Re: Working around xenstored performance?

["Daniel P. Berrange" <berrange@redhat.com>]

On Thu, Jul 31, 2008 at 10:41:15AM +0200, Pim van Riezen wrote:
> Good Day,
> 
> For monitoring purposes, we have a process that sends a 'xend.domains'  
> methodCall to xend at timed intervals. Our problem here is that this  
> call, if the extra parameter is not '0', is pretty slow; xenstored  
> will run with 40% cpu grinding over its database before coming up with  
> a reply, hardly something you want to do like every 15 seconds. With  
> the parameter on 0, the response is instantaneous, but lacks any  
> information beyond the list of currently running guests.
> 
> Is there a less intrusive way to just get the cpu-counter for a  
> specific guest through /proc/xen or some such? I'd also be perfectly  
> happy to fish the information out of xenstored's tdb-file, but all of  
> its records seem to be in an undocumented binary encoding so that's  
> not really helping either.

The quickest workaround is to mount /var/lib/xenstore on tmpfs at boot
time. This should reduce overhead by something like ~50%.

> Our set-up is the Fedora branch of 3.0 (but from what I can read, 3.1  
> still has this problem).

If you use libvirt, then once you have fetched a 'virDomainPtr' object,
you can query the CPU / memory utilization of the guest without it
ever hitting xenstore/xend again. The virDomainGetInfo() method is
optimized so it will either make a direct hypercall (very fast), or
if non-root use a setuid proxy to make the hypercall (fairly fast).

Regards,
Daniel
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