Apache Solr runs on the underlying JVM in the J2EE container. While scaling Solr for processing more requests and indexing data, it becomes important that the underlying JVM is capable of scaling and is optimized well. Choosing the right JVM is an important factor. There are many JVMs available in the market today that can be considered such as Oracle Java HotSpot, BEA JRockit, and Open JDK (the list is available at http://en.wikipedia.org/wiki/List_of_Java_virtual_machines).

Some of these JVMs are commercially optimized for production usage; you may find comparison studies at http://dior.ics.muni.cz/~makub/java/speed.html. Some of the JVM implementations provide server versions, which would be more appropriate than normal ones.

Since Solr runs in JVM, all the standard optimizations for applications are applicable to it. It starts with choosing the right heap size for your JVM. The heap size depends upon the following aspects: