HDUMP - Heap Dump Post-Processor

Overview
Using HDUMP
Delta HDUMP

Overview

HDUMP post-processes the information JPROF collects about the Java heap, in an attempt to locate memory leaks. Because Java uses a Garbage Collector that automatically deallocates unreferenced objects, memory leaks are not caused by forgetting to deallocate an object. Instead, they are the result of unnecessary references which keep objects alive.

HDUMP displays the following types of information to help identify these references:

• the shortest path to any object in the heap from some root of the heap
• the objects furthest from any root in the heap
• all of the roots and objects which reference an object

Using HDUMP

You use HDUMP by simply specifying the log-hd file which contains all of the heap information collected by JPROF:
hdump -f filename [parms]

If none of the optional parameters are specified, this will produce the following information:

• A list of the 10 classes with the most allocated objects
• A heapsum file showing the shortest reference path to the deepest object in each of those 10 classes
• A b1st file that enumerates all of the live objects in the heap, breadth-first, starting with the roots of the heap
• A lattice file that contains the complete lattice of object references.

All output files will use the same file extension as the log-hd file used as input.

After you have executed HDUMP once, you may want to invoke it again in interactive mode, using the -i parameter:

hdump -f filename -i

In interactive mode, you can specify the object id of any object that you find in the b1st file to view the shortest reference path to that object. This path is both displayed on the screen and appended to the heapsum file. These paths are extracted from the information in the b1st file and can be done very quickly.

After you have produced all of the shortest paths you want, you can specify objects using the r command to produce a listing of all of the objects which reference the selected object, using a breadth-first algorithm which works backwards from the object to the roots which reference it. This is actually a list of everything that is keeping this object alive. All of these listings are written to the b2nd file.

The processing for these reverse listings destroys the internal copy of the b1st data, so it is only done after the shortest path listings. These b2nd listings can also be selected by specifying the object id on the command line with the -r parameter for up to 32 objects:

hdump -f filename -r objid [-r objid ...]

The other parameters and files are primarily for debugging, except as noted below. You can display a list of them by using ? as the only parameter.

Delta HDUMP

Delta HDUMP is available when using JPROF with JVMTI. Each log-hd.n file contains information about the highest object tag in use at the time of each preceding heap dump. Thus, we can easily tell which objects were created between heapdumps. By using the "-diff m n" parameter, the user can select which pair of heap dumps to compare. The result of this comparison is the b3rd file.

The format of the b3rd file is the same as the b1st file. However, the roots used in the b3rd file are those found using the b2nd algorithm with all of the objects created between heapdumps m and n. Furthermore, only those objects referenced in the b2nd search that determined those roots are included in the b3rd output. The result is a greatly reduced subset of the b1st file that includes only those references that are keeping these new objects alive.