Re: [Xen-devel] [PATCH] x86/apic/x2apic: Share IRQ vector between cluster members only when no cpumask is specified

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 07/08/17 17:06, Jan Beulich wrote:
>>>> Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 08/07/17 5:40 PM >>>
>> On 07/08/17 16:38, Jan Beulich wrote:
>>> So a single NIC drove the system out of vectors? That's insane, I would
>>> say, i.e. I'd call this a misconfigured system. But yeah, if we really want
>>> to get such a thing to work despite the insanity ...
>> No.  That's one single (dual headed) card with 128 virtual functions each.
> But putting such in a single-core system is, well, not very reasonable. At
> the very least I'd expect the admin to limit the number of VFs then, or not
> load a driver in Dom0 for them (but only for the PF).

Its not a single core system, but cluster mode allocates vectors across
clusters, not cores. This turns even the largest server system into a
single core system as far as vector availability goes.

There was definitely a change in behaviour between Xen 4.1 and Xen 4.4. 
Looking through the ticket, it appears the regression was introduced by
your cluster IPI broadcast optimisation (which iirc was for the 64 vcpu
windows issue), but I'm struggling to locate it in Xens history.

~Andrew

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