Xen devel - Feb 2005 - RE: Calculating real cpu usage of Xen domains correctly! (PATCH)

> OK, here is a patch which provides fine grained cpu utilization 
> reporting. Some notes:
I really can''t see the point of this patch -- it isn''t
necessary to log
every scheduling operation to record CPU usage for a domain over
multiple time scales. 
Simply exporting the cumulative CPU usage for all domains and polling it
from dom0 will enable you to do this. If you do really want to log such
data, why not use the existing trace buffer infrastructure (though
admittedly it could do with some improvements -- it should be possible
for the user space reader to block until the event buffer becomes
half-full rather than having to poll, but I don''t believe anyone has
added this functionality yet -- volunteers?).

Xen is already supposed to be doing the accounting -- though it has been
suggested there is a bug, which needs to be investigated. Its possible
it got broken when SMP guest support was added. If we do per VCPU
accounting the totals can be added to do per-domain. We should also
record the per physical CPU cumulative (non-idle domain) utilization
too.

Calculating utilization averages can be done with a simple user-space
tool. BTW: there are a couple of nice libaries for suporting simple
calculation of multi-scale rolling averages.

Ian
> - part of the code runs in the hypervisor to collect data, 
> and another 
> part of it runs in dom0 userland to process and display the data
> - the code contains vestiges of old features, and partially 
> implemented 
> new features; it is a work in progress.
> - this is the first time I''m sending out a patch to this list 
> so please 
> be gentle on me ;-)
> 
> Feedback appreciated!
> 
> Rob Gardner
> 
> 

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Ian Pratt wrote:
>>OK, here is a patch which provides fine grained cpu utilization 
>>reporting. Some notes:
> 
> 
> I really can''t see the point of this patch -- it isn''t
necessary to log
> every scheduling operation to record CPU usage for a domain over
> multiple time scales. 
Yes, obviously you are correct. I am sorry for not explaining this patch 
better. Let me start from the beginning. We developed this code to 
collect various kinds of data. One of the things it does is measure cpu 
usage. However, that is not all it does, since as you rightly say, such 
complicated code is not necessary to to just measure cpu usage. When 
several people started talking about measuring cpu usage, I just 
volunteered this code to solve the problem. So I do not propose this 
code solely to measure cpu usage.

For instance, one thing the code can do is to measure how much time 
elapses while a domain is runnable, versus how much cpu time it actually 
receives. This metric might be one simple way of expressing ''quality of
service.'' In order to do this, some scheduler activity needs to be 
monitored. Hence the more heavy handed approach. We have other ideas in 
mind that require very fine grained measurements, some have been 
implemented, some not. I hope to share some of the other metrics soon.

Additionally, this is somewhat experimental code in that it was written 
this way in order to facilitate future measurements that we may not have 
  considered yet. Can we let some people try it out and see if it 
provides this framework?

Thanks,
Rob


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