[openstack-dev] [nova] A prototype implementation towards the "shared state scheduler"

Sylvain Bauza sbauza at redhat.com
Wed Feb 17 14:01:39 UTC 2016

Le 17/02/2016 12:59, Chris Dent a écrit :
> On Wed, 17 Feb 2016, Cheng, Yingxin wrote:
>> To better illustrate the differences between shared-state, resource-
>> provider and legacy scheduler, I've drew 3 simplified pictures [1] in
>> emphasizing the location of resource view, the location of claim and
>> resource consumption, and the resource update/refresh pattern in three
>> kinds of schedulers. Hoping I'm correct in the "resource-provider
>> scheduler" part.
> That's a useful visual aid, thank you. It aligns pretty well with my
> understanding of each idea.
> A thing that may be missing, which may help in exploring the usefulness
> of each idea, is a representation of resources which are separate
> from compute nodes and shared by them, such as shared disk or pools
> of network addresses. In addition some would argue that we need to
> see bare-metal nodes for a complete picture.
> One of the driving motivations of the resource-provider work is to
> make it possible to adequately and accurately track and consume the
> shared resources. The legacy scheduler currently fails to do that
> well. As you correctly points out it does this by having "strict
> centralized consistency" as a design goal.

So, to be clear, I'm really happy to see the resource-providers series 
for many reasons :
  - it will help us getting a nice Facade for getting the resources and 
attributing them
  - it will help a shared-storage deployment by making sure that we 
don't have some resource problems when the resource is shared
  - it will create a possibility for external resource providers to 
provide some resource types to Nova so the Nova scheduler could use them 
(like Neutron related resources)

That, I really want to have it implemented in Mitaka and Newton and I'm 
totally on-board and supporting it.

TBC, the only problem I see with the series is [2], not the whole, please.

>> As can be seen in the illustrations [1], the main compatibility issue
>> between shared-state and resource-provider scheduler is caused by the
>> different location of claim/consumption and the assumed consistent
>> resource view. IMO unless the claims are allowed to happen in both
>> places(resource tracker and resource-provider db), it seems difficult
>> to make shared-state and resource-provider scheduler work together.
> Yes, but doing claims twice feels intuitively redundant.
> As I've explored this space I've often wondered why we feel it is
> necessary to persist the resource data at all. Your shared-state
> model is appealing because it lets the concrete resource(-provider)
> be the authority about its own resources. That is information which
> it can broadcast as it changes or on intervals (or both) to other
> things which need that information. That feels like the correct
> architecture in a massively distributed system, especially one where
> resources are not scarce.

So, IMHO, we should only have the compute nodes being the authority for 
allocating resources. They are many reasons for that I provided in the 
spec review, but I can reply again :

  * #1 If we consider that an external system, as a resource provider,
    will provide a single resource class usage (like network segment
    availability), it will still require the instance to be spawned
    *for* consuming that resource class, even if the scheduler accounts
    for it. That would mean that the scheduler would have to manage a
    list of allocations with TTL, and periodically verify that the
    allocation succeeded by asking the external system (or getting
    feedback from the external system). See, that's racy.
  * #2 the scheduler is just a decision maker, by any case it doesn't
    account for the real instance creation (it doesn't hold the
    ownership of the instance). Having it being accountable for the
    instances usage is heavily difficult. Take for example a request for
    CPU pinning or NUMA affinity. The user can't really express which
    pin of the pCPU he will get, that's the compute node which will do
    that for him. Of course, the scheduler will help picking an host
    that can fit the request, but the real pinning will happen in the
    compute node.

Also, I'm very interested in keeping an optimistic scheduler which 
wouldn't lock the entire view of the world anytime a request comes in. 
There are many papers showing different architectures and benchmarks 
against different possibilities and TBH, I'm very concerned by the 
scaling effect.
Also, we should keep in mind our new paradigm called Cells V2, which 
implies a global distributed scheduler for handling all requests. Having 
it following the same design tenets of OpenStack [3] by having a 
"eventually consistent shared-state" makes my guts saying that I'd love 
to see that.

> The advantage of a centralized datastore for that information is
> that it provides administrative control (e.g. reserving resources for
> other needs) and visibility. That level of command and control seems
> to be something people really want (unfortunately).

My point is that while I truly understand the need of getting an API 
resource like "scheduler, get me how much my cloud is free", that 
shouldn't necessarly need to be accurate but rather eventually consistent.
If operators want to do capacity planning, they need trends and 
thresholds, not exactly knowing the precise amounts that can change 
everytime a request comes in.


[2] https://review.openstack.org/#/c/271823/
[3] https://wiki.openstack.org/wiki/BasicDesignTenets
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