Getting a JVM heap dump

This document provides the process of creating a heap-dump on a java machine to investigate potential memory leaks.

Although a heap dump will be auto generated when OutOfMemoryError is thrown by specifying -XX:+HeapDumpOnOutOfMemoryError VM option, we can use the jmap tool to print shared object memory maps or heap memory details of a given process or core file or a remote debug server. If the given process is running on a 64-bit VM, you may need to specify the -J-d64 option, e.g.:

jmap -J-d64 -heap <pid>

To get a list of Java processes running on a machine, use the jps or jcmd commands.

In Windows Systems where dbgeng.dll is not present, ‘Debugging Tools For Windows’ needs to be installed to have these tools working. Also, PATH environment variable should contain the location of jvm.dll used by the target process or the location from which the Crash Dump file was produced. For example, set PATH=<jdk>\jre\bin\client;%PATH%

When no option is used jmap prints shared object mappings. An example of whats normally required for our analysis is:

jmap -dump:[live,]format=b,file=<filename>.bin <pid>

The above will dump the Java heap in binary format to filename. If the optional live suboption is used, live objects in the heap are dumped.

Viewing heap dumps

To browse heap dumps generated as above, one can use jhat (Java Heap Analysis Tool) to read the generated file.

Some useful options to use with jmap are:

  • -heap prints a heap summary instead of a binary dump.
  • -histo[:live] prints a histogram of the heap.
  • -F can be used with jmap -dump or jmap -histo option if the pid does not respond.

We can use the jhat command to parse a java heap dump file and launch a webserver to view heap dumps over a web browser. The default port is 7000, but we can change this by using the -port option

Syntax:

jhat [ options ] <heap-dump-file>

e.g. jhat -port 7001 -debug 1

where -port determines the browser port the output can be viewed and -debug <int> determines the level of debug detail presented.

We can also use the jconsole tool to view cpu, memory and heap utilisation in a graphical format.

jconsole [ options ] [ connection ... ]

e.g. jconsole -interval=5 <pid> 127.0.0.1:7474

where -interval=n determines the report refresh interval in seconds, <pid> is the process id of the target JVM (the JVM must be running with the same user id as that running the jconsole)

References:

  • Last Modified: 2020-10-22 22:13:22 UTC by Umar Muzammil.
  • Relevant for Neo4j Versions: 3.4,3.5.
  • Relevant keywords jvm, memory, heap, dump.