Re: [Xen-devel] Request for input: Extended event channel support

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

On 27 Mar 2013, at 11:23, George Dunlap <dunlapg@xxxxxxxxx> wrote:
> 
> The FIFO solution makes event delivery a matter of adding items to a
> highly structured linked list.  The number of event channels for the
> interface design has a theoretical maximum of 2^28; the current
> implementation is limimited at 2^17, which is over 100,000.  The
> number is the same for both 32-bit and 64-bit kernels.

Is there any reason for such a low default?  If I'm not mistaken,
every guest needs at least 2 event channels (console, xenstore) and
probably has two more for a net and disk device.

With stub-domains in the mix, we could easily imagine running 25,000
VMs with a couple of megabytes of RAM each using Mirage (which can
boot very low memory guests without too much trouble).  This does
run into other problems with CPU scheduling and device scalability,
but it would be nice if any proposed event channel upgrade went well
above this level rather than scrape it.

I personally prefer the 4.3 solution (despite the priority hack for
the timers) just because the existing limitation is so very trivial
to hit.  However, I have no view on the level of technical debt that
would incur if it subsequently required switching to the FIFO
solution in 4.4 and causing yet another round of upgrades.  That's
your problem; I just want the extra domains :-)

-anil