Memory limits?

Memory limits?

[Rene Auberger]

Hi all,

I am relatively new to Xen and have a question regarding memory: I was not able to find out what the limit of memory is I can assign to a guest system. How much memory can I assign to each guest:
* not more than physical memory in the machine?
* not more than physical memory minus memory used by dom0 and other doms?
* not more than 4GB?
* something totally different?

By the way: I am using SuSE 9.2 with the Xen packages from http://www.suse.de/~garloff/linux/xen/ - don't know if that has any influence on which answer applies to my question.

Rene
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Re: Memory limits?

[Mark Williamson]

> I am relatively new to Xen and have a question regarding memory: I was not
> able to find out what the limit of memory is I can assign to a guest
> system. How much memory can I assign to each guest:

> * not more than physical memory in the machine?

Yes, Xen doesn't do swapping.  If swapping is needed, the guests should be 
configured to do it themselves.

> * not more than physical memory minus memory used by dom0 and other doms?

Correct.  Bear in mind Xen uses some memory too.

> * not more than 4GB?

Correct, on x86 we don't support the PAE36 mode required to support more than 
4GB.  This is not an insurmountable technical limitation, it's just that it's 
not deemed worth the development time.  On other systems (e.g. x86_64), 
larger memories does not have the same problems.

Cheers,
Mark

> * something totally different?
>
> By the way: I am using SuSE 9.2 with the Xen packages from
> http://www.suse.de/~garloff/linux/xen/ - don't know if that has any
> influence on which answer applies to my question.
>
> Rene
> ______________________________________________________________
> Verschicken Sie romantische, coole und witzige Bilder per SMS!
> Jetzt bei WEB.DE FreeMail: http://f.web.de/?mc=021193
>
>
>
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> SF email is sponsored by - The IT Product Guide
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> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/xen-devel


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RE: Memory limits?

[Puthiyaparambil, Aravindh]

The 4GB limitation mentioned here is for guests. What about the host?
Does the same limitation apply to the host? If yes... Could you tell me
how much effort is involved in enabling PAE mode in 32-bit Xen? What
components would need to be modified?

Aravindh

-----Original Message-----
From: xen-devel-admin@lists.sourceforge.net
[mailto:xen-devel-admin@lists.sourceforge.net] On Behalf Of Mark
Williamson
Sent: Monday, February 21, 2005 5:57 PM
To: xen-devel@lists.sourceforge.net
Cc: Rene Auberger
Subject: Re: [Xen-devel] Memory limits?

> I am relatively new to Xen and have a question regarding memory: I was
not
> able to find out what the limit of memory is I can assign to a guest
> system. How much memory can I assign to each guest:

> * not more than physical memory in the machine?

Yes, Xen doesn't do swapping.  If swapping is needed, the guests should
be 
configured to do it themselves.

> * not more than physical memory minus memory used by dom0 and other
doms?

Correct.  Bear in mind Xen uses some memory too.

> * not more than 4GB?

Correct, on x86 we don't support the PAE36 mode required to support more
than 
4GB.  This is not an insurmountable technical limitation, it's just that
it's 
not deemed worth the development time.  On other systems (e.g. x86_64), 
larger memories does not have the same problems.

Cheers,
Mark

> * something totally different?
>
> By the way: I am using SuSE 9.2 with the Xen packages from
> http://www.suse.de/~garloff/linux/xen/ - don't know if that has any
> influence on which answer applies to my question.
>
> Rene
> ______________________________________________________________
> Verschicken Sie romantische, coole und witzige Bilder per SMS!
> Jetzt bei WEB.DE FreeMail: http://f.web.de/?mc=021193
>
>
>
> -------------------------------------------------------
> SF email is sponsored by - The IT Product Guide
> Read honest & candid reviews on hundreds of IT Products from real
users.
> Discover which products truly live up to the hype. Start reading now.
> http://ads.osdn.com/?ad_id=6595&alloc_id=14396&op=click
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/xen-devel


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RE: Memory limits?

[M.A. Williamson]

> The 4GB limitation mentioned here is for guests. What about the host?
> Does the same limitation apply to the host?

Yes. Which is to say, memory over and above 4Gig won't be usable (although 
the system would obviously still boot).

> If yes... Could you tell me
> how much effort is involved in enabling PAE mode in 32-bit Xen?

I think the concensus was that it wasn't hugely difficult (relative to 
other tasks in the VM system, i.e. would probably still require an 
experienced kernel hacker) but that it would be a bit too time consuming to 
justify the effort required.

> What
> components would need to be modified?

Take a look under xen/arch/x86/x86/mm.c. You'd need to update this to deal 
with the i686 PAE case, as well as the existing i386 and x86_64 page table 
layouts. I heard there was some commonality between i686 PAE and x86_64 
page tables but I don't know how much.

You'd also need to poke around in the Linux memory management code. You'll 
have to pull in PAE support from x86 Linux, modifying the files to Xen-ify 
them.

You might need to modify some of the userspace tools to understand PAE and 
probably some other bits and bobs within Xen.

It's worth emphasising again that Xen *will* support memory > 4gig on the 
upcoming x86_64 port (as well as the IA64 port and other suitable ports).

Cheers,
Mark

> Aravindh
> 
> -----Original Message-----
> From: xen-devel-admin@lists.sourceforge.net
> [mailto:xen-devel-admin@lists.sourceforge.net] On Behalf Of Mark
> Williamson
> Sent: Monday, February 21, 2005 5:57 PM
> To: xen-devel@lists.sourceforge.net
> Cc: Rene Auberger
> Subject: Re: [Xen-devel] Memory limits?
> 
> > I am relatively new to Xen and have a question regarding memory: I was
> not
> > able to find out what the limit of memory is I can assign to a guest
> > system. How much memory can I assign to each guest:
> 
> > * not more than physical memory in the machine?
> 
> Yes, Xen doesn't do swapping.  If swapping is needed, the guests should
> be 
> configured to do it themselves.
> 
> > * not more than physical memory minus memory used by dom0 and other
> doms?
> 
> Correct.  Bear in mind Xen uses some memory too.
> 
> > * not more than 4GB?
> 
> Correct, on x86 we don't support the PAE36 mode required to support more
> than 
> 4GB.  This is not an insurmountable technical limitation, it's just that
> it's 
> not deemed worth the development time.  On other systems (e.g. x86_64), 
> larger memories does not have the same problems.
> 
> Cheers,
> Mark
> 
> > * something totally different?
> >
> > By the way: I am using SuSE 9.2 with the Xen packages from
> > http://www.suse.de/~garloff/linux/xen/ - don't know if that has any
> > influence on which answer applies to my question.
> >
> > Rene
> > ______________________________________________________________
> > Verschicken Sie romantische, coole und witzige Bilder per SMS!
> > Jetzt bei WEB.DE FreeMail: http://f.web.de/?mc=021193
> >
> >
> >
> > -------------------------------------------------------
> > SF email is sponsored by - The IT Product Guide
> > Read honest & candid reviews on hundreds of IT Products from real
> users.
> > Discover which products truly live up to the hype. Start reading now.
> > http://ads.osdn.com/?ad_id=6595&alloc_id=14396&op=click
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@lists.sourceforge.net
> > https://lists.sourceforge.net/lists/listinfo/xen-devel
> 
> 
> -------------------------------------------------------
> SF email is sponsored by - The IT Product Guide
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> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/xen-devel
> 


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Re: Memory limits?

[Rene Auberger]

>Mark Williamson <maw48@cl.cam.ac.uk> schrieb am 22.02.05 00:04:10:
>
>>How much memory can I assign to each guest:
>>* not more than physical memory in the machine?
>Yes, Xen doesn't do swapping.  If swapping is needed, the guests should be 
>configured to do it themselves.
>
>> * not more than physical memory minus memory used by dom0 and other doms?
>Correct.  Bear in mind Xen uses some memory too.

Are there any plans on implementing swapping? It's really comfortable to start multiple guests with more memory than actually installed in the machine to have it available in case they need it (without manually tweaking something during the systems already run). That's one of the features I really like in VMWare ESX.

Rene

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Re: Memory limits?

[Robin Green]

On Tue, 22 Feb 2005, Rene Auberger wrote:

>> Mark Williamson <maw48@cl.cam.ac.uk> schrieb am 22.02.05 00:04:10:
>>
>>> How much memory can I assign to each guest:
>>> * not more than physical memory in the machine?
>> Yes, Xen doesn't do swapping.  If swapping is needed, the guests should be
>> configured to do it themselves.
>>
>>> * not more than physical memory minus memory used by dom0 and other doms?
>> Correct.  Bear in mind Xen uses some memory too.
>
> Are there any plans on implementing swapping? It's really comfortable to start multiple guests with more memory than actually installed in the machine to have it available in case they need it (without manually tweaking something during the systems already run). That's one of the features I really like in VMWare ESX.

On the other hand, it's great that you don't have swapping, because that 
means that Xen hosting providers won't be able to lie to you about the 
amount of physical RAM they are selling to you.

Please, don't implement swapping! ;-)

-- 
Robin


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Re: Memory limits?

[Mark Williamson]

> Are there any plans on implementing swapping?

I got a vague impression this was being thought about but I don't know.  Is 
anyone out there looking at this at the moment?

> It's really comfortable to 
> start multiple guests with more memory than actually installed in the
> machine to have it available in case they need it (without manually
> tweaking something during the systems already run). That's one of the
> features I really like in VMWare ESX.

The main thing that swapping in the VMM buys you is the ability for VMs to 
grow and shrink in physical memory footprint as different VMs increase / 
decrease their workload.  An (arguably preferable) way to implement this in 
Xen would be to use the balloon driver to automatically adjust memory 
footprints.  We know roughly what this should look like - volunteers would be 
good ;-)

Cheers,
Mark


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