[openstack-dev] realtime kvm cpu affinities

sfinucan at redhat.com sfinucan at redhat.com
Fri Jun 30 13:06:55 UTC 2017

On Thu, 2017-06-29 at 12:20 -0600, Chris Friesen wrote:
> On 06/29/2017 10:59 AM, sfinucan at redhat.com wrote:
> > Thus far, we've no clear conclusions on directions to go, so I've took a
> > stab
> > below. Henning, Sahid, Chris: does the above/below make sense, and is there
> > anything we need to further clarify?
> The above is close enough. :)

Excellent :tentsfingers:

> > # Problem 1
> > 
> >  From the above, there are 3-4 work items:
> > 
> > - Add a 'emulator_pin_set' or 'cpu_emulator_threads_mask' configuration
> > option
> > 
> >    - If using a mask, rename 'vcpu_pin_set' to 'pin_set' (or, better,
> >      'usable_cpus')
> > 
> > - Add a 'emulator_overcommit_ratio', which will do for emulator threads  
> > what
> >    the other ratios do for vCPUs and memory
> If we were going to support "emulator_overcommit_ratio", then we wouldn't 
> necessarily need an explicit mask/set as a config option. If someone wants
> to run with 'hw:emulator_thread_policy=isolate' and we're below the
> overcommit ratio then we run it, otherwise nova could try to allocate a new
> pCPU to add to the emulator_pin_set internally tracked by nova.  This would
> allow for the number of pCPUs in emulator_pin_set to vary depending on the
> number of instances with 'hw:emulator_thread_policy=isolate'on the compute
> node, which should allow for optimal packing.

So we'd now mark pCPUs not only as used, but also as used for a specific
purpose? That would probably be more flexible that using a static pool of CPUs,
particularly if instances are heterogeneous. I'd imagine it would, however, be
much tougher to do right. I need to think on this.

As an aside, what would we do about billing? Currently we include CPUs used for
emulator threads as overhead. Would this change?

> > - Deprecate 'hw:emulator_thread_policy'???
> I'm not sure we need to deprecate it, it would instead signify whether the 
> emulator threads should be isolated from the vCPU threads.  If set to
> "isolate" then they would run on the emulator_pin_set identified above
> (potentially sharing them with emulator threads from other instances) rather
> than each instance getting a whole pCPU for its emulator threads.

I'm confused, I thought we weren't going to need 'emulator_pin_set'? In any
case, it's probably less about deprecating the extra spec and instead changing
how things work under the hood. We'd actually still want something to signify
"I want my emulator overhead accounted for separately".

> > # Problem 2
> > 
> > No clear conclusions yet?
> I don't see any particular difficulty in supporting both RT and non-RT
> instances on the same host with one nova-compute process.  It might even be
> valid for a high-performance VM to make use 
> of 'hw:emulator_thread_policy=isolate' without enabling RT.  (Which is why
> I've been careful not to imply RT in the description above.)

Yeah, I might focus on the above problem for now, as I've no clear ideas or
suggestions on how to proceed here. Happy to work on specs if necessary,


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