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

Reese Faucette
Tue, 06 Feb 2007 22:52:48 -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.

ok, true - got me there.  TCP test with native linux sending to dom0 and
1500 MTU is 9.1GB/s and 23% of 1 CPU used on receiver.

> 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.

of course - i don''t expect there to be no impact, but the numbers i was
getting with mtu=1500 were horrid, like 70Mb/s using netperf with
TCP_SENDFILE.  a slower sender actually resulted in better throughput due to
fewer dropped packets.  (e.g. using TCP_STREAM got 220Mb/s, better but still
stinky)

update:
MTU=9000 does work, I had failed to set one of the vif interfaces - plenty
of interfaces to change ;-)
The performance is better with 9k MTU, as expected:
TCP_STREAM and TCP_SENDFILE both get ~1.8Gb/s, xentop reports 87%
utilization for dom0, so I guess that''s about all I can expect without
code
changes.

If I understand the Xen bridge network flow, though, it''s a real bummer
that

> First off, try switching to 3.0.4 and use the included 2.6.16-xen kernel.

I can try, but the issue i had before was that 2.6.16 did not have the
ethernet driver I need for the main ethernet connection:
0000:06:00.1 Ethernet controller: Intel Corporation Enterprise Southbridge
DPT LAN Copper (rev 01)
It comes with 2.6.18, but not 2.6.16, and the driver does not compile under
2.6.16, so i was stuck with 2.6.18.

before I spend a lot of time converting to 3.0.4, a couple of questions:
1) any reason to expect better perf on 3.0.4 ?
2) if yes, Will the patch "fedora-36252.patch" work on the 2.6.16
kernel
included with 3.0.4, or have conflicts crept in?

thanks!
-reese