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[Guile-commits] 01/01: Revert "futures: Limit the number of nested futur

From: Andy Wingo
Subject: [Guile-commits] 01/01: Revert "futures: Limit the number of nested futures on the same stack."
Date: Tue, 28 Feb 2017 04:53:52 -0500 (EST)

wingo pushed a commit to branch master
in repository guile.

commit 9e28a12121414b4b0508d56c9fe011d9059f48b7
Author: Andy Wingo <address@hidden>
Date:   Tue Feb 28 10:45:21 2017 +0100

    Revert "futures: Limit the number of nested futures on the same stack."
    This reverts commit 8a177d316c0062afe74f9a761ef460e297435e59, though
    keeping the additional tests.  (Guile 2.2 doesn't have a fixed stack
 doc/ref/api-scheduling.texi |  7 -------
 module/ice-9/futures.scm    | 23 +++++++----------------
 2 files changed, 7 insertions(+), 23 deletions(-)

diff --git a/doc/ref/api-scheduling.texi b/doc/ref/api-scheduling.texi
index bf85a64..ff8473a 100644
--- a/doc/ref/api-scheduling.texi
+++ b/doc/ref/api-scheduling.texi
@@ -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
address@hidden(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:
diff --git a/module/ice-9/futures.scm b/module/ice-9/futures.scm
index cc57e5c..4a46283 100644
--- a/module/ice-9/futures.scm
+++ b/module/ice-9/futures.scm
@@ -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
-;; <>.
-(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))))
       (set-future-result! future
@@ -253,16 +246,14 @@ adding it to the waiter queue."
        (unlock-mutex (future-mutex future)))
        (unlock-mutex (future-mutex future))
-       (if (> (%nesting-level) 0)
+       (if (%within-future?)
            (abort-to-prompt %future-prompt future)
-      (else                                       ; queued
+      (else
        (unlock-mutex (future-mutex future))
-       (if (> (%nesting-level) %max-nesting-level)
-           (abort-to-prompt %future-prompt future)
-           (work))
+       (work)
   ((future-result future)))

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