Sending packet at fixed rate really bad performance

[Marco Tizzoni <elibus@gmail.com>]

Hi all,
I'm doing some tests about i/o performance and cpu overhead in dom0.
I've coded a small tool that can sends UDP packets at fixed rate or
flood. While sending packet at fixed rate I'm experiencing performance
problem, related to how Xen  handle signal, I guess.

On my laptop (Core2 duo P8700), with no Xen running, I can send
packets at a rate of 20k, 40k, 60k, 80k/s achieving an actual packet
rate per second pretty near to the requested rate.

On Xen (dom0 with no guest running and no load - Athlon 64 dual core
4000+ ), the performance are really bad.
Here what happen:

Requested | Actual
10             | 10.03
100           | 100.05
200           | 200.04
500           | 250.04
1000         | 250.18
10000        | 247.49
20000        | 250.26
40000        | 250.24

It seems impossibile to go over 250 packets/second, but in flood
configuration (send packet as fast as possible) the actual rate is
quite higher, 39k packets/second about.

The main difference is in the code. Flood loop as been implemented like this:
------------------
for (;;)
  send_pkt();
------------------

while the fixed rate use setitimer to wake up send_pkt() when needed.

Any idea of why this happen?

bye,
Marco