IBM SG24-6526-00 manual Initial Java heap size, Maximum Java heap size

5.4.1 Initial Java heap size

–Xms is an important parameter that controls the initial Java heap size. It is followed by the size, which is usually an integer followed by m (represents megabytes). The default is 32 Mb, which would appear as –Xms32m.

Figure 5-18highlights the Xms parameter on the WebSphere Console’s Application Server panel.

Figure 5-18 Application Server panel of the WebSphere console showing the Xms parameter

If you have only a few users who will not be very busy, then this setting may be sufficient. If, however, you have many users, especially active users, you may benefit from a larger value.

Increasing the value can affect the iSeries in two ways. First, the initial size of the Java heap will be greater. This should speed the startup of the application. Second, it causes garbage collection to run less frequently. The less frequently it runs, the better the average performance should be. If it is too infrequent, then the impact may be significant when it runs.

5.4.2 Maximum Java heap size

It is also possible to specify a maximum Java heap size. The parameter is –Xmx and is followed by a value in the same form as the –Xms parameter.

For the iSeries, we recommend that you do not set this parameter because we have not found it to be useful. If you do not specify it, then the heap grows as required including beyond physical memory.

Chapter 5. Performance tuning 125