Direct I/O performance: direct interrupts to running domU?

[John Byrne <john.l.byrne@hp.com>]

Hi,

I'm testing performance of a NIC assigned directly to a domU. (xen-3.0.3 
x86_64)

After your help straightening out the problems with my test environment, 
my netperf throughput numbers were  close to base Linux (generally with 
0.5%); however, when I did the netperf transaction tests, the numbers on 
Xen (either dom0 and domU) were 10-20% lower than the base Linux 
depending on the test parameters. Since this linear transaction test 
would be very sensitive to the latency/path-length involved in the 
interrupt delivery, I'm making the assumption, for the moment, that this 
is the problem without cranking up xenoprof/oprofile to prove it.

One of the questions that came up in a discussion was whether it might 
be technically possible to have interrupts delivered directly to a 
running domain without the hypervisor overhead. This would presumably 
involve some additional overhead at the time the hypervisor switched 
domains, but it might be a worthwhile trade-off.

Thanks,

John Byrne