Xen devel - Jun 2007 - ballon driver interaction with native page allocator.

hi all

if Balloon driver is not present in the guest OS then how xen control
the memory of the that Guest OS

or there is some other mechanism also with which the guest OS page
allocator can inftate or deflate the balloon.

thanks
TEJ

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> if Balloon driver is not present in the guest OS then how xen control
> the memory of the that Guest OS
It can''t; Xen depends on guest OS co-operation in order to resize the
guest''s
physical memory footprint.
> or there is some other mechanism also with which the guest OS page
> allocator can inftate or deflate the balloon.
The balloon can be controlled from outside Xen using xm mem-set.  It can also 
be controlled within the guest by echoing something into a special file.  If 
there''s no balloon driver, there''s no way to change the memory
footprint (as
of 3.1 the balloon driver can work in HVM, though).

Other hypervisors / virtual machine software is able to page guest memory out 
to disk in order to vary the machine RAM allocation used by each guest; Xen 
doesn''t do this.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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On 6/8/07, Mark Williamson <mark.williamson@cl.cam.ac.uk>
wrote:
> > if Balloon driver is not present in the guest OS then how xen control
> > the memory of the that Guest OS
>
> It can''t; Xen depends on guest OS co-operation in order to resize
the guest''s
> physical memory footprint.
>
ok it means if during domU creation if config file says memory is 64MB
then it can''t be resize.
> > or there is some other mechanism also with which the guest OS page
> > allocator can inftate or deflate the balloon.
>
> The balloon can be controlled from outside Xen using xm mem-set.  It can
also
outside xen means guest OS can use xm-mem set tool to inflate and
deflate the ballon.
> be controlled within the guest by echoing something into a special file. 
If
> there''s no balloon driver, there''s no way to change the
memory footprint (as
> of 3.1 the balloon driver can work in HVM, though).
>
> Other hypervisors / virtual machine software is able to page guest memory
out
> to disk in order to vary the machine RAM allocation used by each guest; Xen
> doesn''t do this.
>
> Cheers,
> Mark
>
I think so many control for the ballon driver may cause the conflit if
xen and guest OS is simultaneously trying to resize the ballon.
> --
> Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
> Mark: To answer a question with a question: What use is a skateboard?
> Dave: Skateboards have wheels.
> Mark: My wheel has a wheel!
>
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> > It can''t; Xen depends on guest OS co-operation in order to
resize the
> > guest''s physical memory footprint.
>
> ok it means if during domU creation if config file says memory is 64MB
> then it can''t be resize.
As long as the guest has the balloon driver, then you can change the memory 
allocation using the command line tools in dom0 (or the domain can 
voluntarily change its allocation).
> > > or there is some other mechanism also with which the guest OS
page
> > > allocator can inftate or deflate the balloon.
> >
> > The balloon can be controlled from outside Xen using xm mem-set.  It
can
> > also
>
> outside xen means guest OS can use xm-mem set tool to inflate and
> deflate the ballon.
xm mem-set is run in dom0 and sends messages to the guest, telling it to 
change its memory footprint (and by how much).
> I think so many control for the ballon driver may cause the conflit if
> xen and guest OS is simultaneously trying to resize the ballon.
Correct - it does cause problems.  It won''t crash, or anything nasty
like
that.  But if the guest does a voluntary resize on its balloon then the dom0 
tools won''t know about it, and may get confused about the current
memory size
of the domain.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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