Re: [Xen-devel] failed to start centos 5 domU with maxmem=30000

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

 On 08/12/2010 05:24 AM, Vasiliy G Tolstov wrote:
> Ð ÐÑÐ, 12/08/2010 Ð 05:17 -0700, Dan Magenheimer ÐÐÑÐÑ:
> > > From: Vasiliy G Tolstov [mailto:v.tolstov@xxxxxxxxx]
> > > Sent: Thursday, August 12, 2010 5:38 AM
> > > To: Pasi KÃrkkÃinen
> > > Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
> > > Subject: Re: [Xen-devel] failed to start centos 5 domU with
> > > maxmem=30000
> > >
> > > Ð ÐÑÐ, 12/08/2010 Ð 14:37 +0300, Pasi KÃrkkÃinen ÐÐÑÐÑ:
> > > > 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 ?
A factor of ~2000 between smallest and largest size won't work.  The 
problem is that lots of kernel structures are scaled with the amount of 
boot-time memory, and they'll simply be overloaded when you add more 
memory.  For example, the page structure array for 60GB needs ~15.3 
million entries, which at 64 bytes per entry will take ~980MB...
> > 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.
This is exactly the use-case for the hotplug memory stuff Daniel is 
working on - which I guess is why you've been testing it.
    J

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