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

[ko0nz <yesiko0nz@xxxxxxxxx>]

Oups!!!
total_memory           : 3322

while i have 8Go of RAM...

i did installed a Debian i686 on a x86_64 Server. It should not be a
problem, isn't it?

If i save all my guests, and install again my Xen Server with Debian
x86_64 instead of i686, then i restore the guests, will there be a
problem?




root@gaia: ~ # xm info
host                   : gaia
release                : 2.6.18-xen
version                : #1 SMP Fri May 18 16:11:33 BST 2007
machine                : i686
nr_cpus                : 4
nr_nodes               : 1
sockets_per_node       : 1
cores_per_socket       : 4
threads_per_core       : 1
cpu_mhz                : 2660
hw_caps                :
bfebfbff:20100000:00000000:00000040:000ce3bd:00000000:00000001
total_memory           : 3322
free_memory            : 2
xen_major              : 3
xen_minor              : 1
xen_extra              : .0
xen_caps               : xen-3.0-x86_32 hvm-3.0-x86_32
xen_scheduler          : credit
xen_pagesize           : 4096
platform_params        : virt_start=0xfc000000
xen_changeset          : Fri May 18 15:52:14 2007 +0100 15042:50fe1a769660
cc_compiler            : gcc version 3.4.4 20050314 (prerelease)
(Debian 3.4.3-13)
cc_compile_by          : shand
cc_compile_domain      : localdomain
cc_compile_date        : Fri May 18 15:53:15 BST 2007
xend_config_format     : 4






On Thu, Jun 19, 2008 at 3:56 PM, Mark Williamson
<mark.williamson@xxxxxxxxxxxx> wrote:
> 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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