RE: [Xen-devel] vif interface dropping 80% packets

Santos, Jose Renato G
Tue, 06 Feb 2007 19:40:26 -0000

You are probably using jumbo packets for native, right? What is the MTU
when you get 9.88 Gb/s? I was thinking MTU=1500  when I said
unrealistic. Not sure why you cannot get higher MTU in Xen to work. I
have not tried MTU > 1500 bytes in Xen myself  but I believe Herbert Xu
has done that successfully.

If you get larger MTU to work, keep in mind that in Xen there is an
extra data copy to transfer the packet from dom0 to guest which will
have a higher impact on performance for larger MTUs when compared to
linux.

Renato


________________________________

	From: xen-devel-bounces@lists.xensource.com
[mailto:xen-devel-bounces@lists.xensource.com] On Behalf Of Reese
Faucette
	Sent: Tuesday, February 06, 2007 10:59 AM
	To: xen-devel@lists.xensource.com
	Subject: Re: [Xen-devel] vif interface dropping 80% packets





		Most likely your dom0 CPU is saturated.. It is
unrealistic to expect full throughput of a 10 Gig NIC. I would be
surprised if even linux could keep up with a receive rate of 10 Gb/s.
Did you try the same experiment on native linux? In  my experiments, on
a 4-way 2.8 Ghz Xeon, dom0 consumes ~75% of one cpu for processing
receive packets while the guest consume ~55% of another CPU.

	With native->native or even native->dom0, I get 9.88 Gb/s with
21% utilization of receive side CPU.  It''s a nice machine and we make
nice NICs ;-)  So, it''s not unrealistic.

		It is  expected that your dual CPU system saturates at a
rate slighlty above 1 Ghz. (You are also consuming extra cycles to drop
packets)
		You could confirm that the CPU is saturating by running
xentop while running your experiments.
		You could avoid dropping packets and get a more
realistic experiment by running a TCP experiment as Ian suggested

	i have a pending note on that, TCP test is actually what I tried
first, and the perf was terrible due to the same packet loss.  more info
to come.
	-reese