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Revert "futures: Limit the number of nested futures on the same stack."

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This reverts commit 8a177d316c, though
keeping the additional tests.  (Guile 2.2 doesn't have a fixed stack
limit).
This commit is contained in:
Andy Wingo 2017-02-28 10:45:21 +01:00
parent 70d4c4b284
commit 9e28a12121
2 changed files with 7 additions and 23 deletions
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7
doc/ref/api-scheduling.texi
View file

@ -830,13 +830,6 @@ future has completed. This suspend/resume is achieved by capturing the
calling future's continuation, and later reinstating it (@pxref{Prompts,
delimited continuations}).
Note that @code{par-map} above is not tail-recursive. This could lead
to stack overflows when @var{lst} is large compared to
@code{(current-processor-count)}. To address that, @code{touch} uses
the suspend mechanism described above to limit the number of nested
futures executing on the same stack. Thus, the above code should never
run into stack overflows.
@deffn {Scheme Syntax} future exp
Return a future for expression @var{exp}. This is equivalent to:

23
module/ice-9/futures.scm
View file

@ -1,6 +1,6 @@
;;; -*- mode: scheme; coding: utf-8; -*-
;;;
;;; Copyright (C) 2010, 2011, 2012, 2013 Free Software Foundation, Inc.
;;; Copyright (C) 2010, 2011, 2012 Free Software Foundation, Inc.
;;;
;;; This library is free software; you can redistribute it and/or
;;; modify it under the terms of the GNU Lesser General Public
@ -90,14 +90,8 @@ touched."
;; A mapping of nested futures to futures waiting for them to complete.
(define %futures-waiting '())
;; Nesting level of futures. Incremented each time a future is touched
;; from within a future.
(define %nesting-level (make-parameter 0))
;; Maximum nesting level. The point is to avoid stack overflows when
;; nested futures are executed on the same stack. See
;; <http://bugs.gnu.org/13188>.
(define %max-nesting-level 200)
;; Whether currently running within a future.
(define %within-future? (make-parameter #f))
(define-syntax-rule (with-mutex m e0 e1 ...)
;; Copied from (ice-9 threads) to avoid circular dependency.
@ -153,8 +147,7 @@ adding it to the waiter queue."
(thunk (lambda ()
(call-with-prompt %future-prompt
(lambda ()
(parameterize ((%nesting-level
(1+ (%nesting-level))))
(parameterize ((%within-future? #t))
((future-thunk future))))
suspend))))
(set-future-result! future
@ -253,16 +246,14 @@ adding it to the waiter queue."
(unlock-mutex (future-mutex future)))
((started)
(unlock-mutex (future-mutex future))
(if (> (%nesting-level) 0)
(if (%within-future?)
(abort-to-prompt %future-prompt future)
(begin
(work)
(loop))))
(else ; queued
(else
(unlock-mutex (future-mutex future))
(if (> (%nesting-level) %max-nesting-level)
(abort-to-prompt %future-prompt future)
(work))
(work)
(loop))))
((future-result future)))