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    <p>CPU time tells you how much time the thread took utilizing the CPU, i.e. actually executing code. Block or wait times increase if the thread had been suspended because of e.g. waiting for I/O or entering a monitor.</p> <p>However, it's up to the JVM and OS to assign CPU time to threads and processes. They may choose to suspend a thread or process and resume another one at any time, if necessary. So the thread may be in a state where it is neither blocked nor waiting but just not executing. This will add to wall time but not blocked/wait time.</p> <p>The CPU time value comes from the operating system. I checked OpenJDK 7 for Linux systems, so it might be different on other platforms. Either <code>fast_thread_cpu_time()</code> or <code>slow_thread_cpu_time()</code> is called to calculate CPU time for threads. Both functions are located in<code>hotspot/src/os/linux/vm/os_linux.cpp</code> of the OpenJDK 7 source. </p> <p>Since <a href="https://stackoverflow.com/questions/1888160/distinguish-java-threads-and-os-threads">OpenJDK uses pthreads on Linux</a>, each Java thread is implemented as a lightweight OS thread. The fast method supported on modern kernels calls <code>pthread_getcpuclockid</code> to retrieve the <code>clock_id</code> for specific thread and then call <code>clock_gettime</code> to retrieve the CPU time for the thread. The slow method will lookup the CPU time values from <code>/proc/self/task/&lt;tid&gt;/stat</code>. Both methods tell you how much time a thread spent consuming CPU in user and system/kernel space.</p> <p><strong>References:</strong></p> <ul> <li><a href="http://download.java.net/openjdk/jdk7/" rel="nofollow noreferrer">OpenJDK 7 source</a></li> <li><a href="http://linux.die.net/man/3/pthread_getcpuclockid" rel="nofollow noreferrer">pthread_getcpuclockid(3) man page</a></li> <li><a href="http://linux.die.net/man/3/clock_gettime" rel="nofollow noreferrer">clock_gettime(3) man page</a></li> <li><a href="http://linux.die.net/man/5/proc" rel="nofollow noreferrer">proc(5) man page</a></li> </ul>
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    1. COI thought it might be this case ... can you (or anyone else) link any source that can support this? (and by "this" I mean "thread is neither running, blocked nor waiting when it can run but is not given time window on a processor core to run on")
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    2. CO@Sethiel: I edited the answer to include some implementation details which might help. You should be able to proof that by creating threads that consume CPU but don't use synchronization or other block/wait functionality. The total count of threads must be greater than the number of CPU cores of the system. Assign the lowest priority to one of the threads. This thread's CPU time should obviously increase much slower than the others' because it was scheduled less often. However, all threads should be actually in the `RUNNING` state all the time.
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