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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 6 Dec 2005, at 00:04, Diwaker Gupta wrote:

> Look forward to it. Whats the deal with the pipelined backend? Whats
> the target scenario there?

It's a form of request-notification avoidance. It allows the frontend 
to avoid sending a notification (virtual ipi) to the backend if any 
previous requests are still in flight (no response received). In some 
situations, pipelining can mean you basically need to send no 
notifications, because the backend will pull down new requests as it 
responds to old ones.
Thing is, it doesn;t work for some forms of packet processing in 
domain0. For example, if you send fragmented IP datagrams and they are 
reassembled in domain0 there may be a dependency between packets. Hence 
you need to send notifications even if buffers are in flight because 
you won't get a response until the datagram is reassembled and 
forwarded.
> Meanwhile, though I agree that SMPs and hyperthreaded processors are
> becoming the norm, it still doesn't solve this problem. Even on an SMP
> machine, I can have dom0 co-located with a VM on the same CPU, and I'm
> not sure how different that would be from the current situation.
With enough hardware contexts, it'll soon become sensible to dedicate a 
context to domain0 or your IDD. It's certainly a very sensible use of a 
hyperthread (almost certainly a better use than multiprocessing within 
a guest, if you do a significant amount of i/o).
> On a related note, has anyone been working on the IDD stuff? Is it
> possible to wrap up a device driver in its own domain? The last time
> I tried, it basically wasn't possible, but I'd really be interested in
> helping out any which way to get it working.
Needs some PCI virtualization in Xen (managed by platform code in 
domain0). Nothing too tricky I think.
 -- Keir


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