Profiling with hprof

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Java 1.2 introduced an experimental profiling interface, called JVMPI (Java Virtual Machine Profiler Interface). The interface comes with a sample profiling agent, called hprof. Any VM that supports JVMPI should be also able to run hprof.

This agent creates profile data files which can be interpreted after the program terminates. However, the format of the files is still evolving. This version of HPjmeter supports only the formats compatible with JavaSoft reference implementations JDK-1.2.1-A or JDK-1.3.0-C for Solaris(TM) or Windows(R) NT(R)), and in particular:

• Java 2 SDK 1.2.2 for HP-UX 11.0, Classic VM (use -classic to select the Classic VM)
• Java 2 SDK 1.3 beta for HP-UX 11.0, Classic or HotSpot 2.0 Client VM (if available).

To run your application with profiling use the following command:

java ... -Xrunhprof:<options> ApplicationClassName

To profile your applet, use:

appletviewer ... -J-Xrunhprof:<options> URL

We have found the following hprof options useful:

1. For performance profiling:
   cpu=samples,thread=y,depth=10,cutoff=0,format=a
2. For analyzing object allocation or solving memory retention problems:
   heap=all,cpu=samples,thread=y,depth=10,cutoff=0,format=a

To see the complete list of available options, use

java ... -Xrunhprof:help

The options which affect the collection of profile data supported in this version of HPjmeter are described below:

• heap=dump|sites|all
  The heap dump shows all objects remaining in memory when the application terminates. The allocation sites show where the objects were created throughout the execution of the program. Heap analysis can be used to locate memory "leaks", and the allocation sites can be used to minimize the memory usage.

• cpu=samples|times|old
  The times collected by hprof are CPU virtual times. This is the time when a thread is runnable, but not necessarily running. cpu=samples uses sampling as the collecting technique, while the two remaining options use tracing (with reduction). The hprof implementation for HP-UX 11.0 is an exception. Unless sampling is used, the collected times are real CPU times rather than virtual CPU times.

  cpu=old outputs inclusive method times, while the two other options output exclusive times. HPjmeter can derive inclusive times from the exclusive times, but only by using estimation. cpu=old cannot produce profiling data separately for each thread.

  Collecting heap data can be very intrusive for time metrics, so we recommend that you do not measure the method times and heap usage at the same time. However, specifying heap=sites and cpu=samples|times for the same run makes sense if you'd like to find the allocation sites in the call graph.

• thread=y|n
  Controls whether the stack traces collected during program execution will be associated with an individual thread. By default, no thread identification information is stored (thread=n). If you do not use cpu=old, we suggest that you always specify thread=y. With the thread identification in the profile file, HPjmeter can calculate the thread profile data for the whole application.

• depth=<size>
  This parameter controls the depth of the stack trace for each stack trace taken. Increasing this value will make the profile data file larger, but simultaneously will enable HPjmeter to better estimate inclusive method times and generate more complete call graphs. If you use sampling, the default value of 4 is too small for most applications, values between 6 and 12 are recommended.

• cutoff=<value>
  This is the cut-off value for printing the list of methods with their exclusive times, or object allocation sites. Decreasing this value will make the profile data file slightly larger, but at the same time will enable HPjmeter to produce more complete call graphs or heap metrics. A value of 0 is recommended for all applications.

• format=a
  HPjmeter supports only the ASCII format of the profile file.

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Trademark Acknowledgement
Solaris is a trademark of Sun Microsystems, Inc. in the U.S. and other countries.
Windows NT is a U.S. registered trademark of Microsoft Corporation.