Re: Scalable Event Channel ABI design (draft A)

[Wei Liu <wei.liu2@citrix.com>]

On Tue, 2013-02-05 at 14:48 +0000, David Vrabel wrote:
> On 04/02/13 19:59, Keir Fraser wrote:
> > On 04/02/2013 17:52, "David Vrabel" <david.vrabel@citrix.com> wrote:
> > 
> >> Hi,
> >>
> >> Here is a design for a scalable event channel ABI for Xen.  It has a
> >> number of benefits over the design currently being proposed by Wei Lui.
> >>
> >> * More event channels (>100,000).
> >> * 16 event priorities.
> >> * Reduced memory requirements (only 1 additional page per domU).
> >> * The use of FIFOs for events ensures fairness, whereas it is difficult
> >> to reason about the fairness with the current bitmap system.
> > 
> > I have some sympathy for this design. It's primary downside compared with
> > the 3-level alternative is its greater space cost (32*#vcpus). However, as
> > you say the fairness and prioritisation features are rather nice. Also
> > having the data structures be per vcpu may well help avoid cacheline
> > contention on busy multi-vcpu guests.
> 
> This design originally (before I posted it) did have per-VCPU event
> arrays but I changed it to per-domain to reduce the memory footprint.
> 
> A hybrid approach might be useful.  Busy guests like dom0 or driver
> domains could use per-VCPU event arrays but other guests could be
> per-domain.  This would be controlled by the toolstack.
> 
> > Interested in what others think, but I may actually prefer a ground-up
> > redesign like this.
> 
> Since the ABI needs to be changed to support more event channels anyway,
> it seems an ideal point to revisit the design.
> 

Right. I do care about better design and good implementation. Can we
build a prototype of this design? We are less than two months away from
4.3 feature freeze, and the event channel scalability is planned for
that release, which means we need to be hurried. :-)


Wei.

> David