Claims in the Scheduler

One of the shortcomings of the current scheduler in OpenStack Nova is that there is a long interval from when the scheduler selects a suitable host for a new instance until the resources on that host are claimed so that they are no longer available. Now that resources are tracked in the Placement service, we want to move the claim closer to the time of host selection, in order to avoid (or eliminate) the race condition. I'm not going to explain the race condition here; if you're reading this, I'm assuming this is well understood, so let me just summarize my concern: the current proposed design, as seen in the series starting with https://review.openstack.org/#/c/465175/, could be made much better with some design changes.

At the recent Boston Summit, which I was unable to attend due to lack of funding by my employer, the design for this change was discussed, and the consensus was to have the scheduler return a list of hosts for each instance to the super conductor, and then have the super conductor attempt to claim the resources for the first host returned. If the allocation fails, the super conductor discards that host and tries to claim the resources on the second host. When it finally succeeds in a claim, it sends a message to that host to start building the instance, and that message will include the list of alternative hosts. If something happens that causes the build to fail, the compute node sends it back to its local conductor, which will unclaim the resources, and then try each of the alternates in order by first claiming the resources on that host, and if successful, sending the build request to that host. Only if all of the alternates fail will the request fail.

I believe that while this is an improvement, it could be better. I'd like to do two things differently:

  1. Have the scheduler claim the resources on the first selected host. If it fails, discard it and try the next. When it succeeds, find other hosts in the list of weighed hosts that are in the same cell as the selected host in order to provide the number of alternates, and return that list.
  2. Have the process asking the scheduler to select a host also provide the number of alternates, instead of having the scheduler use the current max_attempts config option value.

On the first point: the scheduler already has a representation of the resources that need to be claimed. If the super conductor does the claiming, it will have to re-generate that representation. Sure, that's not all that demanding, but it sure makes for cleaner design to not repeat things. It also ensures that the super conductor gets a good host from the start. Let me give an example. If the scheduler returns a chosen host (without claiming) and two alternates (which is the standard behavior using the config option default), the conductor has no guarantee of getting a good host. In the event of a race, the first host may fail to allocate resources, and now there are only the two alternates to try. If the claim was done in the scheduler, though, when that first host failed it would have been discarded, and the the next host tried, until the allocation succeeded. Only then would the alternates be determined, and the super conductor could confidently pass on that build request to the chosen host. Simply put: by having the scheduler do the initial claim, the super conductor is guaranteed to get a good host.

Another problem, although much less critical, is that the scheduler still has the host do consume_from_request(). With the claim done in the conductor, there is no way to keep this working if the initial host fails. We will have consumed on that host, even though we aren't building on it, and have not consumed on the host we actually select.

On the second point: we have spent a lot of time over the past few years trying to clean up the interface between Nova and the scheduler, and have made a great deal of progress on that front. Now I know that the dream of an independent scheduler is still just that: a dream. But I also know that the scheduler code has been greatly improved by defining a cleaner interface between it an Nova. One of the items that has been discussed is that the config option max_attempts doesn't belong in the scheduler; instead, it really belongs in the conductor, and now that the conductor will be getting a list of hosts from the scheduler, the scheduler is out of the picture when it comes to retrying a failed build. The current proposal to not only leave that config option in the scheduler, but to make it dependent on it for its functioning, is something that once again makes the scheduler Nova-centric (and Nova-exclusive). It would be a much cleaner design to simply have the conductor ask for the number of hosts (chosen + alternates), and have the scheduler's behavior use that number. Yes, it requires a change to the RPC interface, but that is to be expected if you are changing a fundamental behavior of the scheduler. And if the scheduler is ever moved into a module, all it is is another parameter. Really, that's not a good reason to follow a poor design.

Since some of the principal people involved in this discussion are not available now, and I'm going to be away at PyCon for the next few days, Dan Smith suggested that I post a summary of my concerns so that all can read it and have an idea what the issues are. Then next week sometime when we are all around and have the time to discuss this, we can hash it out on #openstack-nova, or maybe in a hangout. I also have pushed a series that has all of the steps needed to make this happen, since it's one thing to talk about a design, and it's another to see the actual code. The series starts here: https://review.openstack.org/#/c/464086/. For some of the later patches I haven't finished updating the tests to match the change in method signatures and returned value structures, but you should be able to get a good idea of the code changes I'm proposing.

Leave a Reply