On Jan 30, 6:08 am, Daniel Janus <nath...@gmail.com> wrote:
> Hi,
>
> I've recently heard about the locals clearing feature of Clojure 1.2 (from a
> recent post by Ken Wesson), and decided to test-drive it. Here is a
> contrived example:
>
> (defn garbage []
> (make-array Byte/TYPE 10485760))
>
> (defn -main [& args]
> (let [a (garbage)
> b (garbage)
> c (garbage)
> d (garbage)
> e (garbage)
> f (garbage)
> g (garbage)
> h (garbage)]
> (println "OK")))
>
> Now, when I build this with Leiningen and try to run under -Xmx20M or so, it
> bombs out on me with an OOME. Changing the let to a bunch of nested lets
> doesn't help, nor does migrating to Clojure 1.3alpha4.
>
> Shouldn't the locals clearing feature detect that each of the a-h locals is
> no longer needed and clear them right after allocation? Is this a
> misconception on my part about how this works? Or is something weird going
> on here?
>
Locals clearing happens at the point of last use. Since those locals
are never used, no clearing code is emitted. This java program fails
similarly:
public class Mem {
public static void main (String[] args) {
byte[] a = new byte[10485760];
byte[] b = new byte[10485760];
byte[] c = new byte[10485760];
byte[] d = new byte[10485760];
byte[] e = new byte[10485760];
byte[] f = new byte[10485760];
byte[] g = new byte[10485760];
byte[] h = new byte[10485760];
System.out.println ("Ok");
}
}
This works in Clojure due to locals clearing:
(defn garbage []
(make-array Byte/TYPE 10485760))
(defn -main [& args]
(let [a (garbage)
b (when a (garbage))
c (when b (garbage))
d (when c (garbage))
e (when d (garbage))
f (when e (garbage))
g (when f (garbage))
h (when g (garbage))]
(println "OK")))
Since unused locals serve no purpose, optimizing that case is not
going to become a priority any time soon. (Note: I still think the JVM
GC should be able to figure out that those locals are dead references.
I'm just not going to work around it in this case).
Rich
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