RE: [Xen-users] xennet windows pv performance

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> 
> Was this Linux domU HVM (using PV drivers) or PV?

I knew I'd forget something :)

It's a PV linux domain.

> In this box you have 4 physical cpus.. windows vm has 2 vcpus so your dom0
> has 2 pcpus dedicated only for it..

Well... there are 10 other DomU's on the machine too, but none of them work 
particularly hard (which is why virtualisation is such a good thing!).

> > On another machine with 1 x dual core 1.8Ghz AMD cpu, testing as above I
> > get:
> > Windows (vcpus = 2) - ~65MBits/sec - ~100% cpu utilization (eg 100% on
> > both cpu's)
> > Linux - can't test as I don't have any Linux DomU's set up presently.
> >
> > I don't think there is enough difference in raw CPU power to account for
> > the 7x performance difference between the two physical machines... it
> > must be something else.
> >
> 
> Hmm.. do these CPU's have different sizes of caches? Then again it can't
> cause that big difference..

Probably yes they do.

> Could it be some xen scheduling issue? You're using 2 vcpus for windows,
> and in this second box you only have 2 pcpus.. and they're both 100%
> used so no dedicated pcpus for dom0..

There is probably something in that. The thing that is bugging me is the amount 
of CPU required... I am implementing checksum offload right now which might cut 
some of it down.

I wonder if HVM supports page flipping as opposed to page copying. I don't know 
much about page flipping but if it is what I think it is then we could actually 
implement zero copy.

James


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