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
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