CODEWITHSUNDEEP
javaclojurehashmap
qiuxiafei
qiuxiafei

Reputation: 5977

Why does java native HashMap in clojure run slowly?

I associate a key to a hash map for 10000000 time. Here's the Java code and output：

import java.util.HashMap;

public class TestMap {
    public static void main(String[] args) {
        HashMap<Integer, Integer> mp = new HashMap<Integer, Integer>();
        long  start = System.currentTimeMillis();
        for (int i = 0; i < 10000000; i++) {
            mp.put(1, 1);
        }
        long end = System.currentTimeMillis();
        System.out.println("Elapsed time: " + (end - start) + " msecs");
    }
}


$ javac TestMap.java && java -cp . TestMap
Elapsed time: 38 msecs

And then I call java from clojure in REPL:

user=> (import java.util.HashMap)
java.util.HashMap
user=> (def mp (HashMap.))
#'user/mp
user=>  (time (dotimes [n 10000000] (.put mp 1 1)))
"Elapsed time: 10024.797 msecs"
nil

Both code do the same thing, but the clojure version runs exstreamly slow!!

What's the problem?

Upvotes: 6

Views: 650

Answers (3)

cl-r
cl-r

Reputation: 1264

You can also increase Java code speed with declaring HashMap size in declaration 

HashMap<Integer, Integer> mp = new HashMap<Integer, Integer>(10000000);

I guess it is a also a way for REPL (I do not know it), is it possible to reserve memory space?

Upvotes: 0

Arthur Ulfeldt
Arthur Ulfeldt

Reputation: 91534

The first step with performance problems like this is to turn on reflection warnings and remove any.

 (set! *warn-on-reflection* true)

Also loop and recur have the lowest overhead.

Upvotes: 9

number23_cn
number23_cn

Reputation: 4619

add type hint is better:

user> (import 'java.util.HashMap)
java.util.HashMap
user> (def mp (HashMap.))
#'user/mp
user> (time (dotimes [n 10000000] (.put mp 1 1)))
"Elapsed time: 13932.248126 msecs"
nil
user> (time (dotimes [n 10000000] (.put ^HashMap mp 1 1)))
"Elapsed time: 117.915992 msecs"
nil

Upvotes: 13

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