Xen users - Jan 2006 - confusion over memory, maxmem, balloon, etc

My goal for my xen setup is to allow the dom0 and domU to "share" 
memory, meaning that occasionally one of the domains will hog ram for a 
short time and then relinquish it so it can be used for other domains.  
I get the impression that xen can handle this, but I''m having a hell of
a time getting straight information.

What does the "memory" parameter in a domU config file actually mean?
Is this an amount of physical ram that is permanently reserved for the 
domU?  Is this a minimum setting?  Maximum setting?  Is this the same 
parameter as dom0_mem param given to xen?

What exactly is the role of the balloon driver?  Is this always present 
in the xen linux kernel or do I have to do something to turn it on?  Is 
it something I need to control?  Does it behave the same in dom0 and 
domU kernels?

What is the dom0_min_mem parameter?  How is it set?  Do domU''s have
this
too?

When I type "xm list --long" I see a parameter called
"maxmem".  What
does that mean?  How is it set?  I note that "xm list" will show
maxmem
as say 512, but "xm top" will show "no limit"???


I apologize for the barrage of questions, but the fragments of 
information I have been finding just leave me more confused and frustrated.

Thanks,
Dan.

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I won''t answer all of your questions, but I''ll tell you this.

The memory parameter establishes the amount of member that domU believes
it has. From what I can tell, once you boot domU it cannot use more
memory then this (because this is all that the domU kernel knows about).

Let''s say you set this to 512Mb so your domU kernel is using 512Mb.
Suppose you want 128Mb back. Using the “xm” command in dom0 (in 2.0.7 it
is the “balloon” subcommand and in 3.0 I believe it is “maxmem” [I''m
not
100% on this because I don''t have the documentation handy, if you
attempt to use “xm balloon” in 3.0 it will tell you the new command
name!]) set the domain''s memory to 384Mb (512-128). What happens is
that
dom0 tells the domU''s kernel to reduce its memory usage. It does this
my
kmem alloc''ing 128Mb of memory (so the domU kernel thinks it is using
this memory) and then handing the physical memory back to the
hypervisor. So as far as the linux kernel is concerned in domU, there is
this 128Mb “balloon” of memory that it cannot touch. In fact it isn''t
there [and I suspect attempting to touch it will cause a fault].

When you give back the memory to domU, the balloon driver requests the
memory from the hypervisor to fill in the balloon and then kmem free''s
it. This makes it available again to domU.

This all works the same way in dom0.

So you can hand memory from dom0 to domU and back again...

Hope this helps.

			-Jeff

P.S. I am not a Xen developer, just a user [of 2.0.7 at the moment,
playing with 3.0] who has read some source code :-)

-- 
============================================================================Jeffrey
I. Schiller
MIT Network Manager
Information Services and Technology
Massachusetts Institute of Technology
77 Massachusetts Avenue  Room W92-190
Cambridge, MA 02139-4307
617.253.0161 - Voice
jis@mit.edu
===========================================================================
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Thank you Jeffrey, that was a huge help.  I have some device driver work 
under my belt so that all made perfect sense.

I assume that the balloons are entirely under manual control at the 
moment, ie, you can''t get domains to pump up their balloons when
nothing
is going on?

Cheers,
Dan.

Jeffrey I. Schiller wrote:
>I won''t answer all of your questions, but I''ll tell you
this.
>
>The memory parameter establishes the amount of member that domU believes
>it has. From what I can tell, once you boot domU it cannot use more
>memory then this (because this is all that the domU kernel knows about).
>
>Let''s say you set this to 512Mb so your domU kernel is using 512Mb.
>Suppose you want 128Mb back. Using the “xm” command in dom0 (in 2.0.7 it
>is the “balloon” subcommand and in 3.0 I believe it is “maxmem”
[I''m not
>100% on this because I don''t have the documentation handy, if you
>attempt to use “xm balloon” in 3.0 it will tell you the new command
>name!]) set the domain''s memory to 384Mb (512-128). What happens is
that
>dom0 tells the domU''s kernel to reduce its memory usage. It does
this my
>kmem alloc''ing 128Mb of memory (so the domU kernel thinks it is
using
>this memory) and then handing the physical memory back to the
>hypervisor. So as far as the linux kernel is concerned in domU, there is
>this 128Mb “balloon” of memory that it cannot touch. In fact it
isn''t
>there [and I suspect attempting to touch it will cause a fault].
>
>When you give back the memory to domU, the balloon driver requests the
>memory from the hypervisor to fill in the balloon and then kmem
free''s
>it. This makes it available again to domU.
>
>This all works the same way in dom0.
>
>So you can hand memory from dom0 to domU and back again...
>
>Hope this helps.
>
>			-Jeff
>
>P.S. I am not a Xen developer, just a user [of 2.0.7 at the moment,
>playing with 3.0] who has read some source code :-)
>
>  
>


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Daniel Goertzen wrote:
> I assume that the balloons are entirely under manual control at the
> moment, ie, you can''t get domains to pump up their balloons when
nothing
> is going on?
Don''t believe so. At least I don''t recall seeing any code that
did that,
but I wasn''t looking for it either.

			-Jeff
-- 
============================================================================Jeffrey
I. Schiller
MIT Network Manager
Information Services and Technology
Massachusetts Institute of Technology
77 Massachusetts Avenue  Room W92-190
Cambridge, MA 02139-4307
617.253.0161 - Voice
jis@mit.edu
===========================================================================
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