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Getting JMap Outputs

Date: April 14, 2016

To get the profile of objects that are being generated or used during a JVM. This gives greater control on what objects are being spawned. Here's the command to pipe the output to a file.

$ (sudo -u kafka /usr/jdk64/jdk1.7.0_67/bin/jmap -histo -F <pid>) 2>&1 | tee jmap-output.txt

You'll get a list of the Java object and the amount of instances and total bytes for each object in the java process.

Troubleshooting

Can't attach to a process

Make sure, just like the "Reading JStat" TIL article, to run the jmap command you must ensure 3 things:

  • Use the same user that ran the process.
  • Make sure you use the same java binaries used to launch the same java process. Find out which java the process is using, and use the jmap for that binary. Example: /usr/jdk64/jdk1.7/bin/java was used for launching the java process, then you should use /usr/jdk64/jdk1.7/bin/jmap.
  • If it still can't attach, run this first: echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope. Note: works only on Ubuntu. This enables ptrace attachments over ssh. See https://www.kernel.org/doc/Documentation/security/Yama.txt for more details on this.