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POJMX: CPU + Blocked + Wait time = Wall time?
 

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    <p>I'm trying to use JMX for measuring how long a method call took and how much of that was CPU time, how much was the thread being blocked and how much was waiting. Ideally, I would expect that <strong>CPU time + Block time + Wait time = Wall time</strong>, but I've noticed that it's not the case - and it's not just slightly inaccurate timer. For example:</p> <pre><code>Wall time: 5657.305 ms CPU time: 4060.000 ms (71.77%) User time: 3840.000 ms (67.88%) Block time: 0.000 ms (0.00%) Wait time: 0.000 ms (0.00%) </code></pre> <p>So, the question is ... <strong>is my presumption</strong> that the sum of these times (not User time, that's included in CPU time) should give Wall time <strong>wrong?</strong> Am I missing something?</p> <p>Some more details:</p> <ul> <li><p>Wall time: difference of <code>System.currentTimeMillis()</code> at the method entry and exit</p></li> <li><p>CPU time: difference of <code>ManagementFactory.getThreadMXBean().getCurrentThreadCpuTime()</code> at the method entry and exit</p></li> <li><p>Block and Wait time: analogically to CPU, with <code>ManagementFactory.getThreadMXBean().getThreadInfo(Thread.currentThread().getId()).getBlockedTime()</code> and <code>getWaitedTime()</code></p></li> <li><p>Yes, I do know that these methods return time in different units (ns/ms), I take that into account.</p></li> <li><p>The application is highly hyperthreaded (4000+ threads), but I store all the info per thread, so there <em>shouldn't</em> be any interference between calls from different threads.</p></li> </ul>
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