Hello,
a colleague of mine asked me a question about clojures set
implementation performance. He needed an immutable set on the jvm
platform for a Java/Scala project, and since I've been saying that
Clojures data structures have good performance he tried clojures set.
He was dissapointed though :-(
He sent me this program testing performance:
for (int j = 0; j < 10; j++) {
long t = System.currentTimeMillis();
PersistentHashSet persistentSet = PersistentHashSet.create();
Set<Object> mutableJavaSet = new HashSet<Object>();
for (int i = 0; i < 500000; i++) {
Object o = new Object();
persistentSet = (PersistentHashSet) persistentSet.cons(o);
mutableJavaSet.add(o);
}
System.out.println("filling set took " + (System.currentTimeMillis()
- t) + "ms");
long start = System.currentTimeMillis();
for (Object o : persistentSet) {
o.toString();
}
System.out.println("clojure time: " + (System.currentTimeMillis() -
start) + " ms");
long t1 = System.currentTimeMillis();
for (Object o : mutableJavaSet) {
o.toString();
}
System.out.println("java time: " + (System.currentTimeMillis() - t1)
+ " ms");
}
What was surprising to me wasn't that "inserts" are slower - that is
ok and it could be improved with transients. The surprising thing was
that iterating through the entire set was significantly slower.
I realize that there is some overhead in producing a seq but I wasn't
expecting it to be that much (4-18 times slower). Also, are chunked
seqs used here - as far as I can tell they aren't? IF not, couldn't
they?
Insights wanted :-)
/Karl
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