Re: [Xen-users] Give dom0 2 pinned vcpus, but share one with domU

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2014-05-19 at 09:56 -0700, jumperalex wrote:
> Is it possible to give dom0 two vcpus, pinned, but also allow one of those to
> be shared with a domU?
> 
> So right now I'm setup like this:
> dom0: dom0_max_vcpus=1 dom0_vcpus_pin
> archVM: vcpu="1-7"
> 
> In most of my use cases this works fine, but in a few (rsync specifically)
> my dom0 is just getting hammered and it is slowing a local rsync between
> domU data.img and dom0 raid array.
> 
> Someone else suggested turning off the md5 check in rsync but I'm not
> thrilled about turning off hashing of a backup and I want to see if I can
> solve the problem with good provisioning first.
> 
> So what if I did this:
> dom0: dom0_max_vcpus=*2* dom0_vcpus_pin
> archVM: vcpu = "1-7"
> 
> So I'm giving dom0 2 vcpus which will be pinned to cores 0,1 but I'm still
> allowing archVM access to core 1 but not core 0
> 
> Will this even work?

Yes, it is perfectly fine to have overlapping set of pins.

Note though that depending on your workload pinning (especially dom0)
might be actively harmful. Is there some reason you want to pin rather
than letting dom0's vcpus float?

> How will Xen deal with dividing up core1's cycles?

It will schedule the vcpus according to their affinity (pin) and
workload etc, it's pretty much the normal scheduling but with an
additional constraint.

> If there is contention for core1 will that cause a lot of context switching
> between cores 0 and 1 within dom0?  Or will core 1 just be shared ~50/50
> while core 0 does dom0's heavy lifting?

If both vcpus are busy then they will get approximately a 50/50 fair
share. If one is busy and the other is mostly idle then the split will
reflect that.

> 
> I know I could just give archVM 6 cores instead of 7, but I really want it
> to have access to as many cores as possible for heavy transcoding loads but
> I also know I need to dedicate one to dom0 so this is my compromise
> solution.

Ian.


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