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From "Alex Blewitt" <alex.blew...@gmail.com>
Subject Re: Re: [rant] Memory options in VM -- why is the default not 'unlimited'
Date Sun, 30 Jul 2006 10:56:44 GMT
On 30/07/06, Xiao-Feng Li <xiaofeng.li@gmail.com> wrote:
> Agree that maximum size is still desirable for system stability
> reason. :-)  ( I hope the OS MM can be more intelligent to deal with
> overcommitment.)  But here in this specific discussion context, I
> think the word `unlimit' or `infinite' is meaning `unlimit under
> certain limit'. Whatever the limit is can be defined by the clever OS
> (which returns a ENOMEM, for example) or user heuristics.

Infinite in a computer science, rather than mathematical sense :-)

For example, there may be practical reasons why 4Gb is the maximum (or
2Gb if signed pointers are used). But even if the OS has 512mb of
memory, and (say) 256m free, there's no reason to limit the amount of
memory that the JVM requests. The user will notice a slow-down in
performance as soon as the *practical* limit is transgressed, but
unless the user starts with a -Xmx option, the memory should be
'unlimited' in the 'largest amount of space that is possible'. Given
that most OSs have virtual memory, why should the JVM decide that it's
not going to use it?

I'd argue that if not specified, the limit should be unlimited in the
sense that the largest possible amount that the process can possible
acquire. If the user wants to limit the amount of memory to prevent
swapping, that's fine, but it has to be a user-supplied option and not
a JVM-enforced-default.

Let's face it, the only reason why -Xmx is used at all in almost all
JVMs is not to limit the amount of space, but rather a workaround
because the JVM gets it wrong.


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