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On 6/19/2017 9:17 AM, Jay Pipes wrote:
> On 06/19/2017 09:04 AM, Edward Leafe wrote:
>> Current flow:

As noted in the nova-scheduler meeting this morning, this should have 
been called "original plan" rather than "current flow", as Jay pointed 
out inline.

>> * Scheduler gets a req spec from conductor, containing resource 
>> requirements
>> * Scheduler sends those requirements to placement
>> * Placement runs a query to determine the root RPs that can satisfy 
>> those requirements
> 
> Not root RPs. Non-sharing resource providers, which currently 
> effectively means compute node providers. Nested resource providers 
> isn't yet merged, so there is currently no concept of a hierarchy of 
> providers.
> 
>> * Placement returns a list of the UUIDs for those root providers to 
>> scheduler
> 
> It returns the provider names and UUIDs, yes.
> 
>> * Scheduler uses those UUIDs to create HostState objects for each
> 
> Kind of. The scheduler calls ComputeNodeList.get_all_by_uuid(), passing 
> in a list of the provider UUIDs it got back from the placement service. 
> The scheduler then builds a set of HostState objects from the results of 
> ComputeNodeList.get_all_by_uuid().
> 
> The scheduler also keeps a set of AggregateMetadata objects in memory, 
> including the association of aggregate to host (note: this is the 
> compute node's *service*, not the compute node object itself, thus the 
> reason aggregates don't work properly for Ironic nodes).
> 
>> * Scheduler runs those HostState objects through filters to remove 
>> those that don't meet requirements not selected for by placement
> 
> Yep.
> 
>> * Scheduler runs the remaining HostState objects through weighers to 
>> order them in terms of best fit.
> 
> Yep.
> 
>> * Scheduler takes the host at the top of that ranked list, and tries 
>> to claim the resources in placement. If that fails, there is a race, 
>> so that HostState is discarded, and the next is selected. This is 
>> repeated until the claim succeeds.
> 
> No, this is not how things work currently. The scheduler does not claim 
> resources. It selects the top (or random host depending on the selection 
> strategy) and sends the launch request to the target compute node. The 
> target compute node then attempts to claim the resources and in doing so 
> writes records to the compute_nodes table in the Nova cell database as 
> well as the Placement API for the compute node resource provider.

Not to nit pick, but today the scheduler sends the selected destinations 
to the conductor. Conductor looks up the cell that a selected host is 
in, creates the instance record and friends (bdms) in that cell and then 
sends the build request to the compute host in that cell.

> 
>> * Scheduler then creates a list of N UUIDs, with the first being the 
>> selected host, and the the rest being alternates consisting of the 
>> next hosts in the ranked list that are in the same cell as the 
>> selected host.
> 
> This isn't currently how things work, no. This has been discussed, however.
> 
>> * Scheduler returns that list to conductor.
>> * Conductor determines the cell of the selected host, and sends that 
>> list to the target cell.
>> * Target cell tries to build the instance on the selected host. If it 
>> fails, it unclaims the resources for the selected host, and tries to 
>> claim the resources for the next host in the list. It then tries to 
>> build the instance on the next host in the list of alternates. Only 
>> when all alternates fail does the build request fail.
> 
> This isn't currently how things work, no. There has been discussion of 
> having the compute node retry alternatives locally, but nothing more 
> than discussion.

Correct that this isn't how things currently work, but it was/is the 
original plan. And the retry happens within the cell conductor, not on 
the compute node itself. The top-level conductor is what's getting 
selected hosts from the scheduler. The cell-level conductor is what's 
getting a retry request from the compute. The cell-level conductor would 
deallocate from placement for the currently claimed providers, and then 
pick one of the alternatives passed down from the top and then make 
allocations (a claim) against those, then send to an alternative compute 
host for another build attempt.

So with this plan, there are two places to make allocations - the 
scheduler first, and then the cell conductors for retries. This 
duplication is why some people were originally pushing to move all 
allocation-related work happen in the conductor service.

> 
>> Proposed flow:
>> * Scheduler gets a req spec from conductor, containing resource 
>> requirements
>> * Scheduler sends those requirements to placement
>> * Placement runs a query to determine the root RPs that can satisfy 
>> those requirements
> 
> Yes.
> 
>> * Placement then constructs a data structure for each root provider as 
>> documented in the spec. [0]
> 
> Yes.
> 
>> * Placement returns a number of these data structures as JSON blobs. 
>> Due to the size of the data, a page size will have to be determined, 
>> and placement will have to either maintain that list of structured 
>> datafor subsequent requests, or re-run the query and only calculate 
>> the data structures for the hosts that fit in the requested page.
> 
> "of these data structures as JSON blobs" is kind of redundant... all our 
> REST APIs return data structures as JSON blobs.
> 
> While we discussed the fact that there may be a lot of entries, we did 
> not say we'd immediately support a paging mechanism.

I believe we said in the initial version we'd have the configurable 
limit in the DB API queries, like we have today - the default limit is 
1000. There was agreement to eventually build paging support into the API.

This does make me wonder though what happens when you have 100K or more 
compute nodes reporting into placement and we limit on the first 1000. 
Aren't we going to be imposing a packing strategy then just because of 
how we pull things out of the database for Placement? Although I don't 
see how that would be any different from before we had Placement and the 
nova-scheduler service just did a ComputeNode.get_all() to the nova DB 
and then filtered/weighed those objects.

> 
>> * Scheduler continues to request the paged results until it has them all.
> 
> See above. Was discussed briefly as a concern but not work to do for 
> first patches.
> 
>> * Scheduler then runs this data through the filters and weighers. No 
>> HostState objects are required, as the data structures will contain 
>> all the information that scheduler will need.
> 
> No, this isn't correct. The scheduler will have *some* of the 
> information it requires for weighing from the returned data from the GET 
> /allocation_candidates call, but not all of it.
> 
> Again, operators have insisted on keeping the flexibility currently in 
> the Nova scheduler to weigh/sort compute nodes by things like thermal 
> metrics and kinds of data that the Placement API will never be 
> responsible for.
> 
> The scheduler will need to merge information from the 
> "provider_summaries" part of the HTTP response with information it has 
> already in its HostState objects (gotten from 
> ComputeNodeList.get_all_by_uuid() and AggregateMetadataList).
> 
>> * Scheduler then selects the data structure at the top of the ranked 
>> list. Inside that structure is a dict of the allocation data that 
>> scheduler will need to claim the resources on the selected host. If 
>> the claim fails, the next data structure in the list is chosen, and 
>> repeated until a claim succeeds.
> 
> Kind of, yes. The scheduler will select a *host* that meets its needs.
> 
> There may be more than one allocation request that includes that host 
> resource provider, because of shared providers and (soon) nested 
> providers. The scheduler will choose one of these allocation requests 
> and attempt a claim of resources by simply PUT 
> /allocations/{instance_uuid} with the serialized body of that allocation 
> request. If 202 returned, cool. If not, repeat for the next allocation 
> request.
> 
>> * Scheduler then creates a list of N of these data structures, with 
>> the first being the data for the selected host, and the the rest being 
>> data structures representing alternates consisting of the next hosts 
>> in the ranked list that are in the same cell as the selected host.
> 
> Yes, this is the proposed solution for allowing retries within a cell.
> 
>> * Scheduler returns that list to conductor.
>> * Conductor determines the cell of the selected host, and sends that 
>> list to the target cell.
>> * Target cell tries to build the instance on the selected host. If it 
>> fails, it uses the allocation data in the data structure to unclaim 
>> the resources for the selected host, and tries to claim the resources 
>> for the next host in the list using its allocation data. It then tries 
>> to build the instance on the next host in the list of alternates. Only 
>> when all alternates fail does the build request fail.
> 
> I'll let Dan discuss this last part.
> 
> Best,
> -jay
> 
>> [0] https://review.openstack.org/#/c/471927/
>>
>>
>>
>>
>>
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-- 

Thanks,

Matt