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Re: [Xen-users] Xen dom0 vCPUs pinning for PV/HVM domUs

On 02/14/2018 03:49 PM, Dario Faggioli wrote:
> On Wed, 2018-02-14 at 15:37 +0100, John Naggets wrote:
>> Thank you Dario for your insight. Indeed I now find the
>> dom0_vcpus_ping really confusing. In the wiki documentation it says:
>> "Another interesting approach is pinning Dom0 vCPUs to physical CPUs,
>> this can be done by adding dom0_vcpus_pin to the Xen command line.
>> Then once Dom0 has booted you can see to which CPUs the vCPUs have
>> been pinned and exclude other domains from running on those CPUs."
>> Source: https://wiki.xenproject.org/wiki/Tuning_Xen_for_Performance#D
>> om0_vCPUs
>> I think then this documentation should be adapted to reflect the
>> reality and not inducing people into confusion.
> What is said there is correct.
> And, in the following paragraph, it is also said how to actually
> achieve dom0/domU pCPUs isolation. What I don't like about the
> dom0_vcpus_pin is:
> - it's very inflexible. I.e., it does not allow to say anything like 
>   "pin dom0's vCPUs to pCPUs x,y,z-w". Instead, it will always pin 
>   d0v0 to pCPU 0, d0v1 to pCPU 1, d0v2 to pCPU2, etc;
> - IIRC, if you boot with dom0_vcpus_pin, you can't change dom0 vCPUs
>   affinity (and I never understood why this has been made to be the
>   case...)
> - people may see it and tend to think that, just by using it, they'll 
>   achieve dom0 isolation, without bothering going reading that page, 
>   and actually finding out that they also need to put something in 
>   _all_ their domUs' configs;
> - I do find it very impractical to have to put something in _all_ the 
>   domUs' configs, and although I appreciate that someone may be 
>   willing to do that, and am fine with it, I think we should not 
>   encourage doing it.
> So, although that page does reflect the reality, I agree that it could
> be restructured a bit.

I wonder what the actual thoughts behind recommending it are.

I've been using the alternative, the absurd high scheduler priority for
dom0 for years (after the first time of running into a situation where I
couldn't access a dom0 because guests were exploding).

I've never ran into that problematic situation again afterwards, it does
not need all the pinning config for domUs and it also doesn't keep cpus
idle when dom0 does not need the cpu time.


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