Xen users - May 2006 - detecting paging inside of VMs

This may be more of a developer question, but I''ll ask here first.

I''d like to be able to detect when a VM is pressed for memory and as a
result is paging lots of pages out to disk.  Can anyone famliar with the xen
internals tell me if it is feasible to detect from inside the xen hypervisor
when a VM is doing this?

I''m willing to do some hacking, but I''m not even sure if that
kind of
information can be obtained in the hypervisor.  I''d rather not modify
the
user level xen kernels if possible.

Thanks for any info!
-Tim


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The obvious way to do this would be to detect from inside each domain
that "I''m swapping a lot", and send a message (via networking
most
obviously) to some monitor software within Dom0 that can take some
suitable action, rather than push this task into the less natural place
of the hypervisor. Since this is not something that needs millisecond
resolution (or you''d end up thrashing a lot), I think this is a
workable
solution. The hard part of course is to determine what is "a lot" when
it comes to swapping. 
 
It should be possible to get this info without modifying the kernel,
just adding the suitable service software - which would be no different
from adding some other software to the guest to perform whatever task it
is supposed to perform.
 
For HVM it would be possible for the hypervisor to track page-fault -
I''m not sure if all page-faults are sent to Hypervisor for Para-virtual
guests (I don''t know enough about para-virtual to answer that), but the
problem is that page-faults can happen for many different reasons, not
just swapping - pagefaults are for example used to indicate that a page
has been written to and need to be copied when using a "Copy on Write"
variety of the fork() system call - and if some system is doing a lot of
fork calls, doesn''t mean that it needs more memory... So unless the
hypervisor also understands the REASON for a page-fault, it can''t give
any good indication of whether the guest needs more memory... 
 
--
Mats


________________________________

	From: xen-users-bounces@lists.xensource.com
[mailto:xen-users-bounces@lists.xensource.com] On Behalf Of Tim Wood
	Sent: 19 May 2006 20:25
	To: Xen Users; twwood@gmail.com
	Subject: [Xen-users] detecting paging inside of VMs
	
	
	This may be more of a developer question, but I''ll ask here
first.
	
	I''d like to be able to detect when a VM is pressed for memory
and as a result is paging lots of pages out to disk.  Can anyone famliar
with the xen internals tell me if it is feasible to detect from inside
the xen hypervisor when a VM is doing this?  
	
	I''m willing to do some hacking, but I''m not even sure if that
kind of information can be obtained in the hypervisor.  I''d rather not
modify the user level xen kernels if possible.
	
	Thanks for any info!
	-Tim 
	



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