Setting up and checking your Linux environment

The current working directory of the Java VM process is the default location for the generation of core files, Java™ dumps, heap dumps, and the Java VM trace outputs, including Application Trace and Method trace. Enough free disk space must be available for this directory. Also, the Java VM must have write permission.

When a crash occurs, the most important diagnostic data to obtain is the system dump. To ensure that this file is generated, you must check the following settings.

Operating system settings

Operating system settings must be correct. These settings can vary by distribution and Linux version.

To obtain full core files, set the following ulimit options:

ulimit -c unlimited		 turn on corefiles with unlimited size  
ulimit -n unlimited		 allows an unlimited number of open file descriptors
ulimit -m unlimited		 sets the user memory limit to unlimited
ulimit -f unlimited		 sets the file size to unlimited

For more information about ulimit settings, see Configuring your system in the OpenJ9 user documentation.

From Java 5, the ulimit -c value for the soft limit is ignored and the hard limit value is used to help ensure generation of the core file.

Linux tools such as the Automatic Bug Reporting Tool (ABRT) and Apport tool use settings in the Linux /proc/sys/kernel/core_pattern file to redirect core files to a diagnostic program. These tools and the /proc/sys/kernel/core_pattern file settings they use might prevent the JVM from successfully producing system dumps. The tools are enabled by default in some Linux distributions. You might see this error message:

JVMPORT030W "proc/sys/kernel/core_pattern setting ..... 
specifies that the core dump is to be piped to an external program.
Attempting to rename either core or core.<pid>

Review the configuration of these tools or consider disabling the tools if problems occur when you attempt to generate system dumps from the Java VM.

Java Virtual Machine settings

To generate core files when a crash occurs, check that the Java VM is set to do so.

Run java -Xdump:what, which should produce the following output:

-Xdump:system:
    events=gpf+abort+traceassert+corruptcache,
    label=/mysdk/sdk//jrebin/core.%Y%m%d.%H%M%S.%pid.dmp,
    range=1..0,
    priority=999,
    request=serial

These values are the default settings. At least events=gpf must be set to generate a core file when a crash occurs. You can change and set options with the command-line option -Xdump:system[:name1=value1,name2=value2 ...]

Available disk space

The available disk space must be large enough for the core file to be written.

The JVM allows the core file to be written to any directory that is specified in the label option. For example:

-Xdump:system:label=/mysdk/sdk/jre/bin/core.%Y%m%d.%H%M%S.%pid.dmp

To write the core file to this location, disk space must be sufficient (up to 4 GB might be required for a 32-bit process), and the correct permissions for the Java process to write to that location.

The distributions supported by the IBM Java VM provide the enhanced Native POSIX Threads Library for Linux (NPTL).

You can discover your glibc version by changing to the /lib directory and running the file libc.so.6. The Linux command ldd prints information that should help you to work out the shared library dependencies of your application.