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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 data
> between the two domains?
> I guess what I'm asking is, is there a way to report the amount of cpu
> 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

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


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