mtbox is a suite of software tools designed to monitor Miou applications. There are three tools:

• recd: used to generate a trace (in the Chrome Trace Event Format) which the user can analyse using Perfetto, for example.
• mtop: a TUI that allows you to monitor a Miou application in real time. This can be useful for gaining a clear understanding of what a Miou programme is doing. This can be useful for identifying potential tasks that never end.
• diag: a tool for understanding the errors that a Miou application may throw if the user does not follow Miou's rules (in addition, whenever an exception such as Still_has_children is thrown).

These tools facilitate the debugging of a Miou application and make judicious use of both its own trace system and that provided by OCaml. However, such a system has limitations: indeed, the OCaml event system can lose events, and these tools may therefore miss information that could be important for interpreting events (and errors).

In short, these tools are not designed to evaluate Miou applications under heavy load (such as, for example, a benchmark).

Miou has a few rules regarding task and resource management which, if not followed, will trigger an uncatchable exception when the programme is running. We are referring here to the following exceptions:

• Still_has_children: when a task terminates without having waited for (or cancelled) all its subtasks
• Not_a_child: when attempting to wait for a task that does not belong to us (with which we have no direct parent-child relationship)
• Resource_leaked: when we have not released the resource associated with a task that owns it, even though that task has finished
• Not_owner: when attempting to manipulate a resource that does not belong to our current task.

It can be difficult to know where these errors originate, and the diag tool provides a detailed report if such an exception is raised by your programme. Simply add the Miou and OCaml trace system to your programme and run it with diag. Here is an example:

$ cat >main.ml <<EOF
> let () =
>   Miou.Trace.set_reporter Miou_runtime_events.reporter;
>   Miou_unix.run @@ fun () ->
>   let prm = Miou.async @@ fun () ->
>     let _ = Miou.async ignore in (* <- Error here! *)
>     print_endline "done" in
>   Miou.await_exn prm
> EOF
$ ocamlfind opt -linkpkg -package miou.unix,miou.runtime_events main.ml
$ mtbox.diag -- ./a.out
diag: attached to pid=111xxx dir=/tmp
done
a.out: [0][ERROR] [0] failed with "Miou.Still_has_children"
Fatal error: exception Miou.Still_has_children
[2026-04-21T12:13:44Z] ERROR Still_has_children  
        task #2 main.ml:4 finished while still holding 1 unawaited child (fix: await or cancel every child before returning)
        parent chain: #2
        - child #3 main.ml:5 (parent=#2, runner=dom0)
diag: exit (tasks seen=2, lost=0)

mtop is an (experimental) application that provides a dynamic view of a Miou programme. It allows you to monitor task execution and the usage of domains and resources. You can see an example by running:

$ mtbox.mtop -- dune exec ./demo/demo.exe

You can exit the programme by pressing ESC. The demo programme launches several tasks as well as an 'echo' server (you can follow the tutorial on how to implement an echo server with Miou). The programme also requires at least 4 domains. It is simply designed to demonstrate mtop.

The OCaml runtime events transport is a fixed-size ring buffer as a file, bursts of activity can make the target overwrite events faster than mtop can read them. When that happens, the OCaml runtime reports the number of lost events and mtop displays a red LOST N badge in the status bar. State reconstructed after a loss may be briefly out of sync until the next authoritative event arrives.

mtop was developed using notty-miou (as well as lwd) as a demonstration of what can be achieved in terms of TUI with Miou. It is a good example of a programme that can be written in OCaml, but we would like to warn users that it is purely for cosmetic purposes: recd and diag are more interesting and useful than mtop.