java server cpu usage monitor in linux

Question

I test my server in java, and monitored the cpu usage for it. First I use top command, and it show me as follows:

And then ，I use command

ps -C java -L -o pcpu,cpu,nice,state,cputime,pid,tid | sort to show all the java thread

the first column is cpu usage of each thread.

And I just found that the highest usage is only 0.3 and all cpu usage Sum up much less than 31.6.

I wish some one can tell me why ,THKS

edit:

To see more information , I use top -Hp 1234 (1234is my java pid) and I suddenly found that there exists a java thread(the pidis 1238) and the cpu usage of which usually suddenly promoted to 10-20 and even more, and I think it is this to be blamed.

And then I use

jstack 1234 |grep 4d6 -A 30

to see more detail of the thread, and it showed me :

"VM Thread" prio=10 tid=0x00007ff6ec060000 nid=0x4d6 runnable

"VM Periodic Task Thread" prio=10 tid=0x00007ff6ec091800 nid=0x4dd waiting on condition

JNI global references: 262

And I wish you can help me to analyse it.

THKS for answer, I have installed and used to htop, but it didn't solve my question, what I want is to see the cpu of usage of each java thread, because in my monitor it show too high, but I was given that the java cpu usage of each java thread is less than 0.3% each. — wanghao qin
– wanghao qin, Commented Jan 22, 2016 at 9:25
Because the list of thread is output by ps at a different moment than the sum value (31.6). The two doesn't match. — Ortomala Lokni
– Ortomala Lokni, Commented Jan 22, 2016 at 9:49
Thks，that is true and I found that there exist a java thread, the cpu usage of which suddenly promote to 10-20 percent even more ,and I think it was this thread to be blame, I added the detail in my question, wish you can help more — wanghao qin
– wanghao qin, Commented Jan 22, 2016 at 10:16
Using the small Linux tool "threadcpu" can help you to list the threads with high CPU usage. It does this by using a measuring interval and not by counting all jiffies from process start. tuxad.com/blog/archives/2018/10/01/… — reichhart
– reichhart, Commented Oct 15, 2018 at 20:07

SubOptimal · Accepted Answer · 2016-01-22 09:54:14Z

Maybe there is a misunderstanding of the pcpu value. From man ps

CPU usage is currently expressed as the percentage of time spent running during the entire lifetime of a process. This is not ideal, and it does not conform to the standards that ps otherwise conforms to. CPU usage is unlikely to add up to exactly 100%.

If the process starts and has a high CPU utilization, then the %CPU value is high. When the same process later has a low CPU utilization or is in sleep state this value will decrease. %CPU represents the cputime/realtime ratio.

As a small example

class Loop {
    public static void main(String...args) throws Exception {
        long l = 0;
        for (int i = 0; i >= 0; i++) {
            for (int j = 0; j < 50; j++) {
            l++;
            }
        }
        System.out.println("will sleep now");
        Thread.sleep(30_000);
    }
}

Start in one session following command, which will monitor the process in an interval of one second.

watch -n 1 ps -C java -o pcpu,state,cputime,etimes

Now run in another session the example code.

As long the Java code is inside the foo-loops and has not printed will sleep now the value of %CPU is really high (around 100). When the Java process reach the Thread.sleep the value of %CPU will constantly be decreasd (in this example).

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