Re: [Xen-users] CPU Load Averages Significance

["Luke S. Crawford" <lsc@xxxxxxxxx>]

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.



[1]http://wiki.prgmr.com/mediawiki/index.php/Do_I_have_enough_ram
[2]http://wiki.prgmr.com/mediawiki/index.php/Who_is_using_all_my_I/O%3F



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