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

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

[benian]

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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.

And at the same time, the value reported by “free” and “xm list” , is also
obviously different from “xl list” and “xentop”



For example, dom0_mem=5120, the reports are:

“free”

                   total       used       free     shared    buffers
cached

Mem:       *4489216*    1911468    2577748          0       7976     104940



“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



 “xentop”

Mem: 8378584k total, 3850452k used, 4528132k free    CPUs: 8 @ 2660MHz

NAME  STATE   CPU(sec) CPU(%)  MEM(k) MEM(%)  MAXMEM(k) MAXMEM(%) VCPUS

Domain-0 -----r        144    0.9    *3735552*   44.6   no limit       n/a
8    0





Both of them are less 700MB (even 1400 MB) than the setting in grub.



 Does anyone have idea about

1.      Why system memory short when dom0_mem >=4GB(roughly)?

2.      why the reported value from “free” and ”xm list “ is different from
“xl list” and ”xentop”





any idea is appreciated .

Thanks very much!



Ben

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

[Jeremy Fitzhardinge]

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

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

[Stefano Stabellini]

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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
> > “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.
 
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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