Re: Vanilla Xen total CPU %

[George Dunlap <dunlapg@xxxxxxxxx>]

On Thu, Jun 4, 2020 at 8:34 PM Nick Calvert <nick.calvert@xxxxxxxxx> wrote:

Hi everyone,

I am interested in calculating the approximate total CPU runtime % of
a vanilla Xen project host (without any of the bells and whistles of
XCP-ng or Xen Server). What I have at my disposal is Ubuntu, Xen and
the xl tool stack.

Just to be clear: What you mean, is you want to add up the time all VMs are running?  (i.e., if you have one VM at 150%, another at 25%, and another at 25%, the total would be 200%?)

 

I have been experimenting with writing a parser for xentop output in
batch mode, this is a fairly easy task and I can see other attempts at
parsers across dead and dying github projects... my issue around this
is the precise meaning of the 'CPU/sec' metric given by xentop and how
I could use it to infer a total cpu time.

If you don't mind me asking, what are you (and the projects you mention) using this information for?

Xen is open-source project, so rather than having dozens of projects trying to work around the fact that the core tools don't tell them what they want to know, it seems like it would be better to either modify xentop to tell you what you want to know, or add a new tool to do the same thing.

 

The docs for xentop say "CPU(sec) CPU time which the guest OS has
consumed(cumulated)".

My confusion is around how CPU 'seconds' actually relat to vCPUs, real
cores etc in this context. I can also see a couple of attempts at
figuring out total CPU %, but none look quite right.

If I were able to derive both the CPU seconds for each domu in an
interval, the aggregate CPU seconds in this interval and both total
vCPUs and physical cores what would be the correct formula for
approximating a total CPU runtime %?

Also if I am missing a trick and there is an easier way of calculating
this I would be extremely happy to hear it, as simple is nice :)

I think if I were writing a program, I'd probably use libxl to get the raw data, rather than trying to parse xentop.  libxl_list_domain() will return a list of libxl_dominfo, which has a field "cpu_time", which is (I believe) the number of nanoseconds of cpu time that domain has consumed ever in its lifetime.

So what you'd do is take a timestamp (t1) call libxl_list_domain(), and go through the resulting list, adding up `cpu_time` (c1).  Then at some point later, take a timestamp (t2) and do another sum (c2).  Your total host utilization between t1 and t2 would then be (c2 - c1) / (t2 - t1).

If you're using 4.13 at least, you could use the golang bindings instead, if you didn't want to use C. 

 -George