Multi-core CPU utilization by Java application

Question

I have a program that sorts big files by splitting them into chunks, sort chunks and merge them into final sorted file. Application runs one thread for loading/saving data from/to file - only one thread does I/O operations. Also there are two more threads that receive chunk data, sort it and then send sorted data back to thread that does I/O.

So in general there are 4 threads running - main thread, thread that loads/saves data and two threads that sort data.

I thought during execution i will see 1 sleeping thread (main) that doesn't take any CPU time and 3 active threads that utilize 1 CPU core each.

When i run this program on dual 6 core processor machine with hyper threading (24 CPUs) i see that ALL 24 CPU's are loaded for 100%!

Initially i thought that sort algorithm is mutithreaded, but after looking into java sources i found that it's not.

I'm using simple Collections.sort(LinkedList) to sort the data...

here are some details:

# java -version
java version "1.6.0_26"
Java(TM) SE Runtime Environment (build 1.6.0_26-b03)
Java HotSpot(TM) 64-Bit Server VM (build 20.1-b02, mixed mode)

# uname -a
Linux 2.6.32-28-server #55-Ubuntu SMP Mon Jan 10 23:57:16 UTC 2011 x86_64 GNU/Linux

I was using nmon to monitor processor loading.

I would appreciate any explanation of this case and any advise on how to control CPU loading as i this particular task doesn't leave CPU time for other applications

[UPDATE] I used jvisualvm to count threads - it shows only threads i know about. Also i made a simple test program (see below) that runs only one main thread and got exactly the same results - all 24 processors are busy almost for 100% during code execution

public class Test {

    public void run(){
        Random r = new Random();
        int len = r.nextInt(10) + 5000000;
        LinkedList<String> list = new LinkedList<String>();
        for (int i=0; i<len; i++){
                list.add(new String("test" + r.nextInt(50000000)));
        }
        System.out.println("Inserted " + list.size() + " items");
        list.clear();
    }

    public static void main(String[] argv){
        Test t = new Test();
        t.run();
        System.out.println("Done");
    }
}

[UPDATE]
Here is the screenshot i made while running the program above (used nmon): http://imageshack.us/photo/my-images/716/cpuload.png/

We probably need to see your code to figure this out. My guess is that you're using more threads than you think. — Brendan Long
I used jvisualvm to count threads - it shows only threads i know about. Also i made a simple test program that runs only one main thread and got exactly the same results. please see my update in question. — Eugene M
4 threads can't use 100% utilization of 6 cores. Either you have more threads or your measurements are wrong. — Brendan Long
What program are you using to monitor CPU usage? It's probably showing you some other program that's running in the background. Either that, or your test is so short it's only showing you something related to the JVM startup. — Brendan Long
I used nmon to monitor CPU utilization, here is the screenshot: link Also, while i don't run the java program - it doesn't show any activity. Anyway, i see exactly the same while running my original sorting program. To be honest, i was expecting exactly what everybody would expect - one thread uses one CPU. but looks like java uses green threads that are not linked to underlying OS threads. As in my understanding java uses own thread scheduling. Thus, it might result in multiple native threads serving one java thread??? — Eugene M

Jonathan Jonathan · Accepted Answer · 2011-10-13T19:16:47

I would suggest, that this is rather a nmon than a java question and to solve it, I would take a peek at the top command which provides info about cpu-usage per process. I predict the following result: You will see one java thread using near 100% cpu-time (which is ok, as per-process percentage in top is relative to one (virtual) core), maybe a second and third java thread with much less cpu-usage (the I/O threads). Depending on the choice of the gc you might even spot one or more gc-Threads, however much less than 20.

HotSpot however will not (and even cannot to my knowledge) parallelize a sequential task on its own.

Multi-core CPU utilization by Java application

1 Answers