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On 02/11/2014 03:21 AM, Khanh-Toan Tran wrote:
>> Second, there is nothing wrong with booting the instances (or
> instantiating other
>> resources) as separate commands as long as we support some kind of
>> reservation token.
>
> I'm not sure what reservation token would do, is it some kind of informing
> the scheduler that the resources would not be initiated until later ?

Like a restaurant reservation, it would "claim" the resources for use by 
someone at a later date.  That way nobody else can use them.

That way the scheduler would be responsible for determining where the 
resource should be allocated from, and getting a reservation for that 
resource.  It would not have anything to do with actually instantiating 
the instance/volume/etc.

> Let's consider a following example:
>
> A user wants to create 2 VMs, a small one with 20 GB RAM, and a big one
> with 40 GB RAM in a datacenter consisted of 2 hosts: one with 50 GB RAM
> left, and another with 30 GB RAM left, using Filter Scheduler's default
> RamWeigher.
>
> If we pass the demand as two commands, there is a chance that the small VM
> arrives first. RamWeigher will put it in the 50 GB RAM host, which will be
> reduced to 30 GB RAM. Then, when the big VM request arrives, there will be
> no space left to host it. As a result, the whole demand is failed.
>
> Now if we can pass the two VMs in a command, SolverScheduler can put their
> constraints all together into one big LP as follow (x_uv = 1 if VM u is
> hosted in host v, 0 if not):

Yes.  So what I'm suggesting is that we schedule the two VMs as one call 
to the SolverScheduler.  The scheduler then gets reservations for the 
necessary resources and returns them to the caller.  This would be sort 
of like the existing Claim object in nova/compute/claims.py but 
generalized somewhat to other resources as well.

The caller could then boot each instance separately (passing the 
appropriate reservation/claim along with the boot request).  Because the 
caller has a reservation the core code would know it doesn't need to 
schedule or allocate resources, that's already been done.

The advantage of this is that the scheduling and resource allocation is 
done separately from the instantiation.  The instantiation API could 
remain basically as-is except for supporting an optional reservation token.

> That responses to your first point, too. If we don't mind that some VMs
> are placed and some are not (e.g. they belong to different apps), then
> it's OK to pass them to the scheduler without Instance Group. However, if
> the VMs are together (belong to an app), then we have to put them into an
> Instance Group.

When I think of an "Instance Group", I think of 
"https://blueprints.launchpad.net/nova/+spec/instance-group-api-extension". 
   Fundamentally Instance Groups" describes a runtime relationship 
between different instances.

The scheduler doesn't necessarily care about a runtime relationship, 
it's just trying to allocate resources efficiently.

In the above example, there is no need for those two instances to 
necessarily be part of an Instance Group--we just want to schedule them 
both at the same time to give the scheduler a better chance of fitting 
them both.

More generally, the more instances I want to start up the more 
beneficial it can be to pass them all to the scheduler at once in order 
to give the scheduler more information.  Those instances could be parts 
of completely independent Instance Groups, or not part of an Instance 
Group at all...the scheduler can still do a better job if it has more 
information to work with.

Chris