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Threaddumps-on-JVM.html
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Threaddumps-on-JVM.html
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<title>Thread-dumps on JVM @ JDay Lviv</title>
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</head>
<body class="article">
<div id="header">
<h1>Thread-dumps on JVM @ JDay Lviv</h1>
<span id="author" class="author">Tomasz Borek, @LAFK_pl, lafkblogs.wordpress.com</span><br>
<div id="toc" class="toc">
<div id="toctitle">Table of Contents</div>
<ul class="sectlevel1">
<li><a href="#_promise">Promise</a></li>
<li><a href="#_jvm_overview">JVM overview</a></li>
<li>
<ul class="sectlevel2">
<li><a href="#_threads">Threads</a></li>
<li><a href="#_thread_limits">Thread limits?</a></li>
</ul>
</li>
<li><a href="#_how_can_you_take_a_threaddump">How can you take a threaddump?</a></li>
<li>
<ul class="sectlevel2">
<li><a href="#_about_kill_command">About kill command</a></li>
<li><a href="#_about_signals">About signals</a></li>
<li><a href="#_pid_of_a_java_process">PID of a Java process?</a></li>
</ul>
</li>
<li><a href="#_how_can_you_take_a_threaddump_2">How can you take a threaddump?</a></li>
<li><a href="#_how_can_you_take_a_threaddump_3">How can you take a threaddump?</a></li>
<li><a href="#_printing_threads_then">Printing threads then?</a></li>
<li>
<ul class="sectlevel2">
<li><a href="#_going_native">Going native…</a></li>
<li><a href="#_safepoints">Safepoints</a></li>
<li><a href="#_thread_dump_then">Thread dump then</a></li>
<li><a href="#_comparing_printing_threads">Comparing printing threads</a></li>
</ul>
</li>
<li><a href="#_commands_used_during_presentation">Commands used during presentation</a></li>
<li><a href="#_bibliography_links">Bibliography / Links</a></li>
</ul>
</div>
</div>
<div id="content">
<div class="sect1">
<h2 id="_promise">Promise</h2>
<div class="sectionbody">
<div class="paragraph">
<p>How can you take a threaddump?<br>
Print threads?<br>
What is a safepoint and why you should care?<br>
Which tools can help and how?<br>
And when you have the threaddump… what’s in there?<br>
We’ll talk threads and locks. Dead and alive alike.<br>
We’ll talk CLI and GUI (tools, of course).<br>
We’ll talk about thread lifecycle.<br>
And naming. Oh, the naming!<br>
Example rich talk, though rather aimed at beginners.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_jvm_overview">JVM overview</h2>
<div class="sectionbody">
<div class="paragraph">
<p>From <a href="http://2.bp.blogspot.com/:">http://2.bp.blogspot.com/:</a></p>
</div>
<div class="imageblock">
<div class="content">
<img src="JVM_middleware_interactions.png" alt="Image showing where threads work and from where they originate from">
</div>
</div>
<div class="sect2">
<h3 id="_threads">Threads</h3>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>instances of java.lang.Thread</p>
</li>
<li>
<p>have associated native thread</p>
</li>
<li>
<p>have names (or are named Thread#nextNumberInSequence)</p>
</li>
<li>
<p>have local variables, accessible only to given thread</p>
</li>
</ol>
</div>
<div class="admonitionblock important">
<table>
<tr>
<td class="icon">
<i class="icon-important" title="Important"></i>
</td>
<td class="content">
<div class="title">Thread lifecycle</div>
<div class="paragraph">
<p><strong>Enum: <code>java.lang.Thread.State</code>:</strong></p>
</div>
<div class="dlist">
<dl>
<dt class="hdlist1">NEW</dt>
<dd>
<p>Thread state for a thread which has not yet started.</p>
</dd>
<dt class="hdlist1">RUNNABLE</dt>
<dd>
<p>Thread state for a runnable thread.</p>
</dd>
<dt class="hdlist1">BLOCKED</dt>
<dd>
<p>Thread state for a thread blocked waiting for a monitor lock.</p>
</dd>
<dt class="hdlist1">WAITING</dt>
<dd>
<p>A thread in the waiting state is waiting for another thread to perform a particular action.</p>
</dd>
<dt class="hdlist1">TIMED_WAITING</dt>
<dd>
<p>A thread that is waiting for another thread to perform an action for up to a specified waiting time is in this state.</p>
</dd>
<dt class="hdlist1">TERMINATED</dt>
<dd>
<p>A thread that has exited is in this state.</p>
</dd>
</dl>
</div>
<div class="paragraph">
<p><code>Thread.getState()</code> gives you that.</p>
</div>
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>Native thread gets created in NEW and reclaimed upon thread termination.</p>
</div>
<div class="admonitionblock caution">
<table>
<tr>
<td class="icon">
<i class="icon-caution" title="Caution"></i>
</td>
<td class="content">
<em>RUNNABLE</em> means OK, right?
</td>
</tr>
</table>
</div>
</div>
<div class="sect2">
<h3 id="_thread_limits">Thread limits?</h3>
<div class="paragraph">
<p>Are there any?</p>
</div>
<div class="paragraph">
<p>What limits the threads?</p>
</div>
<div class="imageblock">
<div class="content">
<img src="Heap.png" alt="JVM and RAM">
</div>
</div>
<div class="paragraph">
<p>Let’s add more for just one JVM:</p>
</div>
<div class="imageblock">
<div class="content">
<img src="MemoryInJVM.png" alt="Memory organization in JVM">
</div>
</div>
<div class="paragraph">
<p>See <code>limit</code>, Bash built-in. In my case:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>➜ ThreaddumpsOnJVM git:(master) ✗ limit
cputime unlimited
filesize unlimited
datasize unlimited
stacksize 8MB
coredumpsize 0kB
memoryuse unlimited
maxproc 63635
descriptors 1024
memorylocked 64kB
addressspace unlimited
maxfilelocks unlimited
sigpending 63635
msgqueue 819200
nice 0
rt_priority 0
rt_time unlimited</pre>
</div>
</div>
<div class="paragraph">
<p>Interesting limits are <em>stacksize</em>, <em>descriptors</em> and <em>maxfilelocks</em> when speaking about threads.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_how_can_you_take_a_threaddump">How can you take a threaddump?</h2>
<div class="sectionbody">
<div class="olist arabic">
<ol class="arabic">
<li>
<p><code>kill -3 JAVA_PID_HERE</code></p>
</li>
</ol>
</div>
<div class="sect2">
<h3 id="_about_kill_command">About kill command</h3>
<div class="dlist">
<dl>
<dt class="hdlist1">kill</dt>
<dd>
<p>send a signal to a process</p>
<div class="literalblock">
<div class="content">
<pre>kill -9 111
kill -s SIGKILL 111
kill -KILL 111</pre>
</div>
</div>
</dd>
</dl>
</div>
<div class="paragraph">
<p>May be a shell built-in AND a command: <code>which kill</code><br>
Essentially a wrapper around <em>kill</em> syscall.</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p><code>kill -L</code> - no such option on my Ubuntu/Bash</p>
<div class="olist loweralpha">
<ol class="loweralpha" type="a">
<li>
<p><code>kill -l</code></p>
</li>
</ol>
</div>
</li>
<li>
<p><code>/bin/kill -L</code></p>
<div class="olist loweralpha">
<ol class="loweralpha" type="a">
<li>
<p><code>/bin/kill -l</code> - same as built-in</p>
</li>
</ol>
</div>
</li>
</ol>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="icon-tip" title="Tip"></i>
</td>
<td class="content">
<div class="title">Further reading about <code>kill</code></div>
<div class="paragraph">
<p>Excellent pieces of information, especially the first one:</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p><a href="http://www.linux.org/threads/kill-signals-and-commands-revised.8096/">http://www.linux.org/threads/kill-signals-and-commands-revised.8096/</a></p>
</li>
<li>
<p><a href="http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_12_01.html">http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_12_01.html</a></p>
</li>
</ol>
</div>
</td>
</tr>
</table>
</div>
</div>
<div class="sect2">
<h3 id="_about_signals">About signals</h3>
<div class="paragraph">
<p><code>/bin/kill -L</code></p>
</div>
<div class="paragraph">
<p>Default? <strong>TERM</strong></p>
</div>
<div class="paragraph">
<p><code>man --section 7 signals | grep SIGQUIT</code><br>
<code>man --section 7 signals | grep Core</code></p>
</div>
<div class="admonitionblock caution">
<table>
<tr>
<td class="icon">
<i class="icon-caution" title="Caution"></i>
</td>
<td class="content">
<div class="title">Java and signals</div>
<div class="dlist">
<dl>
<dt class="hdlist1"><code>-Xrs</code></dt>
<dd>
<p>disables default signals for Java applications</p>
</dd>
</dl>
</div>
</td>
</tr>
</table>
</div>
</div>
<div class="sect2">
<h3 id="_pid_of_a_java_process">PID of a Java process?</h3>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>ps aux | grep java</p>
</li>
<li>
<p>pgrep java</p>
</li>
<li>
<p>jps -l</p>
<div class="dlist">
<dl>
<dt class="hdlist1">jps</dt>
<dd>
<p>Lists the instrumented Java Virtual Machines (JVMs) on the target<br>
system. This command is experimental and unsupported.</p>
</dd>
</dl>
</div>
</li>
<li>
<p>jcmd</p>
<div class="dlist">
<dl>
<dt class="hdlist1">jcmd</dt>
<dd>
<p>sends diagnostic command requests to a running JVM.</p>
</dd>
</dl>
</div>
</li>
</ol>
</div>
<div class="imageblock">
<div class="content">
<img src="javapid.gif" alt="Image shows comparison of ways above">
</div>
</div>
<div class="imageblock">
<div class="content">
<img src="java_pid_java_tools.png" alt="Image shows how Java tools offer Java PIDs">
</div>
</div>
<div class="admonitionblock warning">
<table>
<tr>
<td class="icon">
<i class="icon-warning" title="Warning"></i>
</td>
<td class="content">
<div class="title"><code>jps</code> output is empty?</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>which user is running the Java process you are looking for?</p>
</li>
<li>
<p><code>ls -l /tmp/hsperfdata_YourUserHere/</code></p>
</li>
<li>
<p>check the <code>-Djava.io.tmpdir</code> flag or start JVM setting it to <code>=/someDir</code>.</p>
</li>
</ol>
</div>
<div class="paragraph">
<p>More:<br>
. <a href="http://stackoverflow.com/questions/3805376/jps-returns-no-output-even-when-java-processes-are-running">http://stackoverflow.com/questions/3805376/jps-returns-no-output-even-when-java-processes-are-running</a><br>
. <a href="https://devopsengineer.wordpress.com/2014/01/24/java-jps-cannot-see-running-java-processes/">https://devopsengineer.wordpress.com/2014/01/24/java-jps-cannot-see-running-java-processes/</a></p>
</div>
</td>
</tr>
</table>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_how_can_you_take_a_threaddump_2">How can you take a threaddump?</h2>
<div class="sectionbody">
<div class="olist arabic">
<ol class="arabic">
<li>
<p><code>kill -3 JAVA_PID</code></p>
</li>
<li>
<p><code>jstack JAVA_PID</code></p>
<div class="dlist">
<dl>
<dt class="hdlist1">jstack</dt>
<dd>
<p>Prints Java thread stack traces for a Java process, core file,<br>
or remote debug server. This command is experimental and unsupported.</p>
</dd>
</dl>
</div>
</li>
</ol>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="icon-tip" title="Tip"></i>
</td>
<td class="content">
Poor man’s debugger? <code>jstack</code> in a loop… However, can pinpoint live-locks!
</td>
</tr>
</table>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_how_can_you_take_a_threaddump_3">How can you take a threaddump?</h2>
<div class="sectionbody">
<div class="olist arabic">
<ol class="arabic">
<li>
<p><code>kill -3 JAVA_PID</code></p>
</li>
<li>
<p><code>jstack JAVA_PID</code></p>
</li>
<li>
<p><code>Ctrl</code>+<code>\</code></p>
</li>
<li>
<p><code>jconsole</code></p>
</li>
<li>
<p><code>jvisualvm</code></p>
</li>
</ol>
</div>
<div class="admonitionblock caution">
<table>
<tr>
<td class="icon">
<i class="icon-caution" title="Caution"></i>
</td>
<td class="content">
<div class="title">Dump location?</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>Your process' console</p>
</li>
<li>
<p>Current directory</p>
</li>
<li>
<p>JVM settings</p>
<div class="olist loweralpha">
<ol class="loweralpha" type="a">
<li>
<p>different JVMs, different flags</p>
</li>
</ol>
</div>
</li>
<li>
<p>System settings?</p>
<div class="olist loweralpha">
<ol class="loweralpha" type="a">
<li>
<p><code>man core</code></p>
</li>
<li>
<p><code>cat /proc/sys/kernel/core_pattern</code></p>
</li>
</ol>
</div>
</li>
</ol>
</div>
<div class="paragraph">
<p><a href="http://stackoverflow.com/questions/2065912/core-dumped-but-core-file-is-not-in-current-directory">http://stackoverflow.com/questions/2065912/core-dumped-but-core-file-is-not-in-current-directory</a><br>
<a href="http://stackoverflow.com/questions/2062493/jvm-thread-dump-location">http://stackoverflow.com/questions/2062493/jvm-thread-dump-location</a></p>
</div>
</td>
</tr>
</table>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_printing_threads_then">Printing threads then?</h2>
<div class="sectionbody">
<div class="paragraph">
<p><code>jcmd</code></p>
</div>
<div class="sect2">
<h3 id="_going_native">Going native…</h3>
<div class="paragraph">
<p><code>whichThreadIsIt.sh</code></p>
</div>
<div class="paragraph">
<p><code>ps -mo lwp,c -p 14222</code></p>
</div>
</div>
<div class="sect2">
<h3 id="_safepoints">Safepoints</h3>
<div class="paragraph">
<p>What is a safepoint and why you should care?</p>
</div>
</div>
<div class="sect2">
<h3 id="_thread_dump_then">Thread dump then</h3>
<div class="paragraph">
<p>And when you have the threaddump… what’s in there?</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>JVM and Java info</p>
</li>
<li>
<p>Thread dump info</p>
</li>
<li>
<p>Stack traces of threads</p>
</li>
<li>
<p>Heap breakdown</p>
</li>
</ol>
</div>
</div>
<div class="sect2">
<h3 id="_comparing_printing_threads">Comparing printing threads</h3>
<div class="paragraph">
<p>With <code>-l</code> we have also lock information, who holds what:</p>
</div>
<div class="imageblock">
<div class="content">
<img src="comparisonBetweenThreadsAndThreadsWithLocks.png" alt="Shows difference between just printing threads and printing them with lock info via `-l`">
</div>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_commands_used_during_presentation">Commands used during presentation</h2>
<div class="sectionbody">
<div class="listingblock">
<div class="content">
<pre>jconsole
ctrl+break
kill -3 PID
kill -s 3 PID
kill -QUIT PID
type kill
man kill
man -k signal
man --section 7 signals | grep SIGQUIT
man --section 7 signals | grep Core
man core
vim /proc/sys/kernel/core_pattern
ps | grep java
pgrep java
pgrep -u root java
pgrep -u root,tammo java
jps -q
jps
jps -l
jps -m
jcmd
jps -v
jcmd -l
ls /tmp/hsperfdata_tammo/
while true; do; ls /tmp/hsperfdata_tammo/; done
jcmd -options
jcmd 12769 help
jcmd 12769 help Thread.print
jcmd 12769 Thread.print
jcmd 12769 Thread.print > threads
jcmd 12769 Thread.print -l > threadsAndLocks
jcmd 14222 Thread.print -l > threadsAndLocks
jstack 14222 > threaddump
man jcmd
man jinfo
man jstack
jvisualvm -J-Xverbose:gc
for i in $(seq 1 5); do; jstack 16002 > td$i; echo $i; sleep 5; done;
jconsole
jvisualvm</pre>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_bibliography_links">Bibliography / Links</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Besides link already used and added earlier, few others caught my eye:</p>
</div>
<div class="admonitionblock important">
<table>
<tr>
<td class="icon">
<i class="icon-important" title="Important"></i>