Improved Java Memory/Performance Defaults

Binary package hint: tomcat6

Forking an issue from #541520

The current default of a 128MB heap does not make sense. This was chosen when Java 1.4 had a default maximum heap size of 64MB. In Java 1.5 and later, the JVM will automatically size the heap based on available memory. So the current default setting actually unnecessarily sets a limit on the memory available to tomcat, when not setting anything at all would actually allow the modern JVMs to do a better job.

References:
http://java.sun.com/javase/technologies/hotspot/gc/gc_tuning_6.html#par_gc.ergonomics.default_size
http://java.sun.com/javase/6/docs/technotes/guides/vm/gc-ergonomics.html
http://java.sun.com/docs/hotspot/gc5.0/ergo5.html

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initial heap size:
Larger of 1/64th of the machine's physical memory on the machine or some reasonable minimum. Before J2SE 5.0, the default initial heap size was a reasonable minimum, which varies by platform. You can override this default using the -Xms command-line option.

maximum heap size:
Smaller of 1/4th of the physical memory or 1GB. Before J2SE 5.0, the default maximum heap size was 64MB. You can override this default using the -Xmx command-line option.
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