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

On Wed, Mar 27, 2013 at 9:53 PM, David Vrabel <dvrabel@xxxxxxxxxx> wrote:
> On 27/03/2013 19:36, Anil Madhavapeddy wrote:
>> 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.
> 131,072 seemed high enough to me but I'd forgotten about the Mirage use
> case.
> This can be trivially raised to 2^19 (524,288).  Beyond that, the
> implementation becomes slightly more complex as the pointers to the
> event array pages no longer fit in a single page.

Then that would require 512 pages mapped in Xen in the worst case, plus

>> 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).

25,000 pages for domUs if domU uses this ABI as well.

This might require bumping global mapping space in Xen, or we can
restrict domU to only use default 2-level ABI to solve this problem.

But let's not worry about future things for now.


> Having said that, with 25,000 VMs it would seem sensible to disaggregate
> things like the console and xenstore (in addition to the network and
> block backends). Thus reducing the need for event channels for any
> single domain.
> David

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