Re: [Xen-devel] n/w performance degradation

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 5 Dec 2005, at 23:11, Diwaker Gupta wrote:

> N/W performance from dom0 seems fine (though I used to get 930+ until
> a few days back):
>
> [  6]  0.0-20.0 sec  1.95 GBytes    835 Mbits/sec
>
> However, from a VM, the throughput is really bad:
>
> [  5]  0.0-20.0 sec  1.05 GBytes    450 Mbits/sec
>
> The above numbers are using the BVT scheduler. With the SEDF
> scheduler, the numbers are even worse (a VM can't get more than
> 300Mbps in my tests). I can post concrete figures if people are
> interested. I'm _not_ running pipelined netback.
We used to be able to saturate GigE with a single CPU, although 
admittedly burning quite a bit more CPU than using dom0 as the 
endpoint. I guess things have got out of tune, but there are a bunch of 
things we could do to encourage I/O batching ('x packets or y 
milliseconds' style receive batching, and transmitting batches of 
packets every x milliseconds or when the domU goes idle). This, 
together with scheduler tuning, should definitely get the performance 
back, although its a balancing act with one CPU to ensure no stage of 
the I/O processing pipeline gets starved.
 -- Keir


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