Calculating real cpu usage of Xen domains correctly!

[Jerone Young <jyoung5@us.ibm.com>]

Hi all,
	With the new vm-tools we are trying to get top like capabilities going
correctly. Currently we have a program vm-list that has some of this
capability but is dependent on the cpu time given by libxc calls
(xc_get_dom_info & xc_domain_get_cpu_usage). These two functions give
you how much time (in nanoseconds, why is this not documented) the
domain has been actively used. Approaches:

1) CPU time % measured per domain
(The differential of cpu usage time / some differential time) x 100
 new_cpu_time-old_cpu_time           new_time-old_time 
 
This provides us with the % time the domain had activity ...but does not
give us absolute real CPU usage. Another problem here is sometimes you
will get percentages like %103 usage, because the cpu usage returned by
theses functions looks to be measured slightly over a second.

So this one leads to the next

2) Relative usage .. How much of the total of the cpu times is going
toward a particular domain.
(differential of domain cpu time / total differential of all domains cpu
times)
  new_cpu_time-old_cpu_time   total_new_cpu_times-total_old_cpu_times

This one gives you a good idea of what percentage a domain took of the
total active cpu usage time.


Nnot sure this the totally correct way of going about doing this.
Anthony Ligouri and I rea bit perplexed if there is a better way. Does
anyone know if there is a better way and is there a way to get the real
CPU usage.

 
-- 
Jerone Young
Open Virtualization
IBM Linux Technology Center
jyoung5@us.ibm.com
512-838-1157 (T/L: 678-1157)



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