
+  let refill () =
+    buf_off := 0;
+    buf_len := Base.read fd buf 0 (Bytes.length buf)
+  in
+
+  object
+    method input b i len : int =
+      if !buf_len = 0 then refill ();
+      let n = min len !buf_len in
+      if n > 0 then (
+        Bytes.blit buf !buf_off b i n;
+        buf_off := !buf_off + n;
+        buf_len := !buf_len - n
+      );
+      n
+
+    method close () =
+      if close_noerr then (
+        try Unix.close fd with _ -> ()
+      ) else
+        Unix.close fd
+  end
diff --git a/src/picos/IO_out.ml b/src/picos/IO_out.ml
new file mode 100644
index 0000000..b98525f
--- /dev/null
+++ b/src/picos/IO_out.ml
@@ -0,0 +1,118 @@
+open Common_
+
+class type t = object
+  method output_char : char -> unit
+  method output : bytes -> int -> int -> unit
+  method flush : unit -> unit
+  method close : unit -> unit
+end
+
+let create ?(flush = ignore) ?(close = ignore) ~output_char ~output () : t =
+  object
+    method flush () = flush ()
+    method close () = close ()
+    method output_char c = output_char c
+    method output bs i len = output bs i len
+  end
+
+let dummy : t =
+  object
+    method flush () = ()
+    method close () = ()
+    method output_char _ = ()
+    method output _ _ _ = ()
+  end
+
+let of_unix_fd ?(close_noerr = false) ?(buf = Bytes.create _default_buf_size) fd
+    : t =
+  let buf_off = ref 0 in
+
+  let[@inline] is_full () = !buf_off = Bytes.length buf in
+
+  let flush () =
+    if !buf_off > 0 then (
+      Base.write fd buf 0 !buf_off;
+      buf_off := 0
+    )
+  in
+
+  object
+    method output_char c =
+      if is_full () then flush ();
+      Bytes.set buf !buf_off c;
+      incr buf_off
+
+    method output bs i len : unit =
+      let i = ref i in
+      let len = ref len in
+
+      while !len > 0 do
+        (* make space *)
+        if is_full () then flush ();
+
+        let n = min !len (Bytes.length buf - !buf_off) in
+        Bytes.blit bs !i buf !buf_off n;
+        buf_off := !buf_off + n;
+        i := !i + n;
+        len := !len - n
+      done;
+      (* if full, write eagerly *)
+      if is_full () then flush ()
+
+    method close () =
+      if close_noerr then (
+        try
+          flush ();
+          Unix.close fd
+        with _ -> ()
+      ) else (
+        flush ();
+        Unix.close fd
+      )
+
+    method flush = flush
+  end
+
+let of_buffer (buf : Buffer.t) : t =
+  object
+    method close () = ()
+    method flush () = ()
+    method output_char c = Buffer.add_char buf c
+    method output bs i len = Buffer.add_subbytes buf bs i len
+  end
+
+(** Output the buffer slice into this channel *)
+let[@inline] output_char (self : #t) c : unit = self#output_char c
+
+(** Output the buffer slice into this channel *)
+let[@inline] output (self : #t) buf i len : unit = self#output buf i len
+
+let[@inline] output_string (self : #t) (str : string) : unit =
+  self#output (Bytes.unsafe_of_string str) 0 (String.length str)
+
+let output_line (self : #t) (str : string) : unit =
+  output_string self str;
+  output_char self '\n'
+
+(** Close the channel. *)
+let[@inline] close self : unit = self#close ()
+
+(** Flush (ie. force write) any buffered bytes. *)
+let[@inline] flush self : unit = self#flush ()
+
+let output_int self i =
+  let s = string_of_int i in
+  output_string self s
+
+let output_lines self seq = Seq.iter (output_line self) seq
+
+let tee (l : t list) : t =
+  match l with
+  | [] -> dummy
+  | [ oc ] -> oc
+  | _ ->
+    let output bs i len = List.iter (fun oc -> output oc bs i len) l in
+    let output_char c = List.iter (fun oc -> output_char oc c) l in
+    let close () = List.iter close l in
+    let flush () = List.iter flush l in
+    create ~flush ~close ~output ~output_char ()
diff --git a/src/picos/background_thread.ml b/src/picos/background_thread.ml
new file mode 100644
index 0000000..261daad
--- /dev/null
+++ b/src/picos/background_thread.ml
@@ -0,0 +1,7 @@
+let is_setup = Global_.has_bg_thread
+let setup = Global_.setup_bg_thread
+let shutdown = Global_.shutdown_bg_thread
+
+let with_setup ev f =
+  setup ev;
+  Fun.protect ~finally:shutdown f
diff --git a/src/picos/background_thread.mli b/src/picos/background_thread.mli
new file mode 100644
index 0000000..6e4a531
--- /dev/null
+++ b/src/picos/background_thread.mli
@@ -0,0 +1,10 @@
+val setup : Nanoev.t -> unit
+(** Install this event loop in a background thread *)
+
+val shutdown : unit -> unit
+(** Shutdown background thread, assuming {! is_setup} returns [true] *)
+
+val with_setup : Nanoev.t -> (unit -> 'a) -> 'a
+
+val is_setup : unit -> bool
+(** [is_setup()] is [true] iff a background thread is running a nanoev loop *)
diff --git a/src/picos/base.ml b/src/picos/base.ml
new file mode 100644
index 0000000..142e467
--- /dev/null
+++ b/src/picos/base.ml
@@ -0,0 +1,104 @@
+open Common_
+
+let[@inline] get_loop_exn_ () : Nanoev.t =
+  match Atomic.get Global_.st with
+  | None -> failwith "No nanoev loop installed."
+  | Some st -> st.nanoev
+
+let[@inline] unwrap_ = function
+  | None -> ()
+  | Some (exn, bt) -> Printexc.raise_with_backtrace exn bt
+
+let[@unroll 1] rec retry_read_ fd f =
+  match f () with
+  | res -> res
+  | exception
+      Unix.Unix_error
+        ( ( Unix.EAGAIN | Unix.EWOULDBLOCK | Unix.EINTR | Unix.EINPROGRESS
+          | Unix.ECONNRESET ),
+          _,
+          _ ) ->
+    (* Trace_.message "read must wait"; *)
+    let trigger = Picos.Trigger.create () in
+    let closed_r = ref false in
+    let ev = get_loop_exn_ () in
+    Nanoev.on_readable ev fd trigger closed_r (fun ~closed trigger closed_r ->
+        closed_r := closed;
+        Picos.Trigger.signal trigger);
+    Picos.Trigger.await trigger |> unwrap_;
+    if !closed_r then raise Closed;
+    retry_read_ fd f
+
+let[@unroll 1] rec retry_write_ fd f =
+  match f () with
+  | res -> res
+  | exception
+      Unix.Unix_error
+        ( ( Unix.EAGAIN | Unix.EWOULDBLOCK | Unix.EINTR | Unix.EINPROGRESS
+          | Unix.ECONNRESET ),
+          _,
+          _ ) ->
+    (* Trace_.message "write must wait"; *)
+    let ev = get_loop_exn_ () in
+    let trigger = Picos.Trigger.create () in
+    let closed_r = ref false in
+    Nanoev.on_writable ev fd trigger closed_r (fun ~closed trigger closed_r ->
+        closed_r := closed;
+        Picos.Trigger.signal trigger);
+    Picos.Trigger.await trigger |> unwrap_;
+    if !closed_r then raise Closed;
+    retry_write_ fd f
+
+let read fd buf i len : int =
+  try
+    retry_read_ fd (fun () ->
+        (* Trace_.message "read"; *)
+        Unix.read fd buf i len)
+  with Closed -> 0
+
+let close fd =
+  Unix.close fd;
+  let ev = get_loop_exn_ () in
+  Nanoev.close ev fd
+
+let accept fd =
+  try
+    retry_read_ fd (fun () ->
+        (* Trace_.message "accept"; *)
+        Unix.accept fd)
+  with Unix.Unix_error ((Unix.ESHUTDOWN | Unix.ECONNABORTED), _, _) ->
+    raise Closed
+
+let write_once fd buf i len : int =
+  try
+    retry_write_ fd (fun () ->
+        (* Trace_.message "write"; *)
+        Unix.write fd buf i len)
+  with Closed -> 0
+
+let rec write fd buf i len =
+  if len > 0 then (
+    let n = write_once fd buf i len in
+    if n < len then write fd buf (i + n) (len - n)
+  )
+
+let connect fd addr = retry_write_ fd (fun () -> Unix.connect fd addr)
+
+let[@inline] max_fds () =
+  match Atomic.get Global_.st with
+  | None -> 1024
+  | Some st -> Nanoev.max_fds st.nanoev
+
+let sleep t =
+  if t > 0. then (
+    let ev = get_loop_exn_ () in
+    let trigger = Picos.Trigger.create () in
+    Nanoev.run_after_s ev t trigger () (fun trigger () ->
+        Picos.Trigger.signal trigger);
+    Picos.Trigger.await trigger |> unwrap_
+  )
+
+module Raw = struct
+  let retry_read = retry_read_
+  let retry_write = retry_write_
+end
diff --git a/src/picos/base.mli b/src/picos/base.mli
new file mode 100644
index 0000000..7b949e9
--- /dev/null
+++ b/src/picos/base.mli
@@ -0,0 +1,37 @@
+val read : Unix.file_descr -> bytes -> int -> int -> int
+(** Read from the non blocking FD.
+    @raise Nanoev.Closed if the FD is closed
+    @raise Unix.Unix_error for other errors *)
+
+val write_once : Unix.file_descr -> bytes -> int -> int -> int
+(** Write into the non blocking FD.
+    @raise Nanoev.Closed if the FD is closed
+    @raise Unix.Unix_error for other errors *)
+
+val write : Unix.file_descr -> bytes -> int -> int -> unit
+
+val close : Unix.file_descr -> unit
+(** Close the file descriptor
+    @raise Unix.Unix_error when it fails *)
+
+val connect : Unix.file_descr -> Unix.sockaddr -> unit
+(** Connect this FD to the remote address.
+    @raise Nanoev.Closed if the FD is closed.
+    @raise Unix.Unix_error for other errors *)
+
+val accept : Unix.file_descr -> Unix.file_descr * Unix.sockaddr
+(** Accept a connection on this fd.
+    @raise Nanoev.Closed if the FD is closed.
+    @raise Unix.Unix_error for other errors *)
+
+val max_fds : unit -> int
+(** Maximum number of file descriptors one can await on. See {!Nanoev.max_fds}
+*)
+
+val sleep : float -> unit
+(** Suspend current fiber for [n] seconds *)
+
+module Raw : sig
+  val retry_read : Unix.file_descr -> (unit -> 'a) -> 'a
+  val retry_write : Unix.file_descr -> (unit -> 'a) -> 'a
+end
diff --git a/src/picos/common_.ml b/src/picos/common_.ml
new file mode 100644
index 0000000..d35dd22
--- /dev/null
+++ b/src/picos/common_.ml
@@ -0,0 +1,6 @@
+module Trace_ = Nanoev.Trace_
+
+let ( let@ ) = ( @@ )
+let _default_buf_size = 4 * 1024
+
+exception Closed = Nanoev.Closed
diff --git a/src/picos/dune b/src/picos/dune
index db9792d..de66efd 100644
--- a/src/picos/dune
+++ b/src/picos/dune
@@ -1,5 +1,4 @@
 (library
  (name nanoev_picos)
- (public_name nanoev.picos)
- (optional) ; picos
- (libraries threads picos nanoev))
+ (public_name nanoev-picos)
+ (libraries threads picos picos_std.sync iostream nanoev))
diff --git a/src/picos/global_.ml b/src/picos/global_.ml
new file mode 100644
index 0000000..2e590b8
--- /dev/null
+++ b/src/picos/global_.ml
@@ -0,0 +1,59 @@
+open Common_
+
+type st =
+  | None
+  | Some of {
+      active: bool Atomic.t;
+      nanoev: Nanoev.t;
+      th: Thread.t;
+    }
+
+let st : st Atomic.t = Atomic.make None
+let lock = Mutex.create ()
+
+let with_lock lock f =
+  Mutex.lock lock;
+  match f () with
+  | exception e ->
+    Mutex.unlock lock;
+    raise e
+  | x ->
+    Mutex.unlock lock;
+    x
+
+let bg_thread_ ~active ~evloop () : unit =
+  Trace_.set_thread_name "nanoev.picos.bg-thread";
+  while Atomic.get active do
+    Nanoev.step evloop
+  done
+
+let[@inline] has_bg_thread () = Atomic.get st <> None
+
+let setup_bg_thread (ev : Nanoev.t) : unit =
+  let@ () = with_lock lock in
+  (* shutdown existing thread, if any *)
+  (match Atomic.get st with
+  | Some st ->
+    Atomic.set st.active false;
+    Nanoev.wakeup_from_outside st.nanoev;
+    Thread.join st.th
+  | None -> ());
+
+  (* start new bg thread *)
+  let active = Atomic.make true in
+  Atomic.set st
+  @@ Some
+       {
+         active;
+         nanoev = ev;
+         th = Thread.create (bg_thread_ ~active ~evloop:ev) ();
+       }
+
+let shutdown_bg_thread () =
+  let@ () = with_lock lock in
+  match Atomic.exchange st None with
+  | None -> ()
+  | Some st ->
+    Atomic.set st.active false;
+    Nanoev.wakeup_from_outside st.nanoev;
+    Thread.join st.th
diff --git a/src/picos/nanoev_picos.ml b/src/picos/nanoev_picos.ml
index 9920345..8029fda 100644
--- a/src/picos/nanoev_picos.ml
+++ b/src/picos/nanoev_picos.ml
@@ -1,146 +1,7 @@
-open struct
-  module Trace_ = Nanoev.Trace_
-
-  let ( let@ ) = ( @@ )
-end
-
-exception Closed = Nanoev.Closed
-
-module Global_ = struct
-  type st =
-    | None
-    | Some of {
-        active: bool Atomic.t;
-        nanoev: Nanoev.t;
-        th: Thread.t;
-      }
-
-  let st : st Atomic.t = Atomic.make None
-  let lock = Mutex.create ()
-
-  let with_lock lock f =
-    Mutex.lock lock;
-    match f () with
-    | exception e ->
-      Mutex.unlock lock;
-      raise e
-    | x ->
-      Mutex.unlock lock;
-      x
-
-  let bg_thread_ ~active ~evloop () : unit =
-    Trace_.set_thread_name "nanoev.picos.bg-thread";
-    while Atomic.get active do
-      Nanoev.step evloop
-    done
-
-  let[@inline] has_bg_thread () = Atomic.get st <> None
-
-  let setup_bg_thread (ev : Nanoev.t) : unit =
-    let@ () = with_lock lock in
-    (* shutdown existing thread, if any *)
-    (match Atomic.get st with
-    | Some st ->
-      Atomic.set st.active false;
-      Nanoev.wakeup_from_outside st.nanoev;
-      Thread.join st.th
-    | None -> ());
-
-    (* start new bg thread *)
-    let active = Atomic.make true in
-    Atomic.set st
-    @@ Some
-         {
-           active;
-           nanoev = ev;
-           th = Thread.create (bg_thread_ ~active ~evloop:ev) ();
-         }
-end
-
-let has_bg_thread = Global_.has_bg_thread
-let setup_bg_thread = Global_.setup_bg_thread
-
-let[@inline] get_loop_exn_ () : Nanoev.t =
-  match Atomic.get Global_.st with
-  | None -> failwith "No nanoev loop installed."
-  | Some st -> st.nanoev
-
-let[@inline] unwrap_ = function
-  | None -> ()
-  | Some (exn, bt) -> Printexc.raise_with_backtrace exn bt
-
-let retry_read_ fd f =
-  let ev = get_loop_exn_ () in
-  let[@unroll 1] rec loop () =
-    match f () with
-    | res -> res
-    | exception
-        Unix.Unix_error ((Unix.EAGAIN | Unix.EWOULDBLOCK | Unix.EINTR), _, _) ->
-      Trace_.message "read must wait";
-      let trigger = Picos.Trigger.create () in
-      let closed_r = ref false in
-      Nanoev.on_readable ev fd trigger closed_r (fun ~closed trigger closed_r ->
-          closed_r := closed;
-          Picos.Trigger.signal trigger);
-      Picos.Trigger.await trigger |> unwrap_;
-      if !closed_r then raise Closed;
-      loop ()
-  in
-  loop ()
-
-let retry_write_ fd f =
-  let ev = get_loop_exn_ () in
-  let rec loop () =
-    match f () with
-    | res -> res
-    | exception
-        Unix.Unix_error ((Unix.EAGAIN | Unix.EWOULDBLOCK | Unix.EINTR), _, _) ->
-      Trace_.message "write must wait";
-      let trigger = Picos.Trigger.create () in
-      let closed_r = ref false in
-      Nanoev.on_writable ev fd trigger closed_r (fun ~closed trigger closed_r ->
-          closed_r := closed;
-          Picos.Trigger.signal trigger);
-      Picos.Trigger.await trigger |> unwrap_;
-      if !closed_r then raise Closed;
-      loop ()
-  in
-  loop ()
-
-let read fd buf i len : int =
-  try
-    retry_read_ fd (fun () ->
-        Trace_.message "read";
-        Unix.read fd buf i len)
-  with Closed -> 0
-
-let close fd =
-  Unix.close fd;
-  let ev = get_loop_exn_ () in
-  Nanoev.close ev fd
-
-let accept fd =
-  try
-    retry_read_ fd (fun () ->
-        Trace_.message "accept";
-        Unix.accept fd)
-  with Unix.Unix_error ((Unix.ESHUTDOWN | Unix.ECONNABORTED), _, _) ->
-    raise Closed
-
-let write fd buf i len : int =
-  try
-    retry_write_ fd (fun () ->
-        Trace_.message "write";
-        Unix.write fd buf i len)
-  with Closed -> 0
-
-let connect fd addr = retry_write_ fd (fun () -> Unix.connect fd addr)
-
-let sleep t =
-  if t > 0. then (
-    let ev = get_loop_exn_ () in
-    let trigger = Picos.Trigger.create () in
-    Nanoev.run_after_s ev t trigger () (fun trigger () ->
-        Picos.Trigger.signal trigger);
-    Picos.Trigger.await trigger |> unwrap_
-  )
+module Background_thread = Background_thread
+module Base = Base
+include Base
+module IO_in = IO_in
+module IO_out = IO_out
+module Net_client = Net_client
+module Net_server = Net_server
diff --git a/src/picos/nanoev_picos.mli b/src/picos/nanoev_picos.mli
index ef795bb..c89e3a5 100644
--- a/src/picos/nanoev_picos.mli
+++ b/src/picos/nanoev_picos.mli
@@ -1,32 +1,18 @@
 (** Basic interface with picos *)
 
-val setup_bg_thread : Nanoev.t -> unit
-(** Install this event loop in a background thread *)
-
-val has_bg_thread : unit -> bool
-(** [has_bg_thread ()] is [true] iff a background thread is running a nanoev loop *)
+module Background_thread = Background_thread
 
 (** {2 Non blocking IO primitives} *)
 
-val read : Unix.file_descr -> bytes -> int -> int -> int
-(** Read from the non blocking FD.
-  @raise Nanoev.Closed if the FD is closed
-  @raise Unix.Unix_error for other errors *)
+module Base = Base
 
-val write : Unix.file_descr -> bytes -> int -> int -> int
-(** Write into the non blocking FD.
-  @raise Nanoev.Closed if the FD is closed
-  @raise Unix.Unix_error for other errors *)
+include module type of struct
+  include Base
+end
 
-val close : Unix.file_descr -> unit
-(** Close the file descriptor
-  @raise Unix.Unix_error when it fails *)
+(** {2 Building blocks on top of {!Base}} *)
 
-val connect : Unix.file_descr -> Unix.sockaddr -> unit
-
-val accept : Unix.file_descr -> Unix.file_descr * Unix.sockaddr
-(** Accept a connection on this fd.
-  @raise Nanoev.Closed if the FD is closed.
-  @raise Unix.Unix_error for other errors *)
-
-val sleep : float -> unit
+module IO_in = IO_in
+module IO_out = IO_out
+module Net_client = Net_client
+module Net_server = Net_server
diff --git a/src/picos/net_client.ml b/src/picos/net_client.ml
new file mode 100644
index 0000000..20ec2c4
--- /dev/null
+++ b/src/picos/net_client.ml
@@ -0,0 +1,23 @@
+open Common_
+
+let connect addr : Unix.file_descr =
+  let sock = Unix.socket (Unix.domain_of_sockaddr addr) Unix.SOCK_STREAM 0 in
+  Unix.set_nonblock sock;
+  (try Unix.setsockopt sock Unix.TCP_NODELAY true with _ -> ());
+
+  (* connect asynchronously *)
+  Base.connect sock addr;
+  sock
+
+let with_connect addr (f : IO_in.t -> IO_out.t -> 'a) : 'a =
+  let sock = connect addr in
+
+  let ic = IO_in.of_unix_fd sock in
+  let oc = IO_out.of_unix_fd sock in
+
+  let finally () =
+    (try Unix.shutdown sock Unix.SHUTDOWN_ALL with _ -> ());
+    try Unix.close sock with _ -> ()
+  in
+  let@ () = Fun.protect ~finally in
+  f ic oc
diff --git a/src/picos/net_server.ml b/src/picos/net_server.ml
new file mode 100644
index 0000000..642ba48
--- /dev/null
+++ b/src/picos/net_server.ml
@@ -0,0 +1,111 @@
+module Sem = Picos_std_sync.Semaphore.Counting
+
+type client_handler = Unix.sockaddr -> IO_in.t -> IO_out.t -> unit
+
+type t = {
+  active: bool Atomic.t;
+  sock: Unix.file_descr;
+  client_handler: client_handler;
+  spawn: (unit -> unit) -> unit Picos.Computation.t;
+  max_conns: int;
+  sem: Sem.t;
+  mutable running: unit Picos.Computation.t option;
+  exn_handler: exn -> Printexc.raw_backtrace -> unit;
+}
+
+let[@inline] join (self : t) : unit =
+  Option.iter Picos.Computation.await self.running
+
+let[@inline] max_connections self = self.max_conns
+
+let[@inline] n_active_connections (self : t) : int =
+  self.max_conns - Sem.get_value self.sem
+
+let[@inline] running (self : t) : bool = Atomic.get self.active
+let shutdown (self : t) = if Atomic.exchange self.active false then ()
+
+open struct
+  let default_exn_handler exn bt =
+    Printf.eprintf "uncaught exception in network server: %s\n%s%!"
+      (Printexc.to_string exn)
+      (Printexc.raw_backtrace_to_string bt)
+
+  let run (self : t) () : unit =
+    while Atomic.get self.active do
+      let client_sock, client_addr = Base.accept self.sock in
+      Sem.acquire self.sem;
+
+      let cleanup () =
+        (try Unix.shutdown client_sock Unix.SHUTDOWN_ALL with _ -> ());
+        (* TODO: close in nanoev too *)
+        (try Unix.close client_sock with _ -> ());
+        Sem.release self.sem
+      in
+
+      let comp : _ Picos.Computation.t =
+        self.spawn (fun () ->
+            let ic = IO_in.of_unix_fd client_sock in
+            let oc = IO_out.of_unix_fd client_sock in
+            try
+              self.client_handler client_addr ic oc;
+              cleanup ()
+            with exn ->
+              let bt = Printexc.get_raw_backtrace () in
+              cleanup ();
+              self.exn_handler exn bt)
+      in
+      ignore (comp : _ Picos.Computation.t)
+    done
+end
+
+let establish ?backlog ?max_connections ?(exn_handler = default_exn_handler)
+    ~spawn ~(client_handler : client_handler) addr : t =
+  let ev =
+    match Atomic.get Global_.st with
+    | Some { nanoev = ev; _ } -> ev
+    | None -> invalid_arg "Nanoev_picos.Net_server: no event loop installed"
+  in
+
+  let max_connections =
+    match max_connections with
+    | None -> Nanoev.max_fds ev
+    | Some n -> min (Nanoev.max_fds ev) n
+  in
+  let sem = Sem.make max_connections in
+
+  let backlog =
+    match backlog with
+    | Some n -> max 4 n
+    | None -> max 4 max_connections
+  in
+
+  let domain = Unix.domain_of_sockaddr addr in
+  let sock = Unix.socket domain Unix.SOCK_STREAM 0 in
+
+  Unix.bind sock addr;
+  Unix.listen sock backlog;
+  Unix.set_nonblock sock;
+  Unix.setsockopt sock Unix.SO_REUSEADDR true;
+  (try Unix.setsockopt sock Unix.TCP_NODELAY true with _ -> ());
+
+  let server =
+    {
+      active = Atomic.make true;
+      max_conns = max_connections;
+      sem;
+      spawn;
+      sock;
+      client_handler;
+      running = None;
+      exn_handler;
+    }
+  in
+
+  server.running <- Some (spawn (run server));
+  server
+
+let with_ ?backlog ?max_connections ?exn_handler ~spawn ~client_handler addr f =
+  let server =
+    establish ?backlog ?max_connections ?exn_handler ~spawn ~client_handler addr
+  in
+  Fun.protect ~finally:(fun () -> shutdown server) (fun () -> f server)
diff --git a/src/picos/net_server.mli b/src/picos/net_server.mli
new file mode 100644
index 0000000..8f9cf62
--- /dev/null
+++ b/src/picos/net_server.mli
@@ -0,0 +1,37 @@
+type client_handler = Unix.sockaddr -> IO_in.t -> IO_out.t -> unit
+type t
+
+val join : t -> unit
+(** Wait for server to shutdown *)
+
+val shutdown : t -> unit
+(** Ask the server to stop *)
+
+val running : t -> bool
+val max_connections : t -> int
+val n_active_connections : t -> int
+
+val establish :
+  ?backlog:int ->
+  ?max_connections:int ->
+  ?exn_handler:(exn -> Printexc.raw_backtrace -> unit) ->
+  spawn:((unit -> unit) -> unit Picos.Computation.t) ->
+  client_handler:client_handler ->
+  Unix.sockaddr ->
+  t
+(** Create and start a new server on the given socket address.
+    @param spawn used to spawn a new computation per client
+    @param client_handler
+      the logic for talking to a client, will run in its own computation
+    @param backlog number of connections waiting in the listening socket
+    @param max_connections max number of simultaneous connections *)
+
+val with_ :
+  ?backlog:int ->
+  ?max_connections:int ->
+  ?exn_handler:(exn -> Printexc.raw_backtrace -> unit) ->
+  spawn:((unit -> unit) -> unit Picos.Computation.t) ->
+  client_handler:client_handler ->
+  Unix.sockaddr ->
+  (t -> 'a) ->
+  'a
diff --git a/src/posix/dune b/src/posix/dune
new file mode 100644
index 0000000..7387ac6
--- /dev/null
+++ b/src/posix/dune
@@ -0,0 +1,6 @@
+(library
+ (name nanoev_posix)
+ (public_name nanoev-posix)
+ (synopsis "posix backend (poll/ppoll+mtime)")
+ (private_modules heap)
+ (libraries threads nanoev unix iomux mtime mtime.clock.os))
diff --git a/src/posix/heap.ml b/src/posix/heap.ml
new file mode 100644
index 0000000..1a553d3
--- /dev/null
+++ b/src/posix/heap.ml
@@ -0,0 +1,61 @@
+type 'a tree =
+  | E
+  | N of int * 'a * 'a tree * 'a tree
+
+type 'a t = {
+  leq: 'a -> 'a -> bool;
+  mutable t: 'a tree;
+}
+
+let create ~leq () : _ t = { leq; t = E }
+
+let[@inline] is_empty (self : _ t) =
+  match self.t with
+  | E -> true
+  | N _ -> false
+
+exception Empty
+
+open struct
+  (** Rank of the tree *)
+  let[@inline] rank_ = function
+    | E -> 0
+    | N (r, _, _, _) -> r
+
+  (** Make a balanced node labelled with [x], and subtrees [a] and [b]. We
+      ensure that the right child's rank is ≤ to the rank of the left child
+      (leftist property). The rank of the resulting node is the length of the
+      rightmost path. *)
+  let[@inline] mk_node_ x a b =
+    if rank_ a >= rank_ b then
+      N (rank_ b + 1, x, a, b)
+    else
+      N (rank_ a + 1, x, b, a)
+
+  let rec merge ~leq t1 t2 =
+    match t1, t2 with
+    | t, E -> t
+    | E, t -> t
+    | N (_, x, a1, b1), N (_, y, a2, b2) ->
+      if leq x y then
+        mk_node_ x a1 (merge ~leq b1 t2)
+      else
+        mk_node_ y a2 (merge ~leq t1 b2)
+end
+
+let clear self = self.t <- E
+
+let[@inline] insert (self : _ t) x : unit =
+  self.t <- merge ~leq:self.leq self.t (N (1, x, E, E))
+
+let[@inline] peek_min_exn (self : _ t) =
+  match self.t with
+  | E -> raise Empty
+  | N (_, x, _, _) -> x
+
+let[@inline] pop_min_exn (self : _ t) =
+  match self.t with
+  | E -> raise Empty
+  | N (_, x, l, r) ->
+    self.t <- merge ~leq:self.leq l r;
+    x
diff --git a/src/posix/heap.mli b/src/posix/heap.mli
new file mode 100644
index 0000000..3efd4c4
--- /dev/null
+++ b/src/posix/heap.mli
@@ -0,0 +1,13 @@
+type 'a t
+
+val create : leq:('a -> 'a -> bool) -> unit -> 'a t
+
+val is_empty : _ t -> bool
+(** [is_empty h] returns [true] if the heap [h] is empty. *)
+
+exception Empty
+
+val clear : _ t -> unit
+val insert : 'a t -> 'a -> unit
+val peek_min_exn : 'a t -> 'a
+val pop_min_exn : 'a t -> 'a
diff --git a/src/posix/nanoev_posix.ml b/src/posix/nanoev_posix.ml
new file mode 100644
index 0000000..2e9a5ca
--- /dev/null
+++ b/src/posix/nanoev_posix.ml
@@ -0,0 +1,403 @@
+open struct
+  module Trace_ = Nanoev.Trace_
+
+  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
+  open Iomux.Util
+
+  type t = Unix.file_descr
+
+  let equal a b = Int.equal (fd_of_unix a) (fd_of_unix b)
+  let hash a = Hashtbl.hash (fd_of_unix a)
+end)
+
+module P = Iomux.Poll
+module Flags = P.Flags
+
+module Sync_queue = struct
+  type 'a t = {
+    q: 'a Queue.t;
+    mutex: Mutex.t;
+  }
+
+  let create () : _ t = { q = Queue.create (); mutex = Mutex.create () }
+
+  let push (self : _ t) x : unit =
+    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
+
+type timer_ev =
+  | Timer : {
+      deadline: int64;
+      x: 'a;
+      y: 'b;
+      f: 'a -> 'b -> unit;
+    }
+      -> timer_ev
+
+type fd_data = {
+  fd: Unix.file_descr;
+  mutable idx: int;
+      (** 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] 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));
+  !fl
+
+type queued_task =
+  | 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: 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: 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 *)
+  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 *)
+}
+
+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 -> ()
+  | Sub (x, y, f, tail) ->
+    f ~closed x y;
+    perform_cbs ~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 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 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 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;
+
+    (* 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) =
+  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 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.posix.step" in
+
+  self.owner_thread <- Thread.(id (self ()));
+  let now = now_ns () in
+  let timeout_ns : int64 =
+    match next_deadline_ self with
+    | None -> 30_000_000_000L
+    | Some d -> Int64.max 0L (Int64.sub d now)
+  in
+
+  (* run timers *)
+  while
+    if Heap.is_empty self.timer then
+      false
+    else (
+      let (Timer t) = Heap.peek_min_exn self.timer in
+      if t.deadline <= now then (
+        ignore (Heap.pop_min_exn self.timer : timer_ev);
+        t.f t.x t.y;
+        true
+      ) else
+        false
+    )
+  do
+    ()
+  done;
+
+  (* 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__ "poll" ~data:(fun () ->
+          [ "timeout", `Float (ns_to_s timeout_ns); "len", `Int self.len ])
+    in
+    P.ppoll_or_poll self.poll self.len (Nanoseconds timeout_ns)
+  in
+
+  Atomic.set self.in_poll false;
+
+  (* drain notification pipe *)
+  if Atomic.exchange self.wakeup_triggered false then (
+    let b1 = Bytes.create 8 in
+    while try Unix.read self.wakeup_rd b1 0 8 > 0 with _ -> false do
+      ()
+    done
+  );
+
+  (* call callbacks *)
+  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
+      ));
+
+  ()
+
+let ops : st Nanoev.Impl.ops =
+  {
+    step;
+    close;
+    on_readable;
+    on_writable;
+    run_after_s;
+    max_fds;
+    wakeup_from_outside;
+    clear;
+  }
+
+include Nanoev
+
+let create () : t = Impl.build ops (create_st ())
diff --git a/src/posix/nanoev_posix.mli b/src/posix/nanoev_posix.mli
new file mode 100644
index 0000000..60e88ee
--- /dev/null
+++ b/src/posix/nanoev_posix.mli
@@ -0,0 +1,8 @@
+(** Nano event loop using Poll/Ppoll *)
+
+include module type of struct
+  include Nanoev
+end
+
+val create : unit -> t
+(** Create a new nanoev loop using [Iomux] (poll/ppoll). *)
diff --git a/src/tiny_httpd/dune b/src/tiny_httpd/dune
index a372e9d..12a3c11 100644
--- a/src/tiny_httpd/dune
+++ b/src/tiny_httpd/dune
@@ -5,6 +5,7 @@
   threads
   picos
   (re_export nanoev)
-  nanoev.picos
+  (re_export nanoev-picos)
+  picos_std.sync
   (re_export iostream)
   (re_export tiny_httpd)))
diff --git a/src/tiny_httpd/nanoev_tiny_httpd.ml b/src/tiny_httpd/nanoev_tiny_httpd.ml
index 7369fde..b637122 100644
--- a/src/tiny_httpd/nanoev_tiny_httpd.ml
+++ b/src/tiny_httpd/nanoev_tiny_httpd.ml
@@ -5,45 +5,8 @@ module Slice = Iostream.Slice
 module Pool = TH.Pool
 module Buf = TH.Buf
 
-let unwrap_ = function
-  | None -> ()
-  | Some (exn, bt) -> Printexc.raise_with_backtrace exn bt
-
+module Sem_ = Picos_std_sync.Semaphore.Counting
 (** Non blocking semaphore *)
-module Sem_ = struct
-  type t = {
-    mutable n: int;
-    max: int;
-    waiting: Picos.Trigger.t Queue.t;
-    mutex: Mutex.t;
-  }
-
-  let create n =
-    if n <= 0 then invalid_arg "Semaphore.create";
-    { n; max = n; mutex = Mutex.create (); waiting = Queue.create () }
-
-  let acquire self =
-    Mutex.lock self.mutex;
-    while self.n = 0 do
-      let tr = Picos.Trigger.create () in
-      Queue.push tr self.waiting;
-      Mutex.unlock self.mutex;
-      let res = Picos.Trigger.await tr in
-      unwrap_ res;
-      Mutex.lock self.mutex
-    done;
-    assert (self.n > 0);
-    self.n <- self.n - 1;
-    Mutex.unlock self.mutex
-
-  let release self =
-    Mutex.lock self.mutex;
-    self.n <- self.n + 1;
-    Option.iter Picos.Trigger.signal (Queue.take_opt self.waiting);
-    Mutex.unlock self.mutex
-
-  let num_acquired self = self.max - self.n
-end
 
 module Out = struct
   open Iostream
@@ -60,7 +23,7 @@ module Out = struct
         let i = ref i in
         let len = ref len0 in
         while !len > 0 do
-          match EV.write fd bs !i !len with
+          match EV.write_once fd bs !i !len with
           | 0 -> failwith "write failed"
           | n ->
             i := !i + n;
@@ -147,7 +110,7 @@ module Unix_tcp_server_ = struct
     new_thread: (unit -> unit) -> unit;
     timeout: float;
     masksigpipe: bool;
-    mutable running: bool; (* TODO: use an atomic? *)
+    running: bool Atomic.t;
   }
 
   let shutdown_silent_ fd =
@@ -183,7 +146,7 @@ module Unix_tcp_server_ = struct
             let inet_addr = Unix.inet_addr_of_string self.addr in
             Unix.setsockopt sock Unix.SO_REUSEADDR true;
             Unix.bind sock (Unix.ADDR_INET (inet_addr, self.port));
-            let n_listen = 2 * self.max_connections in
+            let n_listen = self.max_connections in
             Unix.listen sock n_listen
           );
 
@@ -191,10 +154,16 @@ module Unix_tcp_server_ = struct
 
           let tcp_server =
             {
-              TH.IO.TCP_server.stop = (fun () -> self.running <- false);
-              running = (fun () -> self.running);
+              TH.IO.TCP_server.stop =
+                (fun () ->
+                  Atomic.set self.running false;
+
+                  (* close accept socket so the main loop will return *)
+                  try Unix.close sock with _ -> ());
+              running = (fun () -> Atomic.get self.running);
               active_connections =
-                (fun () -> Sem_.num_acquired self.sem_max_connections);
+                (fun () ->
+                  self.max_connections - Sem_.get_value self.sem_max_connections);
               endpoint =
                 (fun () ->
                   let addr, port = get_addr_ sock in
@@ -233,7 +202,7 @@ module Unix_tcp_server_ = struct
           in
 
           Unix.set_nonblock sock;
-          while self.running do
+          while Atomic.get self.running do
             match EV.accept sock with
             | client_sock, client_addr ->
               (* limit concurrency *)
@@ -272,7 +241,7 @@ module Unix_tcp_server_ = struct
           done;
 
           (* Wait for all threads to be done: this only works if all threads are done. *)
-          Unix.close sock;
+          (try Unix.close sock with _ -> ());
           (* TODO? *)
           (* Sem_.acquire self.sem_max_connections.max self.sem_max_connections; *)
           ());
@@ -281,7 +250,7 @@ end
 
 open struct
   let get_max_connection_ ?(max_connections = 2048) () : int =
-    let max_connections = max 4 max_connections in
+    let max_connections = min (max 4 @@ EV.max_fds ()) max_connections in
     max_connections
 
   let clear_slice (slice : Slice.t) =
@@ -290,16 +259,22 @@ open struct
     slice.len <- 0
 end
 
-let create ?(masksigpipe = not Sys.win32) ?max_connections ?(timeout = 0.0)
-    ?buf_size ?(get_time_s = Unix.gettimeofday) ?(addr = "127.0.0.1")
-    ?(port = 8080) ?sock ?middlewares ~new_thread () : TH.Server.t =
+let create ?(masksigpipe = not Sys.win32) ?max_connections ?max_buf_pool_size
+    ?(timeout = 0.0) ?buf_size ?(get_time_s = Unix.gettimeofday)
+    ?(addr = "127.0.0.1") ?(port = 8080) ?sock ?middlewares ~new_thread () :
+    TH.Server.t =
   let max_connections = get_max_connection_ ?max_connections () in
+  let max_pool_size =
+    match max_buf_pool_size with
+    | None -> min 4096 max_connections * 2
+    | Some m -> m
+  in
   let server =
     {
       Unix_tcp_server_.addr;
       new_thread;
       buf_pool =
-        Pool.create ~clear:Buf.clear_and_zero
+        Pool.create ~clear:Buf.clear_and_zero ~max_size:max_pool_size
           ~mk_item:(fun () -> Buf.create ?size:buf_size ())
           ();
       slice_pool =
@@ -308,11 +283,11 @@ let create ?(masksigpipe = not Sys.win32) ?max_connections ?(timeout = 0.0)
             (let buf_size = Option.value buf_size ~default:4096 in
              fun () -> Slice.create buf_size)
           ();
-      running = true;
+      running = Atomic.make true;
       port;
       sock;
       max_connections;
-      sem_max_connections = Sem_.create max_connections;
+      sem_max_connections = Sem_.make max_connections;
       masksigpipe;
       timeout;
     }
diff --git a/src/tiny_httpd/nanoev_tiny_httpd.mli b/src/tiny_httpd/nanoev_tiny_httpd.mli
index f0cd9af..7c858e5 100644
--- a/src/tiny_httpd/nanoev_tiny_httpd.mli
+++ b/src/tiny_httpd/nanoev_tiny_httpd.mli
@@ -3,6 +3,7 @@ module TH = Tiny_httpd_core
 val create :
   ?masksigpipe:bool ->
   ?max_connections:int ->
+  ?max_buf_pool_size:int ->
   ?timeout:float ->
   ?buf_size:int ->
   ?get_time_s:(unit -> float) ->
diff --git a/src/unix/dune b/src/unix/dune
index 4f14854..3c0fe50 100644
--- a/src/unix/dune
+++ b/src/unix/dune
@@ -2,4 +2,5 @@
  (name nanoev_unix)
  (public_name nanoev.unix)
  (synopsis "Unix/select backend")
+ (private_modules heap)
  (libraries nanoev unix))
diff --git a/src/unix/heap.ml b/src/unix/heap.ml
index a9a9c9e..1a553d3 100644
--- a/src/unix/heap.ml
+++ b/src/unix/heap.ml
@@ -22,10 +22,10 @@ open struct
     | E -> 0
     | N (r, _, _, _) -> r
 
-  (** Make a balanced node labelled with [x], and subtrees [a] and [b].
-     We ensure that the right child's rank is ≤ to the rank of the
-     left child (leftist property). The rank of the resulting node
-     is the length of the rightmost path. *)
+  (** Make a balanced node labelled with [x], and subtrees [a] and [b]. We
+      ensure that the right child's rank is ≤ to the rank of the left child
+      (leftist property). The rank of the resulting node is the length of the
+      rightmost path. *)
   let[@inline] mk_node_ x a b =
     if rank_ a >= rank_ b then
       N (rank_ b + 1, x, a, b)
diff --git a/src/unix/nanoev_unix.ml b/src/unix/nanoev_unix.ml
index 7d656c1..17b2088 100644
--- a/src/unix/nanoev_unix.ml
+++ b/src/unix/nanoev_unix.ml
@@ -174,17 +174,35 @@ let next_deadline_ (self : st) : float option =
 let step (self : st) : unit =
   let@ _sp = Trace_.with_span ~__FILE__ ~__LINE__ "nanoev.unix.step" in
   (* gather the subscriptions and timeout *)
+  let now = now_ () in
   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_ ())
+      | Some d -> max 0. (d -. now)
     in
     timeout, self.sub_r, self.sub_w
   in
 
+  (* run timers *)
+  while
+    if Heap.is_empty self.timer then
+      false
+    else (
+      let (Timer t) = Heap.peek_min_exn self.timer in
+      if t.deadline <= now then (
+        ignore (Heap.pop_min_exn self.timer : timer_ev);
+        t.f t.x t.y;
+        true
+      ) else
+        false
+    )
+  do
+    ()
+  done;
+
   (* enter [select] *)
   Atomic.set self.in_select true;
   let r_reads, r_writes, _ =
@@ -243,12 +261,16 @@ let step (self : st) : unit =
 
   ()
 
+(* limit for select is fixed and known *)
+let max_fds _ = 1024
+
 let ops : st Nanoev.Impl.ops =
   {
     step;
     close;
     on_readable;
     on_writable;
+    max_fds;
     run_after_s;
     wakeup_from_outside;
     clear;
diff --git a/tests/posix/dune b/tests/posix/dune
new file mode 100644
index 0000000..08e8e1a
--- /dev/null
+++ b/tests/posix/dune
@@ -0,0 +1,3 @@
+(tests
+ (names t1)
+ (libraries nanoev nanoev-posix threads))
diff --git a/tests/posix/echo/README.md b/tests/posix/echo/README.md
new file mode 100644
index 0000000..0fdef34
--- /dev/null
+++ b/tests/posix/echo/README.md
@@ -0,0 +1,6 @@
+
+notes about system limits in Linux:
+- `ulimit -n 100000` will raise the max number of FDs for a process to 100000
+- `/proc/sys/net/core/netdev_max_backlog` controls the kernel backlog size, raise it (default is 1000)
+- `/proc/sys/net/core/somaxconn` is the max size of a socket backlog (as given to `listen()`), raise it (default is 4096)
+
diff --git a/tests/posix/echo/dune b/tests/posix/echo/dune
new file mode 100644
index 0000000..21b26c2
--- /dev/null
+++ b/tests/posix/echo/dune
@@ -0,0 +1,4 @@
+(executables
+ (names echo_server echo_client)
+ (libraries moonpool moonpool.fib nanoev-picos nanoev-posix iostream
+   trace.core trace-tef))
diff --git a/tests/posix/echo/echo_client.ml b/tests/posix/echo/echo_client.ml
new file mode 100644
index 0000000..859dec1
--- /dev/null
+++ b/tests/posix/echo/echo_client.ml
@@ -0,0 +1,139 @@
+module Trace = Trace_core
+module F = Moonpool_fib
+module IO = Nanoev_picos
+module Sem = Picos_std_sync.Semaphore.Counting
+
+[@@@ocaml.alert "-deprecated"]
+
+let ( let@ ) = ( @@ )
+let spf = Printf.sprintf
+let pf = Printf.printf
+let verbose = ref false
+let reset_line = "\x1b[2K\r"
+let n_loops_per_task = 100
+
+let main ~runner:_ ~port ~unix_sock ~n ~n_conn () =
+  Sys.set_signal Sys.sigpipe Sys.Signal_ignore;
+
+  pf "connect on %s n=%d n_conn=%d\n%!"
+    (if unix_sock = "" then
+       spf "localhost:%d" port
+     else
+       spf "unix:%S" unix_sock)
+    n n_conn;
+
+  let addr =
+    if unix_sock = "" then
+      Unix.ADDR_INET (Unix.inet_addr_loopback, port)
+    else
+      Unix.ADDR_UNIX unix_sock
+  in
+
+  Printf.printf "connecting to port %d\n%!" port;
+
+  let all_done = Atomic.make false in
+  let n_queries = Atomic.make 0 in
+
+  (* limit simultaneous number of connections *)
+  let sem = Sem.make n_conn in
+  let n_active_conns = Atomic.make 0 in
+
+  let progress_loop () =
+    while not (Atomic.get all_done) do
+      let n_queries = Atomic.get n_queries in
+      let n_conns = Atomic.get n_active_conns in
+
+      (* progress *)
+      Printf.printf "%sdone %d queries, %d active connections%!" reset_line
+        n_queries n_conns;
+
+      Trace.counter_int ~level:Info "n-conns" n_conns;
+      Trace.counter_int ~level:Info "n-queries" n_queries;
+      let gc = Gc.quick_stat () in
+      Trace.counter_int ~level:Info "gc.major" gc.major_collections;
+      Trace.counter_int ~level:Info "gc.minor" gc.minor_collections;
+      Trace.counter_int ~level:Info "gc.heap-size" (gc.heap_words * 64);
+
+      Thread.delay 0.2
+    done
+  in
+
+  ignore (Thread.create progress_loop () : Thread.t);
+
+  let run_task () =
+    let _task_sp =
+      Trace.enter_manual_toplevel_span ~__FILE__ ~__LINE__ "run-task"
+    in
+    Sem.acquire sem;
+    ( IO.Net_client.with_connect addr @@ fun ic oc ->
+      Atomic.incr n_active_conns;
+      let buf = Bytes.create 32 in
+
+      for _j = 1 to n_loops_per_task do
+        (*let _sp =
+            Trace.enter_manual_sub_span ~parent:_task_sp ~__FILE__ ~__LINE__
+              "write.loop" ~data:(fun () -> [ "iter", `Int _j ])
+          in*)
+        Atomic.incr n_queries;
+
+        Iostream.Out.output_string oc "hello";
+        Iostream.Out_buf.flush oc;
+
+        (* read back what we wrote *)
+        Iostream.In.really_input ic buf 0 (String.length "hello");
+        (* Trace.exit_manual_span _sp; *)
+        F.yield ()
+      done;
+
+      Atomic.decr n_active_conns;
+      Sem.release sem );
+
+    Trace.exit_manual_span _task_sp
+  in
+
+  let t_start = Mtime_clock.now () in
+
+  (* start the first [n_conn] tasks *)
+  let fibers = List.init (n * n_conn) (fun _ -> F.spawn run_task) in
+  List.iter F.await fibers;
+  Atomic.set all_done true;
+
+  let t_stop = Mtime_clock.now () in
+  let elapsed_s =
+    (Mtime.span t_start t_stop |> Mtime.Span.to_uint64_ns |> Int64.to_float)
+    *. 1e-9
+  in
+
+  (* exit when [fut_exit] is resolved *)
+  Printf.printf
+    "%sdone with main (time=%.4fs, n queries=%d, expect=%d, %.3f req/s)\n%!"
+    reset_line elapsed_s (Atomic.get n_queries)
+    (n * n_conn * n_loops_per_task)
+    (float (Atomic.get n_queries) /. elapsed_s)
+
+let () =
+  let@ () = Trace_tef.with_setup () in
+  Trace.set_current_level Info;
+  Trace.set_thread_name "main";
+
+  let port = ref 1234 in
+  let unix_sock = ref "" in
+  let n = ref 1000 in
+  let n_conn = ref 20 in
+  let opts =
+    [
+      "-p", Arg.Set_int port, " port";
+      "-v", Arg.Set verbose, " verbose";
+      "-n", Arg.Set_int n, " number of iterations";
+      "--unix", Arg.Set_string unix_sock, " unix socket";
+      "--n-conn", Arg.Set_int n_conn, " number of simultaneous connections";
+    ]
+    |> Arg.align
+  in
+  Arg.parse opts ignore "echo_client";
+
+  let@ () =
+    Nanoev_picos.Background_thread.with_setup (Nanoev_posix.create ())
+  in
+  F.main @@ fun runner ->
+  main ~runner ~port:!port ~unix_sock:!unix_sock ~n:!n ~n_conn:!n_conn ()
diff --git a/tests/posix/echo/echo_server.ml b/tests/posix/echo/echo_server.ml
new file mode 100644
index 0000000..7ed110b
--- /dev/null
+++ b/tests/posix/echo/echo_server.ml
@@ -0,0 +1,118 @@
+module F = Moonpool_fib
+module IO = Nanoev_picos
+module Trace = Trace_core
+
+[@@@ocaml.alert "-deprecated"]
+
+let ( let@ ) = ( @@ )
+let pf = Printf.printf
+let spf = Printf.sprintf
+let verbose = ref false
+let n_reply_response = Atomic.make 0
+
+let str_of_sockaddr = function
+  | Unix.ADDR_UNIX s -> s
+  | Unix.ADDR_INET (addr, port) ->
+    spf "%s:%d" (Unix.string_of_inet_addr addr) port
+
+let main ~port ~unix_sock ~max_conns ~runner () =
+  Sys.set_signal Sys.sigpipe Sys.Signal_ignore;
+
+  pf "serve on %s\n%!"
+    (if unix_sock = "" then
+       spf "localhost:%d" port
+     else
+       spf "unix:%S" unix_sock);
+
+  let addr =
+    if unix_sock = "" then
+      Unix.ADDR_INET (Unix.inet_addr_loopback, port)
+    else (
+      (* remove leftover unix socket file, if any *)
+      (try Sys.remove unix_sock with _ -> ());
+      Unix.ADDR_UNIX unix_sock
+    )
+  in
+
+  let server =
+    IO.Net_server.establish ?max_connections:max_conns addr
+      ~spawn:(fun f -> Moonpool.spawn ~on:runner f)
+      ~client_handler:(fun client_addr ic oc ->
+        let _sp =
+          Trace.enter_manual_toplevel_span ~__FILE__ ~__LINE__ "serve"
+        in
+
+        if !verbose then
+          pf "handle client on %s\n%!" (str_of_sockaddr client_addr);
+
+        let buf = Bytes.create 256 in
+        let continue = ref true in
+        while !continue do
+          match Iostream.In.input ic buf 0 (Bytes.length buf) with
+          | exception exn ->
+            continue := false;
+            Printf.eprintf "error in client handler: %s\n%!"
+              (Printexc.to_string exn)
+          | 0 -> continue := false
+          | n ->
+            Atomic.incr n_reply_response;
+            Iostream.Out.output oc buf 0 n;
+            Iostream.Out_buf.flush oc;
+            Picos.Fiber.yield ()
+        done;
+
+        Trace.exit_manual_span _sp;
+        if !verbose then
+          pf "done with client on %s\n%!" (str_of_sockaddr client_addr))
+  in
+
+  Printf.printf "max number of connections: %d\n%!"
+    (IO.Net_server.max_connections server);
+
+  if Trace.enabled () then
+    ignore
+      (Thread.create
+         (fun () ->
+           while IO.Net_server.running server do
+             Trace.counter_int ~level:Info "n-conns"
+               (IO.Net_server.n_active_connections server);
+             let gc = Gc.quick_stat () in
+             Trace.counter_int ~level:Info "gc.major" gc.major_collections;
+             Trace.counter_int ~level:Info "gc.minor" gc.minor_collections;
+             Trace.counter_int ~level:Info "n-reply-response"
+               (Atomic.get n_reply_response);
+             Trace.counter_int ~level:Info "gc.heap-size" (gc.heap_words * 64);
+
+             Thread.delay 0.2
+           done)
+         ()
+        : Thread.t);
+
+  IO.Net_server.join server;
+  IO.Net_server.shutdown server;
+  print_endline "exit"
+
+let () =
+  let@ () = Trace_tef.with_setup () in
+  Trace.set_current_level Info;
+  let port = ref 1234 in
+  let unix_sock = ref "" in
+  let max_conns = ref None in
+  let opts =
+    [
+      "-p", Arg.Set_int port, " port";
+      "--unix", Arg.Set_string unix_sock, " unix socket";
+      ( "--max-conns",
+        Arg.Int (fun i -> max_conns := Some i),
+        " max number of connections" );
+      "-v", Arg.Set verbose, " verbose";
+    ]
+    |> Arg.align
+  in
+  Arg.parse opts ignore "echo_server";
+
+  let@ () =
+    Nanoev_picos.Background_thread.with_setup (Nanoev_posix.create ())
+  in
+  F.main @@ fun runner ->
+  main ~port:!port ~unix_sock:!unix_sock ~max_conns:!max_conns ~runner ()
diff --git a/tests/posix/t1.expected b/tests/posix/t1.expected
new file mode 100644
index 0000000..a36183e
--- /dev/null
+++ b/tests/posix/t1.expected
@@ -0,0 +1,3 @@
+writing
+can read
+done writing
diff --git a/tests/posix/t1.ml b/tests/posix/t1.ml
new file mode 100644
index 0000000..366a5c9
--- /dev/null
+++ b/tests/posix/t1.ml
@@ -0,0 +1,29 @@
+module E = Nanoev_posix
+
+let mkpipe () : Unix.file_descr * Unix.file_descr =
+  let f1, f2 = Unix.pipe () in
+  Unix.set_nonblock f1;
+  Unix.set_nonblock f2;
+  f1, f2
+
+let loop (e : E.t) =
+  while true do
+    E.step e
+  done
+
+let () =
+  let ev = E.create () in
+  ignore (Thread.create loop ev : Thread.t);
+  let rd, wr = mkpipe () in
+  E.on_readable ev rd () () (fun ~closed () () ->
+      if closed then
+        print_endline "closed!"
+      else
+        print_endline "can read");
+  Thread.delay 0.05;
+  print_endline "writing";
+  ignore
+    (Unix.write wr (Bytes.unsafe_of_string "hello") 0 (String.length "hello")
+      : int);
+  Thread.delay 0.1;
+  print_endline "done writing"