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RE: [Xen-devel] system load

I'm only wondering from a monitoring point of view. If I have a few
physical servers it will be useful to get an accurate picture of how
hard each is working with a view to balancing the virtual servers
sensibly across them.


> -----Original Message-----
> From: Ian Pratt [mailto:Ian.Pratt@xxxxxxxxxxxx]
> Sent: Friday, 10 September 2004 17:32
> To: James Harper
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxxx; Ian.Pratt@xxxxxxxxxxxx
> Subject: Re: [Xen-devel] system load
> > Where does system load live for xen related tasks?
> > If a packet is received by dom0, then bridged into dom1, how is the
> > system load proportioned between domains for the actual transfer of
> > between the two domains?
> >
> > I guess what I'm asking is, is there a way to report the amount of
> > time is spent in xen itself and the corresponding system load?
> Very little time will be spent in Xen itself, but what there is
> will appear to be accounted to the domain that was running at the
> time.
> If you have a domain that is doing a lot of I/O, this will be
> generating work for your driver domains (usually dom0). We've
> gone to every effort to do as little work in the driver domain as
> possible, but it does take some CPU to execute the hardware
> device driver, the bridging/firewalling code, and the 'backend'
> virtual driver.
> It's hard to account and apportion exactly how much time the
> driver domain spends working on behalf of each of the other
> domains. If you're worried about a domain hogging too much of
> this resource then use can use tools like Linux's 'tc' to rate
> limit the amount of IO a particularly domain is allowed to do.
> > It would be nice to be able to see the load on the physical
> > server as a whole for monitoring purposes, or is it sufficient
> > to simply sum up the load on all the domains?
> Xen/xend already export the information you need to sum up load
> over each CPU, and hence see the total system load.
> As Keir says, the load figures reported internally within each
> domain can be confused due to pre-emption. One trick we could do
> would be to hack Linux to create a dummy process to which we
> account all time when the domain isn't running. This would enable
> the load figures to add up, but I'm pretty sure that this is not
> what many Xen users want: In a 'virtual dedicated server'
> environment the owner of the physical server doesn't want to giveq
> customers too much information about what else is going on on the
> server...
> Ian

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