Xen users - Jul 2008 - Java memory allocation in guest domain with mem and max-mem properties

Hi, I''ve tried to search the list archives but I didn''t found
anything
about this and I''m experiencing some problems I need to solve quickly
so here follows my question and related doubts.

* The configuration

I''ve an 8GB equipped with 2 dual core xeon cpus server running CentOS
5.1 (x64) and xen 3.0.3-41.el5 on kernel 2.6.18-53

I''m running 4 DomU each one with mem = 512M and max-mem=2GB

Each of my DomU runs a java application (JBoss 4.2.2-GA) using the Sun
JDK 1.5.0.14 with options to grow java heap size from 256M to 1G on
demand

* The problem

My JVM runs out of memory without balloing the Xen memory, or at least
this is what I think I''m experiencing.

If I run free in the DomU or xm top in the Dom0 I see my DomUs memory
standing still at 512M, if I run top in the DomU I see the java
process running with 300M on memory and 700M on swap space (which is
something I want to avoid and let the app run in memory) and in some
occasions going in OutOfMemoryError without seeing the xen memory
balloing out

* 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!)


-- 
   Roberto Lo Giacco

_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

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...

robert



_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

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.

Cheers,
Mark

-- 
Push Me Pull You - Distributed SCM tool (http://www.cl.cam.ac.uk/~maw48/pmpu/)

_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users