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Re: [Xen-users] CPU Load Averages Significance

On 30/06/2012 22:45, Luke S. Crawford wrote:
On Sat, Jun 30, 2012 at 10:32:05PM +0100, Jonathan Tripathy wrote:
Hi Everyone,

When running a Xen system with only PV guests, what is the significance
of the load averages figure as reported by the uptime command, both in
the Dom0 and each DomU?

Specifically, can the CPU usage of one DomU affect the reading of the
load average on the other?
If it is the CPU, it's usually pretty clear.   Look for %steal or %st
in top;  if that is non-zero, that means your guest is not getting
all the CPU it could use because it's waiting on another guest
that is using that CPU.

But, usually?  the way that guests interfere with oneanother is
through I/O contention, not CPU contention.  Look at the %wa or iowait
values;   that's the percentage of time that your cpu is idle because
it's waiting for I/O.

It's harder, though, to determine if it's you or your neighbor
causing the problem when it's I/O.   The usual troubleshooting
steps should be taken to make sure you aren't swapping[1]

If you know it's not your domain (or if you are only the dom0
admin and don't have access to the guests)  xentop is moderately
useful (though, it only has total blocks read and written, not
blocks read and written in the last X seconds.)  -  iostat[2]
can be useful for that.

Hi Luke,

Thanks for the tips. I love your (or your colleague's) book btw!

Yes, I am a Dom0 admin with customer DomUs hosted on the server. However, we also host our own DomUs on the same host and are experiencing the following:

Sometimes, on a DomU with a single vCPU, the "Load Average" appears just over 1, even though CPU idle is nearly 100%. It is this phenomenon that I am trying to understand. We aren't experiencing any slow-downs (at least that we can notice when we use the DomU), however I worry that sometimes the LA spikes to 1...

Load Averages are generally very low in the Dom0 - usually 0.3 - 0.4 with the Dom0 having access to all cores in the machine (4 physical + 4 HT)


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