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doc: Document nested futures.

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* doc/ref/api-scheduling.texi (Futures): Mention nested futures.
  Explain what happens upon `touch'.
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Ludovic Courtès 2012-11-29 21:49:49 +01:00
parent 8eccf6cdbd
commit a638be152c
1 changed files with 29 additions and 1 deletions
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doc/ref/api-scheduling.texi
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@ -982,6 +982,24 @@ machine, though, the computation of @code{(find prime? lst2)} may be
done in parallel with that of the other @code{find} call, which can
reduce the execution time of @code{find-prime}.
Futures may be nested: a future can itself spawn and then @code{touch}
other futures, leading to a directed acyclic graph of futures. Using
this facility, a parallel @code{map} procedure can be defined along
these lines:
@lisp
(use-modules (ice-9 futures) (ice-9 match))
(define (par-map proc lst)
(match lst
(()
'())
((head tail ...)
(let ((tail (future (par-map proc tail)))
(head (proc head)))
(cons head (touch tail))))))
@end lisp
Note that futures are intended for the evaluation of purely functional
expressions. Expressions that have side-effects or rely on I/O may
require additional care, such as explicit synchronization
@ -995,6 +1013,15 @@ pool contains one thread per available CPU core, minus one, to account
for the main thread. The number of available CPU cores is determined
using @code{current-processor-count} (@pxref{Processes}).
When a thread touches a future that has not completed yet, it processes
any pending future while waiting for it to complete, or just waits if
there are no pending futures. When @code{touch} is called from within a
future, the execution of the calling future is suspended, allowing its
host thread to process other futures, and resumed when the touched
future has completed. This suspend/resume is achieved by capturing the
calling future's continuation, and later reinstating it (@pxref{Prompts,
delimited continuations}).
@deffn {Scheme Syntax} future exp
Return a future for expression @var{exp}. This is equivalent to:
@ -1024,7 +1051,8 @@ Return the result of the expression embedded in future @var{f}.
If the result was already computed in parallel, @code{touch} returns
instantaneously. Otherwise, it waits for the computation to complete,
if it already started, or initiates it.
if it already started, or initiates it. In the former case, the calling
thread may process other futures in the meantime.
@end deffn