note
ikegami
<p>Good question! I've been working on it to see what the real issue is. I don't have an answer for you; I'm just documenting what I've found for everyone's benefit.
<p>First, I presume that "inherently recursive" means "cannot be rewritten to be tail-recursive". Fibonacci is an example of an inherently recursive algorithm.
<c>
sub fibonacci {
my ($n) = @_;
return 0 if $n == 0;
return 1 if $n == 1;
return sum fibonacci($n-2), fibonacci($n-1);
}
</c>
<p>At face value, it's quite easy to make it threaded:
<c>
sub fibonacci {
my ($n) = @_;
return 0 if $n == 0;
return 1 if $n == 1;
return sum map $_->join(),
async { fibonacci($n-2) },
async { fibonacci($n-1) };
}
</c>
<p>But what if you wanted to limit the number of threads (perhaps to limit overhead)? One solution would be to have a pool of threads and to use them if available.
<c>
sub fibonacci {
my ($n) = @_;
return 0 if $n == 0;
return 1 if $n == 1;
return sum map $_->get(),
async_maybe { fibonacci($n-2) },
async_maybe { fibonacci($n-1) };
}
</c>
<p>(The above assumes closures can be shared, but it can be rewritten to not use closures.)
<p>When implementing <c>async_maybe</c> (and the <c>get</c> of the object it returns), one must be extra careful to avoid the situation where a thread is waiting to have it's result collected.
<p>But what if you want a worker model (perhaps to distribute the work to other machines)? Now, that's hard. One would need some kind of a callback system.
<c>
sub fibonacci {
my ($n) = @_;
my $result;
process_and_wait(sub {
fibonacci_task(sub { $result = $_[0]; }, $n );
});
return $result;
}
sub fibonacci_task {
my ($on_complete, $n) = @_;
return $on_complete->(0) if $n == 0;
return $on_complete->(1) if $n == 1;
my ($x,$y);
process(sub { fibonacci_task(sub { $x = $_[0] }, $n-2) });
process(sub { fibonacci_task(sub { $y = $_[0] }, $n-1) });
#TODO: The last of the two tasks to complete
# must call $on_complete->($x+$y).
}
</c>
<p>The TODO item is hard to do cleanly. And of course, I'm still using closures even though I think that's not an option for [mod://threads] threads. Rewriting to avoid closures will likely make the code even longer.
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