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Increasing the maximum size of the Java heap might not always solve this type of problem
because the problem might be memory overuse. Conversely, if response times are too long
because of GC pause times, decrease the heap size. If both problems are observed, an
analysis of the application heap usage is required.
4.16.3 Setting the heap size when running multiple JVMs on one system
Each running Java program has a heap associated with it. If you have a configuration where
more than one Java program is running on a single physical system, setting the heap sizes
appropriately is of particular importance. Setting the heap size is also important for 32-bit
systems, where the total amount of addressable memory is limited.
An example of one such configuration is when the Integration Designer is on the same
physical system as a Business Process Manager server using a 32-bit JVM. Each of these
applications is a separate Java program that has its own Java heap. If the sum of all of the
virtual memory usage (including both Java heaps and all other virtual memory allocations)
exceeds the size of addressable physical memory, the Java heaps are subject to paging.
Such paging causes total system performance to degrade significantly. To minimize the
possibility of total system degradation, use the following guidelines:
► First, collect a verbosegc trace for each running JVM.
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Based on the verbosegc trace output, set the initial heap size to a relatively low value. For
example, assume that the verbosegc trace output shows that the heap size grows quickly
to 256 MB, and then grows more slowly to 400 MB and stabilizes at that point. Based on
this change, set the initial heap size to 256 MB (-Xms256m).
Also based on the verbosegc trace output, set the maximum heap size appropriately. Be
careful not to set this value too low, or out-of-memory errors occur. The maximum heap
size must be large enough to allow for peak throughput. Using the same example, a
maximum heap size of 768 MB might be appropriate (-Xmx768m). Correct sizing of
maximum heap gives the Java heap room to expand beyond its current size of 400 MB, if
required. The Java heap grows only if required (for example, if a period of peak activity
drives a higher throughput rate), so setting the maximum heap size higher than current
requirements is generally a good policy.
Be careful not to set the heap sizes too low, or garbage collections will occur frequently,
which might reduce throughput. Again, a verbosegc trace assists in determining garbage
collection frequency. You must ensure that the heap sizes are large enough that garbage
collections do not occur too often. At the same time, you must still ensure that the heap
sizes are not cumulatively so large as to cause the heap to page to the file system. This
balancing act is depends on configuration.
4.16.4 Reducing or increasing heap size if OutOfMemory errors occur
The java.lang.OutOfMemory exception is used by the JVM in various circumstances, so that
finding the source of the exception is difficult. There is no conclusive mechanism for telling the
difference between these potential error sources, but a good start is to collect a trace using
verbosegc. If the problem is a lack of memory in the heap, you can easily see this condition in
the output. For more information about verbosegc output, see 4.16.1, “Monitoring garbage
collection” on page 92. Many garbage collections that produce little free heap space generally
occur preceding this exception. If this lack of free heap space is the problem, increase the
size of the heap.
If there is enough free memory when the java.lang.OutOfMemory exception occurs, the next
item to check is the finalizer count from the verbosegc (only the IBM JVM provides this
information). If this count appears high, a subtle effect might be occurring whereby resources
94 IBM Business Process Manager V8.0 Performance Tuning and Best Practices