RE: [Xen-devel] I/O descriptor ring size bottleneck?

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> -----Original Message-----
> From: Diwaker Gupta [mailto:diwakergupta@xxxxxxxxx] 
> Sent: 21 March 2005 00:07
> To: Ian Pratt
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxxx; ian.pratt@xxxxxxxxxxxx
> Subject: Re: [Xen-devel] I/O descriptor ring size bottleneck?
> 
> > BTW, I'd actually be very suspicious of dummynet's ability 
> to operate at
> > 500Mb/s. It's possible that the reduced bandwidth is due to some bad
> > interaction between burstiness caused by Xen's context switching and
> > dummynet.
> 
> Could you elaborate a bit more on this? Even if dummynet can't operate
> at 500Mbps, we should atleast see the same degradation in performance
> right?

Because of the context switching between dom0 and domU, the packets tend
to come out in a more bursty fashion. It's conceivable that this might
cause hickups.

It might be worth looking at the number of interupts that are occuring,
and also the Xen s/w perf counters to see domain switch rate.

BTW, what Ethernet card are you using?

> > Are your dom0 and domU running on the same processor? Could you try
> > using hyperthreading or SMP?
> 
> Yep, same processor. For various other reasons, I wanted to avoid SMP,
> so I was running with the nosmp option. I'll try running with SMP and
> post an update.

That'll be interesting. 

Ian


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