Use the J2SE version 5 for running the Instant Messaging server, because it performs better and does not require command line tuning as it supports ergonomic features. More information on the use of this version of Java can be found in the following documents:
The Instant Messaging server uses the iim.jvm.maxmemorysize parameter in iim.conf to set the maximum size of the JVM heap to allocate. The default value of this parameter is 256 MB, but a large active deployment of Instant Messaging will need more memory. Determining how much memory to allocate for the Instant Messaging servers in the server pool is dependent on how many concurrent active users you need to support. Each Instant Messaging server in the server pool needs 256 MB along with 65 KB for each connected/active user with daily usage like the following:
Two presence updates
Five chats with durations of 10 minutes
One multiuser conference with a duration of 15 minutes
One logout and login
Additional load per user, use of additional Instant Messaging services like news or file transfer, and use of features such as message filters, archiving, or SSL will consume more memory. You should perform load profiling of typical user activity before deploying Instant Messaging into a production environment. Contact Sun Support Services for more information about load profiling an Instant Messaging deployment.