RE: failed to start centos 5 domU with maxmem=30000

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: Vasiliy G Tolstov [mailto:v.tolstov@selfip.ru]
> Sent: Thursday, August 12, 2010 5:38 AM
> To: Pasi Kärkkäinen
> Cc: xen-devel@lists.xensource.com
> Subject: Re: [Xen-devel] failed to start centos 5 domU with
> maxmem=30000
> 
> В Чтв, 12/08/2010 в 14:37 +0300, Pasi Kärkkäinen пишет:
> > On Thu, Aug 12, 2010 at 02:24:05PM +0400, Vasiliy G Tolstov wrote:
> > > Hello. I'm try to start centos domU pvm under sles 11 sp1 xen
> kernel
> > > with memory=512 and maxmem=30000. Centos doing boot and freeze.
> > > If i'm try to use maxmem=15000 all works.
> > >
> > > Can You tell me what's wrong?
> > >
> >
> > Is the centos PV domU 32bit or 64bit ?
> 
> 64 bit kernel-xen.
> 
> I found this in xen sources:
> 
> /* Simple continuous piecewiese linear function:
>          *  max MiB -> min MiB  gradient
>          *       0         0
>          *      16        16
>          *      32        24
>          *     128        72    (1/2)
>          *     512       168    (1/4)
>          *    2048       360    (1/8)
>          *    8192       552    (1/32)
>          *   32768      1320
>          *  131072      4392
>          */
> 
> Why this is not provided in documentation or on web site?

Hi Vasily --

This function limits how far memory can be reduced when
ballooning a guest (including dom0).  It is only a heuristic
but is intended to take into account the various overheads
a guest Linux kernel requires to manage memory to avoid
out-of-memory conditions.

But I think you are correct... the same (or similar)
function should be published as it also serves as a
guideline for the ratio between memory= and maxmem=
parameters when creating a guest:  If the ratio
of maxmem divided by memory is too high, the guest
will not even boot.

I am curious as to why you would specify memory= so
much smaller than maxmem=.  Are you trying to overcommit
memory for guests that are often idle but sometimes use
a very large amount of memory?

Thanks,
Dan