Inspecting Java Threads in Linux Using Top

Which Java thread is hogging the CPU?

Try looking at the Hot Thread Detector plugin for visual VM -- it uses the ThreadMXBean API to take multiple CPU consumption samples to find the most active threads. It's based on a command-line equivalent from Bruce Chapman which might also be useful.

Obtaining CPU thread usage in Java

You can use ThreadMXBean to get cpu usage statistics from all running threads. In the example below the CPU usage per thread is calculated:

private int sampleTime = 10000;
private ThreadMXBean threadMxBean = ManagementFactory.getThreadMXBean();
private RuntimeMXBean runtimeMxBean = ManagementFactory.getRuntimeMXBean();
private OperatingSystemMXBean osMxBean = ManagementFactory.getOperatingSystemMXBean();
private Map<Long, Long> threadInitialCPU = new HashMap<Long, Long>();
private Map<Long, Float> threadCPUUsage = new HashMap<Long, Float>();
private long initialUptime = runtimeMxBean.getUptime();

ThreadInfo[] threadInfos = threadMxBean.dumpAllThreads(false, false);
for (ThreadInfo info : threadInfos) {
    threadInitialCPU.put(info.getThreadId(), threadMxBean.getThreadCpuTime(info.getThreadId()));
}

try {Thread.sleep(sampleTime);} catch (InterruptedException e) {}

long upTime = runtimeMxBean.getUptime();

Map<Long, Long> threadCurrentCPU = new HashMap<Long, Long>();
ThreadInfo[] threadInfos = threadMxBean.dumpAllThreads(false, false);
for (ThreadInfo info : threadInfos) {
    threadCurrentCPU.put(info.getThreadId(), threadMxBean.getThreadCpuTime(info.getThreadId()));
}

// CPU over all processes
//int nrCPUs = osMxBean.getAvailableProcessors();
// total CPU: CPU % can be more than 100% (devided over multiple cpus)
long nrCPUs = 1;
// elapsedTime is in ms.
long elapsedTime = (upTime - initialUptime);
for (ThreadInfo info : threadInfos) {
    // elapsedCpu is in ns
    Long initialCPU = threadInitialCPU.get(info.getThreadId());
    if (initialCPU != null) {
        long elapsedCpu = threadCurrentCPU.get(info.getThreadId()) - initialCPU;
        float cpuUsage = elapsedCpu / (elapsedTime * 1000000F * nrCPUs);
        threadCPUUsage.put(info.getThreadId(), cpuUsage);
    }
}

// threadCPUUsage contains cpu % per thread
System.out.println(threadCPUUsage);
// You can use osMxBean.getThreadInfo(theadId) to get information on every thread reported in threadCPUUsage and analyze the most CPU intentive threads

How to find a Java thread running on Linux with ps -axl?

Use

jps -v

for finding your java process. Sample Output:

3825 RemoteMavenServer -Djava.awt.headless=true -Xmx512m -Dfile.encoding=MacRoman
6172 AppMain -Didea.launcher.port=7533 -Didea.launcher.bin.path=/Applications/IntelliJ IDEA 10.app/bin -Dfile.encoding=UTF-8
6175 Jps -Dapplication.home=/Library/Java/JavaVirtualMachines/1.6.0_31-b04-411.jdk/Contents/Home -Xms8m

Then use

jstack 6172

(6172 is id of your process) to get stack of threads inside jvm. Thread priority could be found from it. Sample output:

.....
"main" **prio=5** tid=7ff255800800 nid=0x104bec000 waiting on condition [104beb000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
    at java.lang.Thread.sleep(Native Method)
    at au.com.byr.Sample.main(Sample.java:11)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

..... 

Enjoy!

EDIT: If application running under different user than yourself (typical case on production and other non-local environments) then jps/jstack should be run via sudo. Examples:

sudo jps -v

sudo jstack 6172

High CPU Utilization in java application - why?

If a profiler is not applicable in your setup, you may try to identify the thread following steps in this post.

Basically, there are three steps:

1. run top -H and get PID of the thread with highest CPU.
2. convert the PID to hex.
3. look for thread with the matching HEX PID in your thread dump.

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