AW: Customization of nova-scheduler

Stephen Finucane stephenfin at
Mon May 31 16:21:19 UTC 2021

On Mon, 2021-05-31 at 13:44 +0200, levonmelikbekjan at wrote:
> Hello Stephen,
> I am a student from Germany who is currently working on his bachelor thesis. My job is to build a cloud solution for my university with Openstack. The functionality should include the prioritization of users. So that you can imagine exactly how the whole thing should work, I would like to give you an example.
> Two cases should be solved!
> Case 1: A user A with a low priority uses a VM from Openstack with half performance of the available host. Then user B comes in with a high priority and needs the full performance of the host for his VM. When creating the VM of user B, the VM of user A should be deleted because there is not enough compute power for user B. The VM of user B is successfully created.
> Case 2: A user A with a low priority uses a VM with half the performance of the available host, then user B comes in with a high priority and needs half of the performance of the host for his VM. When creating the VM of user B, user A should not be deleted, since enough computing power is available for both users.
> These cases should work for unlimited users. In order to optimize the whole thing, I would like to write a function that precisely calculates all performance components to determine whether enough resources are available for the VM of the high priority user.

What you're describing is commonly referred to as "preemptible" or "spot"
instances. This topic has a long, complicated history in nova and has yet to be
implemented. Searching for "preemptible instances openstack" should yield you
lots of discussion on the topic along with a few proof-of-concept approaches
using external services or out-of-tree modifications to nova.

> I’m new to Openstack, but I’ve already implemented cloud projects with Microsoft Azure and have solid programming skills. Can you give me a hint where and how I can start?

As hinted above, this is likely to be a very difficult project given the fraught
history of the idea. I don't want to dissuade you from this work but you should
be aware of what you're getting into from the start. If you're serious about
pursuing this, I suggest you first do some research on prior art. As noted
above, there is lots of information on the internet about this. With this
research done, you'll need to decide whether this is something you want to
approach within nova itself, via out-of-tree extensions or via a third party
project. If you're opting for integration with nova, then you'll need to think
long and hard about how you would design such a system and start working on a
spec (a design document) outlining your proposed solution. Details on how to
write a spec are discussed at [1]. The only extension points nova offers today
are scheduler filters and weighers so your options for an out-of-tree extension
approach will be limited. A third party project will arguably be the easiest
approach but you will be restricted to talking to nova's REST APIs which may
limit the design somewhat. This Blazar spec [2] could give you some ideas on
this approach (assuming it was never actually implemented, though it may well
have been).

> My university gave me three compute hosts and one control host to implement this solution for the bachelor thesis. I’m currently setting up Openstack and all the services on the control host all by myself to understand all the functionality (sorry for not using Packstack) 😉. All my hosts have CentOS 7 and the minimum deployment which I configure is Train.
> My idea is to work with nova schedulers, because they seem to be interesting for my case. I've found a whole infrastructure description of the provisioning of an instance in Openstack  
> The nova scheduler is the first component, where it is possible to implement functions via Python and the Compute API,list-servers-detail to check for active VMs and probably delete them if needed before a successful request for an instantiation can be made. 
> What do you guys think about it? Does it seem like a good starting point for you or is it the wrong approach? 

This could potentially work, but I suspect there will be serious performance
implications with this, particularly at scale. Scheduler filters are
historically used for simple things like "find me a group of hosts that have
this metadata attribute I set on my image". Making API calls sounds like
something that would take significant time and therefore slow down the schedule
process. You'd also have to decide what your heuristic for deciding which VM(s)
to delete would be, since there's nothing obvious in nova that you could use.
You could use something as simple as filter extra specs or something as
complicated as an external service.

This should be lots to get you started. Once again, do make sure you're aware of
what you're getting yourself into before you start. This could get complicated
very quickly :)


> I'm very happy to have found you!!! 
> Thank you really much for your time!


> Best regards
> Levon
> -----Ursprüngliche Nachricht-----
> Von: Stephen Finucane <stephenfin at> 
> Gesendet: Montag, 31. Mai 2021 12:34
> An: Levon Melikbekjan <levonmelikbekjan at>; openstack at
> Betreff: Re: Customization of nova-scheduler
> On Wed, 2021-05-26 at 22:46 +0200, Levon Melikbekjan wrote:
> > Hello Openstack team,
> > 
> > is it possible to customize the nova-scheduler via Python? If yes, how? 
> Yes, you can provide your own filters and weighers. This is documented at [1].
> Hope this helps,
> Stephen
> [1]
> > 
> > Best regards
> > Levon
> > 

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