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

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

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,


[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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[2.1] https://github.com/openstack/nova/blob/master/nova/compute/resources/__init__.py#L75 [2.2] https://github.com/openstack/nova/blob/master/nova/compute/claims.py#L134

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