On 08/14/2014 03:21 AM, Nikola Đipanov wrote:
On 08/13/2014 06:05 PM, Sylvain Bauza wrote:
Le 13/08/2014 12:21, Sylvain Bauza a écrit :
Le 12/08/2014 22:06, Sylvain Bauza a écrit :
Le 12/08/2014 18:54, Nikola Đipanov a écrit :
On 08/12/2014 04:49 PM, Sylvain Bauza wrote:
(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.


Yes - the problem is there is no clear API to get all the needed
bits to
do so - especially the user supplied one from image and flavors.
On top of that, in current implementation we only pass a hand-wavy
'usage' blob in. This makes anyone wanting to use this in conjunction
with some of the user supplied bits roll their own
'extract_data_from_instance_metadata_flavor_image' or similar which is
horrible and also likely bad for performance.
I see your concern where there is no interface for user-facing
resources like flavor or image metadata.
I also think indeed that the big 'usage' blob is not a good choice
for long-term vision.

That said, I don't think as we say in French to throw the bath
water... ie. the problem is with the RT, not the ERT (apart the
mention of third-party API that you noted - I'll go to it later below)
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 ?

Will look at these - but none of them seem to hit the issue I am
complaining about, and that is that it will need to consider other
request data for claims, not only data available for on instances.

Also - the fact that you don't implement test() in flavor ones tells me
that the implementation is indeed racy (but it is racy atm as well) and
two requests can indeed race for the same host, and since no claims are
done, both can succeed. This is I believe (at least in case of single
flavor hosts) unlikely to happen in practice, but you get the idea.
Agreed, these 2 patches probably require another iteration, in
particular how we make sure that it won't be racy. So I need another
run to think about what to test() for these 2 examples.
Another patch has to be done for aggregates, but it's still WIP so
not mentioned here.

Anyway, as discussed during today's meeting, these 2 patches will not
be based on ERT because of the risk it goes for Juno so let's scope
them out of this thread.


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.

RT does not miss info about the host, but about the particular request
which we have to fish out of different places like image_metadata
extra_specs etc, yet - it can't really work without them. This is
definitely a RT issue that is not specific to ERT.
+1, I agree with you, that's an API issue for RT : how do we pass out
user-defined metrics ?
I still need to figure out which kind of usecases are requiring such
examples, albeit the NUMA usecase of course.

After a night and morning of thinking about this problem, I'm writing
down some ideas that could help fixing the problem (although I don't
think it's a long-term scenario, rather a workaround) :

Considering you need the metadata of the flavor asked by user when
claiming a resource :
1. Compute Manager knows request_spec when building an instance, so it
could pass out request_spec as paramater (optional or not) to
rt.instance_claim() in the claim context
2. As ERT does, when claiming, it calls the ERT plugin .test() method
so with request_spec as extra parameter
3. On the ERT side, a plugin is responsible for calling (and caching)
all flavors metadata, or even only the key you need to compare
4. Scheduler knows flavors metadata thanks to ERT plugin and blueprint
isolate-scheduler-db so it makes decisions on the same subset as the RT
5. When claiming in ERT plugin test(), it extracts the flavor_id,
looks at the internal in-memory representation of flavors to get the
metadata, and calls the NUMA _test_numa_topology() method for checking
wrt this flavor metadata


The idea is that so filters and claims are doing separate decisions on
the same source of knowledge, here being ERT.

-Sylvain


Just made a draft over my thoughts here :
https://review.openstack.org/113936

I have commented on the review itself in more detail, but in short -
this solution won't work for me, and thus in the general case as well, as

1) we need a way to persist the request data (and not make it a horrible
performance bottleneck), as caching won't cut it.

There's a proposal to persist some data already at http://git.openstack.org/cgit/openstack/nova-specs/tree/specs/juno/persistent-resource-claim.rst . Based on this discussion it seems like we might want to revisit that to ensure it's going to do the right thing, or can be extended to later.


2) We also need a way to use it in every periodic task run.

None of this is only ERT problem, they are problems of the RT itself,
but as I have said in the thread already - adding a plugin API on top of
something that is broken is just asking for all sorts of trouble.

Still leaning towards revert really - anything else would be short
sighted. I would be happy to help come up with a baseline of things to
fix before we can add it back, to avoid deferring it needlesly due to
scope creep.

N.

-S

However, I still see several issues with the current ERT
implementation,
but the one I am getting at here, and why I think we should revert it,
is that we are building a 3rd party plugin API that is tightly coupled
to an obviously flawed internal API (RT and Claims).

We have no policy AFIK about what guarantees we provide to 3rd party
plugins, but if we are going to be breaking them all the time, or in
this case providing very limited usefulness - I see little value in the
current implementation of ERT, and see issues with it staying, which
means future work will be based on it.

N.
*This* is to me IMHO the biggest problem with ERT : if we say that we
externalize an API for 3rd-party plugins, we need to make sure that
everything is handled.

That said, instead of reverting the whole patch, could we just
consider to only accept changes that wouldn't require user-facing
metrics ?
The existing VCPU implementation still sounds good to me, so we can
just consider a clear communication on what is acceptable and what
not (ie. a docstring in the plugin API or so., plus -2/-1 reviews if
needed)

-Sylvain

Thanks,
-Sylvain

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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[2.1]
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

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



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