Re: [Xen-users] Why limit dom0's memory?

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

Hi there,

> It's strange for me.
> I've got 8Go of RAM.
> I've defined 1024M to Dom0 on the boot (/boot/grub/menu.lst)
> ...
> kernel          /xen-3.1.0.gz dom0_mem=1024M console=vga
> ...
>
> I've got some Guests with some Go of RAM et it still remain a lot of
> RAM to use that i attibue to the news guests.
> But the problem that when i tried to create it, i had this error:
>
> # xm create toto.cfg
> Using config file "/etc/xen/toto.cfg".
> Error: I need 1073152 KiB, but dom0_min_mem is 200704 and shrinking to
> 200704 KiB would leave only 675672 KiB free.
>
> # cat /etc/xen/toto.cfg
> ---
> memory  = '1048'
> ---
>
> If i shutdown one of my other guest, it will ok.
> It's strange since i defined 1Go to Dom0, i get only 853Mo
> # free -mo
>
>              total       used       free     shared    buffers     cached
> Mem:           853        190        662          0         23         32
>
>
> Would someone explain me about this?
> i'm with Debian Etch, Xen 3.1, 8Go RAM.

Is that "free" output from when dom0 is freshly booted, or after you've 
started some domains?

Although you're setting dom0_mem=1024M, which means dom0 *initially* has 1GB 
of RAM, it looks like your dom0-min-mem setting in /etc/xen/xend-config.sxp 
is set to 200704, which will permit dom0 to shrink in order to accommodate 
starting other domains.  If dom0 has already shrunk then the value given 
by "free" will be correspondingly reduced.

The error messages you're seeing are due to dom0 not being able to shrink 
enough to accommodate your new domain, suggesting that you're running short 
on RAM.

xm info can be used to find out the total amount of memory that's free for Xen 
to allocate without dom0 needing to shrink.

Cheers,
Mark

> Thank you in advance.
>
> On Thu, Jun 19, 2008 at 4:24 AM, Mark Williamson
>
> <mark.williamson@xxxxxxxxxxxx> wrote:
> > That said, I think many people find that for a minimal dom0, not doing
> > much other work about 256MB is a reasonable amount of memory.  Maybe
> > you'd want to go to 512MB if you had many guests and / or memory to
> > spare.  dom0's requirements aren't extravagent, as long as you're not
> > running loads of things in it (which on a server you shouldn't, for
> > security reasons).
> >
> > Of course if you start running X and a modern desktop, you can expect
> > dom0 to have significantly higher memory requirements, just as a normal
> > machine would ;-)
> >
> > Cheers,
> > Mark
> >
> > On Monday 16 June 2008, Sandor W. Sklar wrote:
> >> On Jun 15, 2008, at 7:37 PM, Tim Post wrote:
> >> > On Sun, 2008-06-15 at 17:45 -0700, Sandor W. Sklar wrote:
> >> >> Well, of course, the amount of memory that a dom0 needs depends upon
> >> >> the service running within it.  I guess I didn't word my question
> >> >> explicitly enough.  Is there a formula for determine how much
> >> >> memory a
> >> >> dom0 might need, given a system with X amount of RAM, and X number of
> >> >> guests, assuming there are no other services running in the dom0?
> >> >
> >> > You are hoping to calculate the best possible density? I.e. give dom-0
> >> > xx MB per pv guest, xx MB per HVM guest?
> >>
> >> Indeed, exactly!  Not exact numbers, but a guideline that would let me
> >> maximize the memory available to guests without running the risk of
> >> starving the dom0.
> >>
> >> > Even that is too broad to really pin down, it would really depend on
> >> > what you give the guests and how much they exercise the disks.
> >>
> >> OK, thanks, I guess I assumed as much, given that if such information
> >> existed, Google would have told me.  :-)
> >>
> >>       -s-
> >>
> >>
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> >
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