[openstack-dev] [nova] Proposal: Move CPU and memory allocation ratio out of scheduler

Day, Phil philip.day at hp.com
Fri Jun 6 09:36:22 UTC 2014

From: Scott Devoid [mailto:devoid at anl.gov]
Sent: 04 June 2014 17:36
To: OpenStack Development Mailing List (not for usage questions)
Subject: Re: [openstack-dev] [nova] Proposal: Move CPU and memory allocation ratio out of scheduler

Not only live upgrades but also dynamic reconfiguration.

Overcommitting affects the quality of service delivered to the cloud user.  In this situation in particular, as in many situations in general, I think we want to enable the service provider to offer multiple qualities of service.  That is, enable the cloud provider to offer a selectable level of overcommit.  A given instance would be placed in a pool that is dedicated to the relevant level of overcommit (or, possibly, a better pool if the selected one is currently full).  Ideally the pool sizes would be dynamic.  That's the dynamic reconfiguration I mentioned preparing for.

+1 This is exactly the situation I'm in as an operator. You can do different levels of overcommit with host-aggregates and different flavors, but this has several drawbacks:

  1.  The nature of this is slightly exposed to the end-user, through extra-specs and the fact that two flavors cannot have the same name. One scenario we have is that we want to be able to document our flavor names--what each name means, but we want to provide different QoS standards for different projects. Since flavor names must be unique, we have to create different flavors for different levels of service. Sometimes you do want to lie to your users!
[Day, Phil] I agree that there is a problem with having every new option we add in extra_specs leading to a new set of flavors.    There are a number of changes up for review to expose more hypervisor capabilities via extra_specs that also have this potential problem.    What I’d really like to be able to ask for a s a user is something like “a medium instance with a side order of overcommit”, rather than have to choose from a long list of variations.    I did spend some time trying to think of a more elegant solution – but as the user wants to know what combinations are available it pretty much comes down to needing that full list of combinations somewhere.    So maybe the problem isn’t having the flavors so much, but in how the user currently has to specific an exact match from that list.
If the user could say “I want a flavor with these attributes” and then the system would find a “best match” based on criteria set by the cloud admin (for example I might or might not want to allow a request for an overcommitted instance to use my not-overcommited flavor depending on the roles of the tenant) then would that be a more user friendly solution ?

  1.  If I have two pools of nova-compute HVs with different overcommit settings, I have to manage the pool sizes manually. Even if I use puppet to change the config and flip an instance into a different pool, that requires me to restart nova-compute. Not an ideal situation.
[Day, Phil] If the pools are aggregates, and the overcommit is defined by aggregate meta-data then I don’t see why you  need to restart nova-compute.
3.      If I want to do anything complicated, like 3 overcommit tiers with "good", "better", "best" performance and allow the scheduler to pick "better" for a "good" instance if the "good" pool is full, this is very hard and complicated to do with the current system.
[Day, Phil]  Yep, a combination of filters and weighting functions would allow you to do this – its not really tied to whether the overcommit Is defined in the scheduler or the host though as far as I can see.

I'm looking forward to seeing this in nova-specs!
~ Scott
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