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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 1 Dec 2010, Jeremy Fitzhardinge wrote:
> 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
> > âfreeâ âxm listâ âxl listâ and âxentopâ 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.
> 
> > âxm listâ
> >
> > Name                 ID   Mem VCPUs      State   Time(s)
> >
> > Domain-0              0 * 4384*     8     r-----    136.8
> >
> >  
> >
> >  
> >
> >  
> >
> > âxl listâ
> >
> > 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.
 
I am sure that libxl is doing the right thing; my guess would be that
the difference is due to xend printing max_pages instead of tot_pages.

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