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Subject |
Re: [GLIF controlplane] RE: Network Control Architecture |
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From |
Bert Andree <handree@xxxxxxxxxxxxxx> |
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Date |
Thu, 19 Apr 2007 22:05:22 +0200 |
Hello Steve,
Well, it is easy to make the pre-reservation of a single resource
atomic. Even the reservation of multiple resources in a single domain is
not very hard. But in the multidomain case, the pre-reservation of all
resources together needs to be atomic. This can only be achieved if
there is some kind of RB, and only one RB is active at the same time.
This process can be compared with the semaphore in parallel computing.
If not *all* resources can be pre-reserved in a single atomic action the
following might occur:
RB-A pre-reserves RS-1
RB-B pre-reserves RS-2
RB-A pre-reserves RS-2 (fail)
RB-B pre-reserves RS-1 (fail)
RB-A releases RS-1
RB-B releases RS-2
RB-A pre-reserves RS-1
RB-B pre-reserves RS-2
RB-A pre-reserves RS-2 (fail)
RB-B pre-reserves RS-1 (fail)
RB-A releases RS-1
RB-B releases RS-2
etc.
Now, although the above scenario is not very likely, in a complex system
where a lot of reservations are made similar processes might and will
happen, just like in the early supercomputers.
Now, if there is a single RB (or at least a single queue for requests
from resource brokers), than it is possible to *develop* a system that
does not lead to deadlock or individual starvation.
Of course, if we assume that resources are not scarse, the above is not
important, but then: why do we need reservations.
Best regards,
Bert
Steve Thorpe wrote:
Hello Bert, everyone,
The point Bert made "...if the pre-reservation of resources is not an
atomic action..." is very important.
My belief is the pre-reservation of resources, or Phase 1 of a 2-phase
commit protocol, *must* be atomic. That is, there must be a guarantee
that at most one requestor will ever be granted a pre-reservation of a
given resource. Then, the requestor should come back with a subsequent
"Yes, commit the pre-reservation", or "No, I release the
pre-reservation". In the case where the requestor does not come back
within a certain amount of time, then the pre-reservation could expire
and some other requestor could then begin the 2-phase commit process on
the given resource.
There may be situations where a resource broker can not get the desired
resource reservation(s) booked. But, I don't see deadlocking here -
where both resources can *never* be booked. Unless of course, a
resource broker books them once and is allowed on to them forever.
The atomicity of the pre-reservation (phase 1) stage of the 2-phase
commit process is a very critical part for this to work.
Steve
PS I have also added Jon MacLaren to this thread, as I'm not sure he's
on the GLIF email list(s).
Bert Andree wrote:
Hi Gigi,
What exactly dou you mean with one RB per request.
Suppose there are two independant RB's,RB-A and RB-B and two
resources, RS-1 and RS-2.
Suppose that there is a request to RB-A to book both resources and a
request to RB-B to do the same. Now, if the pre-reservation of
resources is not an atomic action, two different strategies may
introduce specific problems.
Stategy 1: an availibility request does not reserve the resource:
RB-A asks for RS-1 (available)
RB-B asks for RS-2 (available)
RB-A asks for RS-2 (available)
RB-B asks for RS-1 (available)
RB-A confirms RS-1 (success)
RB-B confirms RS-2 (success)
RB-A confirms RS-2 (fail)
RB-B confirms RS-1 (fail)
The obvious solution would be to free all resources and try again. In
complex systems there is a fair chance that both resources can never
be booked (deadlock).
Stategy 2: an availibility request reserves the resource:
RB-A asks for RS-1 (available)
RB-B asks for RS-2 (available)
RB-A asks for RS-2 (not available)
RB-B asks for RS-1 (not available)
RB-A and RB-B free all resources and try again. In complex systems
there is a fair chance that both resources can never be booked
(deadlock).
The only way to prevent is, is to have some queing of requests and
even then "individual starvation", e.g. RB-A can never book any
resources is possible in complex systems.
Best regards,
Bert
Gigi Karmous-Edwards wrote:
Hi Admela,
I agree, there are two phases, 1) check availability from xRM, and 2)
If all xRMs give an ack. then go th second phase of commit, 2') if
one or more xRM gives a nack, then do not proceed to the phase two
commit. In the architecture sent out, the responsibility of
coordinating and administering the two phases is in ONE RB per
request. Each xRM will rely on the RB to tell them whether to
proceed to a commit or not. If they get a commit from an RB, it then
becomes the xRM's responsibility to make the reservation and
allocation in the actual resources. I think if for example RB-A
talks to an xRM in domain "B", then it may be the responsibility of
the xRM-B to tell its own RB-B of its interaction with RB-A. Is this
in line with your thoughts?
Gigi
