Why are -Xms and -Xmx limits so small?

Question

On a Win XP machine with 3GB of installed memory, the limits for -Xms and -Xmx are 1GB. My knapsack program works for small inputs (< 20 objects). But for real world problems it crashes immediately with an out-of-memory error. What possible harm could it do to allow a Java user to increase -Xms and -Xmx to 2 or 3 GB, or even larger? I would not care if it thrashed, if it finally came up with a good answer.

Answer 1

CHE22 wrote:
CHE22 wrote:
On a Win XP machine with 3GB of installed memory, the limits for -Xms and -Xmx are 1GB.
tschodt wrote:
How did you arrive at that?
By reading this:
http://java.sun.com/j2se/reference/whitepapers/memorymanagement_whitepaper.pdf.
I guess you are referring to
Automatic Selection of Collector, Heap Sizes, and Virtual Machine
A server-class machine is defined to be one with
• 2 or more physical processors and
• 2 or more gigabytes of physical memory
13 Ergonomics - Automatic Selections and Behavior Tuning Sun Microsystems, Inc.
This definition of a server-class machine applies to all platforms, with the exception of 32-bit platforms running a version of the Windows operating system.
On machines that are not server-class machines, the default values for JVM, garbage collector, and heap sizes are
• the client JVM
• the serial garbage collector
• Initial heap size of 4MB
• Maximum heap size of 64MB
On a server-class machine, the JVM is always the server JVM unless you explicitly specify the -client
command line option to request the client JVM. On a server-class machine running the server JVM, the default
garbage collector is the parallel collector. Otherwise, the default is the serial collector.
On a server-class machine running either JVM (client or server) with the parallel garbage collector, the default initial and maximum heap sizes are
• Initial heap size of 1/64th of the physical memory, up to 1GB. (Note that the minimum initial heap size is 32MB, since a server-class machine is defined to have at least 2GB of memory and 1/64th of 2GB is 32MB.)
• Maximum heap size of 1/4th of the physical memory, up to 1GB.
Otherwise, the same default sizes as for non-server-class machines are used (4MB initial heap size and 64MB maximum heap size). Default values can always be overridden by command line options. Relevant options are shown in Section 8.And you missed that it said default.
And by adding any small integer to 1 in -Xms1G.Show us exactly what you specified, do you realize that-Xms11Gmeans 11 GB, much larger than 32-bit Windows can handle.