package moonpool

  1. Overview
  2. Docs

A simple thread pool in FIFO order.

FIFO: first-in, first-out. Basically tasks are put into a queue, and worker threads pull them out of the queue at the other end.

Since this uses a single blocking queue to manage tasks, it's very simple and reliable. The number of worker threads is fixed, but they are spread over several domains to enable parallelism.

This can be useful for latency-sensitive applications (e.g. as a pool of workers for network servers). Work-stealing pools might have higher throughput but they're very unfair to some tasks; by contrast, here, older tasks have priority over younger tasks.

  • since 0.5
include module type of Runner
type task = unit -> unit
type t

A runner.

If a runner is no longer needed, shutdown can be used to signal all worker threads in it to stop (after they finish their work), and wait for them to stop.

The threads are distributed across a fixed domain pool (whose size is determined by Domain.recommended_domain_count on OCaml 5, and simple the single runtime on OCaml 4).

val size : t -> int

Number of threads/workers.

val num_tasks : t -> int

Current number of tasks. This is at best a snapshot, useful for metrics and debugging.

val shutdown : t -> unit

Shutdown the runner and wait for it to terminate. Idempotent.

val shutdown_without_waiting : t -> unit

Shutdown the pool, and do not wait for it to terminate. Idempotent.

exception Shutdown
val run_async : ?ls:Task_local_storage.t -> t -> task -> unit

run_async pool f schedules f for later execution on the runner in one of the threads. f() will run on one of the runner's worker threads/domains.

  • parameter ls

    if provided, run the task with this initial local storage

  • raises Shutdown

    if the runner was shut down before run_async was called.

val run_wait_block : ?ls:Task_local_storage.t -> t -> (unit -> 'a) -> 'a

run_wait_block pool f schedules f for later execution on the pool, like run_async. It then blocks the current thread until f() is done executing, and returns its result. If f() raises an exception, then run_wait_block pool f will raise it as well.

NOTE be careful with deadlocks (see notes in Fut.wait_block about the required discipline to avoid deadlocks).

  • raises Shutdown

    if the runner was already shut down

val dummy : t

Runner that fails when scheduling tasks on it. Calling run_async on it will raise Failure.

  • since 0.6

Implementing runners

module For_runner_implementors : sig ... end

This module is specifically intended for users who implement their own runners. Regular users of Moonpool should not need to look at it.

val get_current_runner : unit -> t option

Access the current runner. This returns Some r if the call happens on a thread that belongs in a runner.

  • since 0.5
val get_current_storage : unit -> Task_local_storage.t option

get_current_storage runner gets the local storage for the currently running task.

type ('a, 'b) create_args = ?on_init_thread:(dom_id:int -> t_id:int -> unit -> unit) -> ?on_exit_thread:(dom_id:int -> t_id:int -> unit -> unit) -> ?on_exn:(exn -> Stdlib.Printexc.raw_backtrace -> unit) -> ?around_task:((t -> 'b) * (t -> 'b -> unit)) -> ?num_threads:int -> ?name:string -> 'a

Arguments used in create. See create for explanations.

val create : (unit -> t, _) create_args

create () makes a new thread pool.

  • parameter on_init_thread

    called at the beginning of each new thread in the pool.

  • parameter min

    minimum size of the pool. See Pool.create_args. The default is Domain.recommended_domain_count(), ie one worker per CPU core. On OCaml 4 the default is 4 (since there is only one domain).

  • parameter on_exit_thread

    called at the end of each worker thread in the pool.

  • parameter around_task

    a pair of before, after functions ran around each task. See Pool.create_args.

  • parameter name

    name for the pool, used in tracing (since 0.6)

val with_ : (unit -> (t -> 'a) -> 'a, _) create_args

with_ () f calls f pool, where pool is obtained via create. When f pool returns or fails, pool is shutdown and its resources are released. Most parameters are the same as in create.


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