Obtaining CPU thread usage in Java

Question

I have a question around getting CPU utilization for a given JNI block. I'm making some intensive CPU computation in the underlying C++ JNI native method. I'm in the process of optimizing this computation and want to benchmark it against varying inputs.

I need some guidance on how to go about measuring this. The alternatives I have considered so far are

Using JMX ThreadMXBean to measure system CPU usage for the current thread that invokes call into JNI method. However, I am not sure if JNI code is executed within the invoking thread context. What happens when the thread spawns more threads?
Using JMX OperatingSystemMXBean to get the CPU usage for the entire JVM. Ideally, this is not want I want as there could be parallel executions in JVM that might tweak the benchmarking.
Measure externally using getrusage(..). What I want to know here is that how is it different than using OperatingSystemMXBean.

ThreadMXBean#getCurrentThreadCpuTime() is fine for non-spawning JNI (if your Java/OS supports it). If it spanws you would have to dig deep into the system and cannot monitor it with Java means anymore. (yes I am basically agreeing with your fear :) — eckes
– eckes, Commented Apr 28, 2015 at 23:17
On Linux it uses either /proc/<pid>/cpu (slow) or getcpu_clockid/clock_gettime if supported. This is fast and high precision supported by pthreads. — eckes
– eckes, Commented Apr 28, 2015 at 23:35
If the operation is CPU-intensive, this means that what you want to measure is total running time, not ratio of CPU vs. I/O. In this case, why is a simple "end time minus start time" measurement not good enough? Sure, you will have greater accuracy if you exclude other threads, but on the other hand, it may be a big pain to exclude those threads, so is it worth the hassle? Besides, one of these threads will be the GC, and if your java code is producing a lot of garbage, then this might be something you'd want to take into account and optimize too, no? — Mike Nakis
– Mike Nakis, Commented Oct 1, 2015 at 18:15

R. Oosterholt · Accepted Answer · 2015-10-01 14:41:37Z

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

Amila · Accepted Answer · 2015-08-28 09:00:54Z

3

On Linux, this method may help you:

Run kill -3 <pid of Java process> to get the thread dump. Thread dump will be added standard output.
Open the dump and search for the thread you're interested in. (You can search using class name)
Get the thread PID (it's in hex) and convert it to decimal.
Run top command.
Press Shift-H to enable Threads View
Identify the thread using decimal value you obtained above. You can see cpu / memory usage of your thread there.

answered Aug 28, 2015 at 9:00

Amila

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