Difference between thread user time and thread cpu time in Java

Question

The ThreadMXBean has two methods for retrieving thread time usage:

• getThreadUserTime
• getThreadCpuTime

What is the difference between the two?

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Update 2: If I'm able to link to the javadocs, please don't quote them - I've read them already.

Update: here's some code which I tried to use to learn what these times mean, with little success:

ThreadMXBean threadMXBean = ManagementFactory.getThreadMXBean();
threadMXBean.setThreadContentionMonitoringEnabled(true);
long mainThreadId = getMainThreadId(threadMXBean);

logTimes("Start", threadMXBean, mainThreadId);

URL url = new URL("https://hudson.dev.java.net");
URLConnection connection = url.openConnection();

connection.getContent();

logTimes("After loading", threadMXBean, mainThreadId);

and the output is:

Start Tue Jun 16 16:13:40 EEST 2009 Cpu time : 80, user time: 60, waited: 0, blocked: 0
After loading Tue Jun 16 16:13:43 EEST 2009 Cpu time : 1,020, user time: 960, waited: 0, blocked: 0

So the difference between cpu and user time increased from 20 to 60 milliseconds. Is that because using a HttpUrlConnection does include some network I/O?

Answer 1

As the API docs you linked to yourself already point out

getThreadCpuTime

If the implementation distinguishes between user mode time and system mode time, the returned CPU time is the amount of time that the thread has executed in user mode or system mode.

If the implementation of the JVM distinguishes between user mode and kernel mode time there could be a difference in the results of the two functions.

Further the value is only precise to the nanosecond and the value has an overflow problem if the offset is > 2^63. The JVM must also support measuring the CPU time for the current thread and it must be enabled.

On Win32 the return values should be the same as the ones you get from the GetThreadTimes Function

getThreadUserTime() -> lpUserTime * 100 //or something like this

getThreadCpuTime() -> (lpKernelTime + lpUserTime) * 100 //or something like this

And a more clear reference to User Mode vs Kernel Mode

Answer 2

It's explained in the javadocs :-).

getThreadCpuTime: Returns the total CPU time for a thread of the specified ID.

getThreadUserTime: Returns the CPU time that a thread of the specified ID has executed in user mode.

The difference is that getThreadCpuTime also includes time that the thread used the CPU but was not in user mode. That would be things like being inside a device driver, polling for I/O or similar.