Module Lwt_preemptive

This module allows to mix preemptive threads with Lwt cooperative threads. It maintains an extensible pool of preemptive threads to which you can detach computations.

See Mwt for a more modern implementation.

val detach : ('a -> 'b) -> 'a -> 'b Lwt.t

detach f x runs the computation f x in a separate preemptive thread. detach evaluates to an Lwt promise, which is pending until the preemptive thread completes.

detach calls simple_init internally, which means that the number of preemptive threads is capped by default at four. If you would like a higher limit, call init or set_bounds directly.

Note that Lwt thread-local storage (i.e., Lwt.with_value) cannot be safely used from within f. The same goes for most of the rest of Lwt. If you need to run an Lwt thread in f, use run_in_main.

val run_in_main : (unit -> 'a Lwt.t) -> 'a

run_in_main f can be called from a detached computation to execute f () in the main preemptive thread, i.e. the one executing Lwt_main.run. run_in_main f blocks until f () completes, then returns its result. If f () raises an exception, run_in_main f raises the same exception.

Lwt.with_value may be used inside f (). Lwt.get can correctly retrieve values set this way inside f (), but not values set using Lwt.with_value outside f ().

val init : int -> int -> (string -> unit) -> unit

init min max log initialises this module. i.e. it launches the minimum number of preemptive threads and starts the dispatcher.

  • parameter min

    is the minimum number of threads

  • parameter max

    is the maximum number of threads

  • parameter log

    is used to log error messages

    If Lwt_preemptive has already been initialised, this call only modify bounds and the log function.

val simple_init : unit -> unit

simple_init () checks if the library is not yet initialized, and if not, does a simple initialization. The minimum number of threads is set to zero, maximum to four, and the log function is left unchanged, i.e. the default built-in logging function is used. See Lwt_preemptive.init.

Note: this function is automatically called by detach.

val get_bounds : unit -> int * int

get_bounds () returns the minimum and the maximum number of preemptive threads.

val set_bounds : (int * int) -> unit

set_bounds (min, max) set the minimum and the maximum number of preemptive threads.

val set_max_number_of_threads_queued : int -> unit

Sets the size of the waiting queue, if no more preemptive threads are available. When the queue is full, detach will sleep until a thread is available.

val get_max_number_of_threads_queued : unit -> int

Returns the size of the waiting queue, if no more threads are available