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Re: [Xen-users] Java memory allocation in guest domain with mem and max-

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Subject: Re: [Xen-users] Java memory allocation in guest domain with mem and max-mem properties
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Thu, 17 Jul 2008 19:11:16 +0100
Cc: Roberto Lo Giacco <rlogiacco@xxxxxxxxxxxx>, Robert Read <rread@xxxxxxxxxxxxx>
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On Thursday 17 July 2008, Robert Read wrote:
> On Jul 17, 2008, at 09:28 , Roberto Lo Giacco wrote:
> > * The question
> >
> > Have I missed something (I'm quite new to xen) in my configuration
> > files, like using mem-min and mem-max instead of mem and mem-max?
> > Anyone out there has already encountered this behavior and solved it?
> > Does this error depends on the used JVM or is there any solution which
> > is not just "set your DomU memory to a fixed value of 2GB" (btw, this
> > is the only solution which comes to me)?
> >
> > Thank you in advance for your patience guys (and ladies, if any!)
> It's possible this has be implemented in more recent releases, but 3.0
> does not support
> guest autoballooning.  In other words, you'll need to track guest
> memory and add more manually, at least in 3.0.
> Automating this shouldn't be too difficult, though, so if this hasn't
> been done yet in xen 3.3 then we really should find some volunteers to
> take this on...

It's still the case that guests will not balloon themselves "on demand".  
mem-max is a limit on how large the domain can grow but it won't grow unless 
you tell it to.

Also, you can't normally grow a domain beyond its initial allocation unless 
you pass "mem=<foo>" on its kernel command-line to tell it how big its memory 
might get.  You can of course start a domain large and then balloon it down 
to normal operating size.

There have been patches floated for automatically dynamically reszing domains.  
Dan Magenheimer has posted some to the xen-devel list in the last few 
months - those threads would be worth looking at for anyone wishing to get 
some functionality like this into mainline or just to try it out.

This is all due to the fact that (unlike VMware and many other VMMs) Xen opts 
not to swap domains but leave it up to them to implement a sensible swapping 
policy.  This strategy would be usefully complemented by an "autoballooning" 
feature for domUs that could redistribute memory when required and it's been 
on the roadmap for a long time.

I hope that helps.


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