Use `current-processor-count' to determine the number of future-workers.

* module/ice-9/futures.scm (%worker-count): Use
  `current-processor-count'.

* doc/ref/api-scheduling.texi (Futures): Add note about side-effects and
  I/O.  Mention `current-processor-count'.

doc/ref/api-scheduling.texi

@@ -848,12 +848,18 @@ 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}.
 
+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
+(@pxref{Mutexes and Condition Variables}).
+
 Guile's futures are implemented on top of POSIX threads
 (@pxref{Threads}).  Internally, a fixed-size pool of threads is used to
 evaluate futures, such that offloading the evaluation of an expression
 to another thread doesn't incur thread creation costs.  By default, the
-pool contains one thread per CPU core, minus one, to account for the
+pool contains one thread per available CPU core, minus one, to account
-main thread.
+for the main thread.  The number of available CPU cores is determined
+using @code{current-processor-count} (@pxref{Processes}).
 
 @deffn {Scheme Syntax} future exp
 Return a future for expression @var{exp}.  This is equivalent to:

module/ice-9/futures.scm

@@ -146,9 +146,7 @@ touched."
 
 (define %worker-count
   (if (provided? 'threads)
-      (if (defined? 'getaffinity)
+      (- (current-processor-count) 1)
-          (- (bit-count #t (getaffinity (getpid))) 1)
-          3) ;; FIXME: use Gnulib's `nproc' here.
       0))
 
 (define %workers