Re: [Xen-devel] Advanced questions

[Rolf Neugebauer <rolf.neugebauer@xxxxxxxxx>]

On Tue, 2003-11-11 at 15:25, Tvrtko A. Uršulin wrote:
> Hello all,
> 
> As I have reported earlier, everything works for me now!
> 
> Therefore :), it is time for some more advanced questions:
> 
> Is it possible to have more IP addresses per virtual domain?
> Can a virtual domain reconfigure it's IP address?
> 
> What kind of CPU time control is possible (or planned)? To be more precise, 
> how can one stop one domain to hog the cpu? (some kind of minimum guaranteded 
> CPU time setting?) Is there a description for xi_sched* helpers?

the current scheduler is a weighted proportional fair scheduler. The
scheduler is called BVT (Borrowed Virtual Time) and is described in this
paper: http://citeseer.nj.nec.com/duda99borrowedvirtualtime.html.

by default all domains have the same weight and should therefore share
the CPU equally if they all need the CPU. xi_sched_domain allows you to
change the weight of a domain. the first parameter is the domid, the
second is the _inverse_ of the weight. For an explanation of the other
parameters I'd suggest to read the paper. Essentially, they control the
wake-up latency of a blocking domain. 

xi_sched_global sets a global parameter for the scheduler called context
switch allowance. The default value is sensible and you shouldn't need
to change it.

Be aware that the xi_sched_* and the corresponding hypercalls do not do
any range checking and allow you to shoot yourself in the foot. I guess
this should be fixed :)

To answer your other questions more directly: if you don't assign
weights in a very skewed way one domain should not be able to hog the
CPU. You can "guarantee" shares of CPU if you control the weights and
the sum of weights carefully. there are vague plans to also provide a
scheduler which gives absolute guarantees, rather than weighted
proportional fairness. But that is not yet very high on the list but
should be reasonably straight forward.


> What happens if user inside virtual domain issues a reboot? (nothing eg. 
> stuck 
> - if my testing is correct). IMHO, it would be good if user could reboot.
>
> If I understand the documentation correctly, the memory size given with 
> xi_create is minimum guaranteed? Guest can grow beyond that, and under that 
> limit? There is a mention about absolute maximum limit. How is this set?

My understanding is that all memory a domains gets is guaranteed. a
domain might _voluntarily_ relinguish some of its memory (e.g., using
the balloon driver) but will get it back when it asks for it. This also
means that xen can't allocate the memory it gets back from a domain
permanently and guaranteed to another domain. 

Rolf



> 
> Many thanks for all the help!
> 
> Best regards,
> Tvrtko A. Ursulin



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