Re: Scalable Event Channel ABI design (draft A)

[David Vrabel <david.vrabel@citrix.com>]

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.

David