After the introduction of the Fair I/O Queueing mechanism in Seastar,
it is possible to add requests to a specific priority class, that will
end up being serviced fairly.
This patch introduces a Priority Manager service, that manages the priority
each class of request will get. At this moment, having a class for that may
sound like an overkill. However, the most interesting feature of the Fair I/O
queue comes from being able to adjust the priorities dynamically as workloads
changes: so we will benefit from having them all in the same place.
This is designed to behave like one of our services, with the exception that
it won't use the distributed interface. This is mainly because there is no
reason to introduce that complexity at this point - since we can do thread local
registration as we have been doing in Seastar, and because that would require us
to change most of our tests to start a new service.
Signed-off-by: Glauber Costa <glauber@scylladb.com>
261c272178