Memory not seen in Dom0

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Memory not seen in Dom0

rah600
I have a machine with 64G of ram. Running CentOS 5.4 with kernel 2.6.18-164, I only see 32G in /proc/meminfo. All 64G show up for the stock kernel. There are no vm's running.

I'm new to xen, so my first question is: is this normal? If not, anyone know what the issue might be? Any help or pointers to a doc is appreciated.

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Re: Memory not seen in Dom0

Itamar Reis Peixoto
32 or 64 bit ?


On Sat, Nov 7, 2009 at 8:50 AM,  <[hidden email]> wrote:

> I have a machine with 64G of ram. Running CentOS 5.4 with kernel 2.6.18-164,
> I only see 32G in /proc/meminfo. All 64G show up for the stock kernel. There
> are no vm's running.
>
> I'm new to xen, so my first question is: is this normal? If not, anyone know
> what the issue might be? Any help or pointers to a doc is appreciated.
>
> _______________________________________________
> Xen-users mailing list
> [hidden email]
> http://lists.xensource.com/xen-users
>



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Itamar Reis Peixoto

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Re: Memory not seen in Dom0

Steve Thompson
In reply to this post by rah600
On Sat, 7 Nov 2009, [hidden email] wrote:

> I have a machine with 64G of ram. Running CentOS 5.4 with kernel 2.6.18-164,
> I only see 32G in /proc/meminfo. All 64G show up for the stock kernel. There
> are no vm's running.
>
> I'm new to xen, so my first question is: is this normal? If not, anyone know
> what the issue might be? Any help or pointers to a doc is appreciated.

Yes, this is normal, or rather it is a limitation of the xen kernel. Note
that it probably is not a real issue in practice, as any VM's that you
create will come from the "missing" 32GB, only overflowing into the "real"
32GB when the first 32GB is used. I have a similar system with 48GB
physical memory, and with 26 guests, each having 1 GB, the amount of
memory shown in Dom0 is around 22GB.

Steve

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Re: Memory not seen in Dom0

Pasi Kärkkäinen
In reply to this post by rah600
On Sat, Nov 07, 2009 at 05:50:45AM -0500, [hidden email] wrote:
>    I have a machine with 64G of ram. Running CentOS 5.4 with kernel
>    2.6.18-164, I only see 32G in /proc/meminfo. All 64G show up for the stock
>    kernel. There are no vm's running.
>
>    I'm new to xen, so my first question is: is this normal? If not, anyone
>    know what the issue might be? Any help or pointers to a doc is
>    appreciated.

Dom0 shouldn't see all the memory !!
You should limit dom0 memory to, say, 1 GB.

Add dom0_mem=1G option for xen.gz in /boot/grub/grub.conf and reboot.

then your dom0 should have around 1 GB in /proc/meminfo.

You can check the Xen hypervisor memory info with "xm info". It'll list
the hypervisor available memory.

And "xm list" will list how much memory each domain/guest is using.

If you have full amount (32G or 64G) of memory visible to dom0 linux
kernel, then linux will set network, memory etc defauls based on that
(huge) boot time amount of memory.. and then when you create guests,
that dom0 memory will be "ballooned" down, and later when you have
much smaller amount of memory in dom0, it'll have problems since many
internal kernel parameters were based on having 32G or 64G of memory..
and weird things start to happen.

So it's best to limit and dedicate for example 1 GB of memory to dom0
and prevent ballooning that way.

Xen always dedicated physical memory to each guest.. dom0 is also a guest.

-- Pasi



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