Re: [Xen-users] Very slow domU network performance

[Winston Chang <winston@xxxxxxxxxx>]

On Apr 4, 2006, at 1:59 PM, Stephen C. Tweedie wrote:

> > The packet loss is as follows:
> > domU --> domU  0% (using 127.0.0.1)
> > domU --> domU  0% (using domU eth0 IP address)
> > dom0 --> domU  ~100% (only 7 of 38464 made it!)
> Yow.
>
> There have been a number of weird checksum problems identified in the
> past with Xen's networking; a checkin was just made a day or two ago
> which cleans up the checksum handling in a way which may well help  
> here.
> We'll have to see whether an updated dom0/domU kernel improves things
> much.
I ran the test with the latest xen-unstable build.  The results are  
the same.
When I ran 'xm sched-sedf 0 0 0 0 1 1' to prevent  domU CPU  
starvation, network performance was good.  The numbers in this case  
are the same as in my other message where I detail the results using  
the week-old xen build -- it could handle 90Mb/s with no datagram  
loss.  So it looks like the checksum patches had no effect on this  
phenomenon; the only thing that mattered was the scheduling.
I also did some lower data-rate UDP tests with iperf (without the  
scheduling change).  At 500 Kb/s it loses about 48% of the datagrams,  
at 2 Mb/s it loses 81%, and at 4 Mb/s it loses 99%.  Ouch.  iperf  
also manages to chew up 100% of CPU time doing this, so that might  
explain why domU chokes even at low bandwidths.  Perhaps its timing  
is implemented with a while loop.
There's still the odd thing with domU<->dom0 communication being  
about 1/10 the speed of dom0<->dom0 or domU<->domU.  It's roughly 170  
Mb/s versus 1.7 Gb/s.
--Winston

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