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

Sylvain Bauza sbauza at redhat.com
Tue Aug 12 14:49:42 UTC 2014

(sorry for reposting, missed 2 links...)

Hi Nikola,

Le 12/08/2014 12:21, Nikola Đipanov a écrit :
> 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.
> """
> 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.

Well, ERT does provide a plugin mechanism for testing resources at the 
claim level. This is the plugin responsibility to implement a test() 
method [2.1] which will be called when test_claim() [2.2]

So, provided this method is implemented, a local host check can be done 
based on the host's view of resources.

> 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.

And here is where I agree with you : at the moment, ResourceTracker (and 
consequently Extensible RT) only provides the view of the resources the 
host is knowing (see my point above) and possibly some other resources 
are missing.
So, whatever your choice of going with or without ERT, your patch [3] 
still deserves it if we want not to lookup DB each time a claim goes.

> 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.

Two distinct implementations of that spec (ie. instances and flavors) 
have been proposed [2.3] [2.4] so reviews are welcome. If you see the 
test() method, it's no-op thing for both plugins. I'm open to comments 
because I have the stated problem : how can we define a limit on just a 
counter of instances and flavors ?

> I propose to revert [2] ASAP since it is still fresh, and see how we can
> come up with a cleaner design.
> Would like to hear opinions on this, before I propose the patch tho!

IMHO, I think the problem is more likely that the regular RT misses some 
information for each host so it requires to handle it on a case-by-case 
basis, but I don't think ERT either increases complexity or creates 
another issue.


> 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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[2.3] https://review.openstack.org/112578
[2.4] https://review.openstack.org/113373

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