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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.

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.

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.

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