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;;; GNU Guix --- Functional package management for GNU
;;; Copyright © 2017 Ludovic Courtès <ludo@gnu.org>
;;;
;;; This file is part of GNU Guix.
;;;
;;; GNU Guix is free software; you can redistribute it and/or modify it
;;; under the terms of the GNU General Public License as published by
;;; the Free Software Foundation; either version 3 of the License, or (at
;;; your option) any later version.
;;;
;;; GNU Guix is distributed in the hope that it will be useful, but
;;; WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
;;; GNU General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with GNU Guix. If not, see <http://www.gnu.org/licenses/>.
(define-module (guix workers)
#:use-module (ice-9 threads)
#:use-module (ice-9 match)
#:use-module (ice-9 q)
#:use-module (srfi srfi-1)
#:use-module (srfi srfi-9)
#:use-module (srfi srfi-26)
#:use-module ((guix build syscalls) #:select (set-thread-name))
#:export (pool?
make-pool
pool-enqueue!
pool-idle?
eventually))
;;; Commentary:
;;;
;;; This module implements "worker pools". Worker pools are the low-level
;;; mechanism that's behind futures: there's a fixed set of threads
;;; ("workers") that one can submit work to, and one of them will eventually
;;; pick the submitted tasks.
;;;
;;; Unlike futures, these worker pools are meant to be used for tasks that
;;; have a side-effect. Thus, we never "touch" a task that was submitted like
;;; we "touch" a future. Instead, we simply assume that the task will
;;; eventually complete.
;;;
;;; Code:
(define-record-type <pool>
(%make-pool queue mutex condvar workers busy)
pool?
(queue pool-queue)
(mutex pool-mutex)
(condvar pool-condition-variable)
(workers pool-workers)
(busy pool-busy))
(define-syntax-rule (without-mutex mutex exp ...)
(dynamic-wind
(lambda ()
(unlock-mutex mutex))
(lambda ()
exp ...)
(lambda ()
(lock-mutex mutex))))
(define* (worker-thunk mutex condvar pop-queue
#:key idle busy (thread-name "guix worker"))
"Return the thunk executed by worker threads."
(define (loop)
(match (pop-queue)
(#f ;empty queue
(idle)
(wait-condition-variable condvar mutex)
(busy))
((? procedure? proc)
;; Release MUTEX while executing PROC.
(without-mutex mutex
(catch #t proc
(const #f)
(lambda (key . args)
;; XXX: In Guile 2.0 ports are not thread-safe, so this could
;; crash (Guile 2.2 is fine).
(display-backtrace (make-stack #t) (current-error-port))
(print-exception (current-error-port)
(and=> (make-stack #t)
(cut stack-ref <> 0))
key args))))))
(loop))
(lambda ()
(catch 'system-error
(lambda ()
(set-thread-name thread-name))
(const #f))
(with-mutex mutex
(loop))))
(define* (make-pool #:optional (count (current-processor-count))
#:key (thread-name "guix worker"))
"Return a pool of COUNT workers. Use THREAD-NAME as the name of these
threads as reported by the operating system."
(let* ((mutex (make-mutex))
(condvar (make-condition-variable))
(queue (make-q))
(busy count)
(procs (unfold (cut >= <> count)
(lambda (n)
(worker-thunk mutex condvar
(lambda ()
(and (not (q-empty? queue))
(q-pop! queue)))
#:busy (lambda ()
(set! busy (+ 1 busy)))
#:idle (lambda ()
(set! busy (- busy 1)))
#:thread-name thread-name))
1+
0))
(threads (map (lambda (proc)
(call-with-new-thread proc))
procs)))
(%make-pool queue mutex condvar threads (lambda () busy))))
(define (pool-enqueue! pool thunk)
"Enqueue THUNK for future execution by POOL."
(with-mutex (pool-mutex pool)
(enq! (pool-queue pool) thunk)
(signal-condition-variable (pool-condition-variable pool))))
(define (pool-idle? pool)
"Return true if POOL doesn't have any task in its queue and all the workers
are currently idle (i.e., waiting for a task)."
(with-mutex (pool-mutex pool)
(and (q-empty? (pool-queue pool))
(zero? ((pool-busy pool))))))
(define-syntax-rule (eventually pool exp ...)
"Run EXP eventually on one of the workers of POOL."
(pool-enqueue! pool (lambda () exp ...)))
;;; Local Variables:
;;; eval: (put 'without-mutex 'scheme-indent-function 1)
;;; End:
;;; workers.scm ends here
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