Re: Xen Hypervisor's memory get roughly less 700MB(even 1400MB) memory than actual grub setting when >4GB

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 12/01/2010 12:00 AM, benian wrote:
>
> Hi,
>
> I am using Xen 4.0.1-6 and fedora2.6.32.23-170.xendom0.fc12.x86_64
>
>  
>
> When I try to set the a fix value for memory on grub
>
>  
>
> For example
>
>  
>
> title Xen_Fedora (2.6.32.23-170.xendom0.fc12.x86_64)
>
>         root (hd0,0)
>
>         kernel /xen-4.0.1.gz dom0_mem=5120M
>
>  
>
> I found that when I set dom0_mem<=3GB, the system memory report by
> “free” “xm list” “xl list” and “xentop” are both approximately conform
> with the setting in grub.
>
> But when dom_mem>=4GB(roughly), the system memory is obviously
> different from the grub.
>

Yes, that's expected.  The BIOS reserves a chunk of address space
between 3-4G to map PCI devices into.  Since dom0 needs to get access to
those devices, we free the Xen-supplied memory in that range back to the
hypervisor.  You should see that reflected in the free memory output of
"xl info".

In recent versions of xen/stable-2.6.32.x you can balloon dom0 back up
again to its full size with "xl mem-set 0 <size>", but I don't think
that's implemented in the version you're using.

> “xm list”
>
> Name                 ID   Mem VCPUs      State   Time(s)
>
> Domain-0              0 * 4384*     8     r-----    136.8
>
>  
>
>  
>
>  
>
> “xl list”
>
> Name                 ID   Mem VCPUs       State        Time(s)
>
> Domain-0              0  *3648*     8        r--    139.8
>

That's just strange: xl and xm should show the same results here.

    J