[openstack-dev] [Nova] Concerns around the Extensible Resource Tracker design - revert maybe?

Michael Still mikal at stillhq.com
Wed Aug 13 23:44:33 UTC 2014

So, there's been a lot of email in the last few days and I feel I am
not keeping up.

Sylvain, can you summarise for me what the plan is here? Can we roll
forward or do we need to revert? Time is running out for Juno.


On Thu, Aug 14, 2014 at 3:40 AM, Sylvain Bauza <sbauza at redhat.com> wrote:
> Le 13/08/2014 18:40, Brian Elliott a écrit :
>> On Aug 12, 2014, at 5:21 AM, Nikola Đipanov <ndipanov at redhat.com> wrote:
>>> Hey Nova-istas,
>>> While I was hacking on [1] I was considering how to approach the fact
>>> that we now need to track one more thing (NUMA node utilization) in our
>>> resources. I went with - "I'll add it to compute nodes table" thinking
>>> it's a fundamental enough property of a compute host that it deserves to
>>> be there, although I was considering  Extensible Resource Tracker at one
>>> point (ERT from now on - see [2]) but looking at the code - it did not
>>> seem to provide anything I desperately needed, so I went with keeping it
>>> simple.
>>> So fast-forward a few days, and I caught myself solving a problem that I
>>> kept thinking ERT should have solved - but apparently hasn't, and I
>>> think it is fundamentally a broken design without it - so I'd really
>>> like to see it re-visited.
>>> The problem can be described by the following lemma (if you take 'lemma'
>>> to mean 'a sentence I came up with just now' :)):
>>> """
>>> Due to the way scheduling works in Nova (roughly: pick a host based on
>>> stale(ish) data, rely on claims to trigger a re-schedule), _same exact_
>>> information that scheduling service used when making a placement
>>> decision, needs to be available to the compute service when testing the
>>> placement.
>>> “""
>> Correct
>>> This is not the case right now, and the ERT does not propose any way to
>>> solve it - (see how I hacked around needing to be able to get
>>> extra_specs when making claims in [3], without hammering the DB). The
>>> result will be that any resource that we add and needs user supplied
>>> info for scheduling an instance against it, will need a buggy
>>> re-implementation of gathering all the bits from the request that
>>> scheduler sees, to be able to work properly.
>> Agreed, ERT does not attempt to solve this problem of ensuring RT has an
>> identical set of information for testing claims.  I don’t think it was
>> intended to.
>> ERT does solve the issue of bloat in the RT with adding
>> just-one-more-thing to test usage-wise.  It gives a nice hook for inserting
>> your claim logic for your specific use case.
> I think Nikola and I agreed on the fact that ERT is not responsible for this
> design. That said I can talk on behalf of Nikola...
>>> This is obviously a bigger concern when we want to allow users to pass
>>> data (through image or flavor) that can affect scheduling, but still a
>>> huge concern IMHO.
>> I think passing additional data through to compute just wasn’t a problem
>> that ERT aimed to solve.  (Paul Murray?)  That being said, coordinating the
>> passing of any extra data required to test a claim that is *not* sourced
>> from the host itself would be a very nice addition.  You are working around
>> it with some caching in your flavor db lookup use case, although one could
>> of course cook up a cleaner patch to pass such data through on the “build
>> this” request to the compute.
> Indeed, and that's why I think the problem can be resolved thanks to 2
> different things :
> 1. Filters need to look at what ERT is giving them, that's what
> isolate-scheduler-db is trying to do (see my patches [2.3 and 2.4] on the
> previous emails
> 2. Some extra user request needs to be checked in the test() method of ERT
> plugins (where claims are done), so I provided a WIP patch for discussing it
> : https://review.openstack.org/#/c/113936/
>>> As I see that there are already BPs proposing to use this IMHO broken
>>> ERT ([4] for example), which will surely add to the proliferation of
>>> code that hacks around these design shortcomings in what is already a
>>> messy, but also crucial (for perf as well as features) bit of Nova code.
>>> I propose to revert [2] ASAP since it is still fresh, and see how we can
>>> come up with a cleaner design.
>> I think the ERT is forward-progress here, but am willing to review
>> patches/specs on improvements/replacements.
> Sure, your comments are welcome on https://review.openstack.org/#/c/113373/
> You can find an example where TypeAffinity filter is modified to look at
> HostState and where ERT is being used for updating HostState and for
> claiming resource.
>>> Would like to hear opinions on this, before I propose the patch tho!
>>> Thanks all,
>>> Nikola
>>> [1]
>>> https://blueprints.launchpad.net/nova/+spec/virt-driver-numa-placement
>>> [2] https://review.openstack.org/#/c/109643/
>>> [3] https://review.openstack.org/#/c/111782/
>>> [4] https://review.openstack.org/#/c/89893
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