If you suspect that your MapViewer instance is having memory related issues when pre-fetching large number of map tiles for a tile layer, and your OS is Linux, then maybe this tip will help you.
Basically on Linux, the underlying file system itself will often aggressively use memory to cache file handles et al. When you start a large tile pre-fetching task, it is not uncommon for MapViewer to create hundreds of thousands of files (map tile images) in a short period. If the OS keeps caching these file system related "things" in memory, sooner or later your system will experience memory shortages, and this may cause MapViewer instance's JVM to crash or throw OutOfMemory exceptions.
Fortunately there is an easy workaround (thanks to Ji who found and tested it). All you (or your system admin) need to do is execute the following shell command as root:
#> sync; echo 3 > /proc/sys/vm/drop_caches
This command will force the Linux kernel to release file system related caches. It is best to run this script periodically (by using a cron job) like every 3 hours.