PRSTAT Linux – How to pinpoint high CPU Java VM Threads
This article will provide you with an equivalent approach, for JVM on
Linux OS, of the powerful Solaris OS prstat command; allowing you to quickly
pinpoint the high CPU Java VM Thread contributors.
One key concept to understand for a Java VM running on the Linux OS is
that Java threads are implemented as native
Threads, which results in each thread being a separate Linux process.
Ok thanks for the info but why is this related to
prstat?
Well this key concept means that you don’t need a prstat command
for Linux. Since each Java VM Thread is implemented as a native Thread, each
Thread CPU % can simply be extracted out-of-the-box using the top command.
You still need to generate Thread Dump data of your JVM process in order
to correlate with the Linux top command output.
Thanks for this explanation. Now please show me how to
do this
Please simply follow the instructions below:
1) Execute
the top command
(press SHIFT-H to get the Threads toggle view) or use -H option (to show all
Threads) and find the PID associated with your affected / high CPU WLS
process(es) (remember, many may show up since each Java Thread is
implemented as a separate Linux process)
2) Immediately
after, generate a few Thread Dump snapshots using kill –3 <PID> of the
parent WLS process. Thread Dump provides you with the complete list with
associated Stack Trace of each Java Thread within your JVM process
3) Now,
convert the PID(s) extracted from the top command output to HEX format
4) The
next step is to search from the Thread Dump data for a match nid=<HEX
PID>
5) The
final step is to analyze the affected Thread(s) and analyze the Stack Trace so
you can determine where in the code is the problem (application code,
middleware itself, JDK etc.)
Example: top command captured of a Weblogic Server Java Thread running
at 40% CPU
utilization along with Thread Dump data via kill -3 <PID>
## top output sample
PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM
TIME COMMAND
...........
22111 userWLS 9 0 86616 84M 26780 S 0.0 40.1 0:00
java
- Decimal
to HEX conversion of Java Thread (native Thread) 22111 >> 0x565F
- Now
using the HEX value, we can search within the Thread Dump for the
following keyword: nid=0x565F
## Thread Dump output sample Thread as per the above search criteria nid=0x565F
"ExecuteThread: '0' for queue: 'default'"
daemon prio=1 tid=0x83da550 nid=0x565Fwaiting
on monitor [0x56138000..0x56138870]
at java.util.zip.ZipFile.getEntry(Native
Method)
at
java.util.zip.ZipFile.getEntry(ZipFile.java:172)
at
java.util.jar.JarFile.getEntry(JarFile.java:269)
at
java.util.jar.JarFile.getJarEntry(JarFile.java:252)
at
sun.misc.URLClassPath$JarLoader.getResource(URLClassPath.java:989)
at sun.misc.URLClassPath$JarLoader.findResource(URLClassPath.java:967)
at
sun.misc.URLClassPath.findResource(URLClassPath.java:262)
at
java.net.URLClassLoader$4.run(URLClassLoader.java:763)
at
java.security.AccessController.doPrivileged(AccessController.java:224)
at
java.net.URLClassLoader.findResource(URLClassLoader.java:760)
at
java.lang.ClassLoader.getResource(ClassLoader.java:444)
at
java.lang.ClassLoader.getResourceAsStream(ClassLoader.java:504)
............................................
In the above example,
the problem was related to an excessive class loading / IO problem.
As you can see, the
approach did allow us to quickly pinpoint high CPU Thread contributor but you
will need to spend additional time to analyze the root cause which is now your
job.
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