Re: [Xen-users] Xen, Memory and prelink interaction

[Massimo Mongardini <massimo.mongardini@xxxxxxxxx>]

John Madden wrote:

On Fri, 2008-07-25 at 17:35 +0100, Massimo Mongardini wrote:
  

The servers are HP ProLiant BL465c G1
cpus: 2xDual-Core AMD Opteron(tm) Processor 2216 HE
os: RHEL 4.6 32bit version (Linux server-0007 2.6.9-67.0.15.ELsmp #1
SMP 
Tue Apr 22 13:50:33 EDT 2008 i686 athlon i386 GNU/Linux)
ram: 8GB

I have several java apps that start with -Xmx2048m option and they
work 
just fine on 12 blades, but on some of them that were used as dom0s.
    

Really?  With a 32-bit JVM?  I've never been able to get it to start
with any more than the ~1530MB limit and I found docs on Sun's site to
back that up -- different OS's place different limits under 2048MB for
various overheads.  I'd say you should count yourself very, very lucky
that the 2048MB's never ran into OOM conditions.  Still, odd that the
JVM won't even *start* under Xen...

John



  

Sorry my post wasn't clear enough... the problem arises on a non-xen kernel (redhat kernel 2.6.9-67.0.15.ELsmp) after the server was "xenified" and then reverted back to a plain redhat kernel.
Not sure if and how this can matter but when running as a dom0 I gave the single domU that it had running 6GB of memory out of 8GB.

On my site I have various java instances like that one happily running on various non dom0/U servers, for instance
[massimo@server-0006 ~]$ ps auxw | grep Xmx2048M | grep -v grep | wc -l
6

I believe the java heap size limit for a 32-bit is 4GB if not more constrained by the os.
http://java.sun.com/performance/reference/whitepapers/tuning.html#section4.1.2

I still have 5 servers that are running a xen kernel and since this is the second server that has this issue in the same circumstances,  I am quite confident that if I switch those back to a rh kernel I will face the same issue.... If any of you wants to do some debugging I am more than willing to switch one of them back to run "plain" kernels.

cheers
Massimo

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