Re: [Xen-devel] [PATCHv4 0/11] Xen: FIFO-based event channel ABI

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Tue, Oct 01, 2013 at 11:25:59AM +0100, Jan Beulich wrote:
> >>> On 30.09.13 at 20:41, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> 
> >>> wrote:
> > On Fri, Sep 27, 2013 at 11:55:48AM +0100, David Vrabel wrote:
> >> This is a complete implementation of the hypervisor and xl toolstack
> >> parts of the FIFO-based event channel ABI described in this design
> >> document:
> >> 
> >> http://xenbits.xen.org/people/dvrabel/event-channels-F.pdf 
> >> 
> >> Changes in draft F are:
> >> 
> >> - READY field in the control block is now 32-bits (so guests only need
> >>   to support atomic bit ops on 32-bit words).  This is only a
> >>   documentation change as the implementation already used a uint32_t.
> >> 
> >> - DOMCTL_set_max_evtchn replaces EVTCHNOP_set_limit.
> >> 
> >> - DomUs default to unlimited number of event channels requiring
> >>   the toolstack to set a limit.
> >> 
> >> The toolstack defaults to limiting guests to 127 event channels if the
> >> event_channels option is omitted.  This means the minimum amount of
> >> both Xen heap and global mapping space is used regardless of which ABI
> >> is used.  If this is considered too restrictive a limit, 1023 would be
> >> another sensible default (limits the guest to a single event array
> >> page but 5 xenheap pages for the struct evtchns).
> > 
> > I would say 1023 (so the same value as the existing event mechanism)
> > would be a sensible default.
> 
> That's the existing 32-bit default; 64-bit has 4095 (yet that surely
> would be needlessly high as the new default).

127 is too little I think. For example for every VCPU there are 6 events
being consumed (VIRQ_TIMER, VIRQ_DEBUG, CALLFUNCSINGLE, CALLFUNC, RESCHED
and IRQWORK). If you launch a 32 VCPU guest you are already at 224.

Then there is the blk event channel, the tx/rx of the vif. With the possibility
of per-cpu tx/rx of vifs you would have 2*VCPU, so now we are at 288.

If you want even more LUNS (say 16), you are at 304.

1023 being the universal value looks OK to me.
> 
> Jan
> 

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