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

[Vasiliy G Tolstov <v.tolstov@selfip.ru>]

В Чтв, 12/08/2010 в 05:17 -0700, Dan Magenheimer пишет:
> > 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.
> 

Is that possible to use memory=32 and maxmem=60000 ?

> 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?

We want to provide ability to use small as possible memory if the guest
is idle. And much as possible when the guest under heavy load.

Thank You.
-- 
Vasiliy G Tolstov <v.tolstov@selfip.ru>
Selfip.Ru