Question

I’m having issues with a Java algorithm I created to convert nanosecond CPU time usage (obtained via JMX) to a percentage out of 100%. The algorithm appears to be giving numbers greater than 100%, which I assume to be due to multiple available processors, although the code should sort this out. The algorithm can be seen below. cpuTimeDiffNS is the amount of CPU time used in nanoseconds, while periodMS is the sampled period.

public static final double getCPUPerc(long cpuTimeDiffNS, long periodMS) {
    if (periodMS == 0) return 0;
    double cpuTimeDiffMS = cpuTimeDiffNS / 1000000d;
    int procs = Runtime.getRuntime().availableProcessors();
    long availableTime = periodMS * procs;
    double perc = cpuTimeDiffMS / availableTime;
    return perc * 100d;
}

Here’s some samples from the data acquisition:

0
87.5
133.8288232
160.8231707
197.7896341
209.6036585
248.822774
274.3902439
267.9115854
271.3414634
277.1067759
283.1554878
272.1036585
279.4000734
283.9176829
283.5365854
275.9146341
282.4578033
278.9634146
261.0536937
254.6071775
286.662182
278.9634146
276.7245597
288.4908537
281.6933708
286.9664634
279.7822896
276.2957317
280.4878049
275.5335366
271.7557485
280.8689024
287.2689689
281.6933708
267.5097276
273.2469512
286.1735835
289.6341463
296.875
279.4000734
289.2530488
282.8400196
288.4908537
287.4266145
288.1097561
286.5853659
288.9554795
238.1207192
288.4908537
288.7063531
290.3963415
286.662182
277.4390244
290.4843444
281.6310976
271.7557485
272.8658537
283.2222358
250.7621951

---

Edit: as requested, the input gathering functions (you can probably ignore this):

// returns CPU time in NS for a thread group (recursively)
public static long getCPUTime(ThreadGroup tg) {
    synchronized (TGLOCK) {
        int size;
        do {
            size = tg.enumerate(tgThreads, true);
            if (size <= tgThreads.length) continue;
            tgThreads = new Thread[size];
        } while (size > tgThreads.length);

        long totalTime = 0;
        for (int i = 0; i < size; i++) {
            totalTime += getCPUTime(tgThreads[i]);
        }
        return totalTime;
    }
}

public static long getCPUTime(Thread t) {
    return threadMXBean.getThreadCpuTime(t.getId());
}

public static ThreadGroup getRootThreadGroup() {
    // Find the root thread group
    ThreadGroup root = Thread.currentThread().getThreadGroup().getParent();
    while (root.getParent() != null) {
        root = root.getParent();
    }
    return root;
}

and the inputs (again, you can probably ignore this):

    simCPUTimeNS     = getCPUTime(kks.getSimThreadGroup());
    appsCPUTimeNS    = getCPUTime(kks.getAppThreadGroup());
    lwjns3CPUTimeNS  = getCPUTime(kks.getKKSThreadGroup());
    simCoreCPUTimeNS = getCPUTime(kks.getSimThread());
    totalCPUTimeNS   = getCPUTime(getRootThreadGroup());

    simCPUTimeNSDiff  = simCPUTimeNS - lastSimCPUTimeNS;
    appsCPUTimeNSDiff = appsCPUTimeNS - lastAppsCPUTimeNS;
    lwjns3CPUTimeNSDiff = lwjns3CPUTimeNS - lastLwjns3CPUTimeNS;
    simCoreCPUTimeNSDiff = simCoreCPUTimeNS - lastSimCoreCPUTimeNS;
    totalCPUTimeNSDiff = totalCPUTimeNS - lastTotalCPUTimeNS;

    lastSimCPUTimeNS     = simCPUTimeNS;
    lastAppsCPUTimeNS    = appsCPUTimeNS;
    lastLwjns3CPUTimeNS  = lwjns3CPUTimeNS;
    lastSimCoreCPUTimeNS = simCoreCPUTimeNS;
    lastTotalCPUTimeNS   = totalCPUTimeNS;

    simCPUPerc     = getCPUPerc(simCPUTimeNSDiff, currDiffMS);
    appsCPUPerc    = getCPUPerc(appsCPUTimeNSDiff, currDiffMS);
    lwjns3CPUPerc  = getCPUPerc(lwjns3CPUTimeNSDiff, currDiffMS);
    simCoreCPUPerc = getCPUPerc(simCoreCPUTimeNSDiff, currDiffMS);
    totalCPUPerc   = getCPUPerc(totalCPUTimeNSDiff, currDiffMS);

Cheers for any help, I’m sure the answer is obvious 😉
Chris

Answer 1

I’m running on a time-dilated JVM (5x slowdown), and it looks like I forgot to dilate in one part of the C++ JVM code in os_windows.cpp (os::thread_cpu_time) when making adjustments. Oops. It uses timeGetTime(), a Windows time function. That’d explain it.