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