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Virtual networks
- Consider a two-node system with one incoming and one outgoing queue on each node
- Single queue is not enough to avoid deadlock
- Single queue forms a single virtual network
- Similar deadlock issues as multi-level caches
- An incoming message may generate another message e.g., request generates reply, ReadX generates reply and invalidation requests, request may generate intervention request
- Memory controller refuses to schedule a message if the outgoing queue is full
- Same situation may happen on all nodes: deadlock
- One incoming and one outgoing queue is not enough
- What if we have two in each direction?: one for request and one for reply
- Replies can usually sink
- Requests generating requests?
- What is the length of the longest transaction in terms of number of messages?
- This decides the number of queues needed in each direction (Origin 2000 uses a different scheme)
- One type of message is usually assigned to a queue
- One queue type connected across the system forms a virtual network of that type e.g. request network, reply network, third party request (invalidations and interventions) network
- Virtual networks are multiplexed over a physical network
- Sink message type must get scheduled eventually
- Resources should be sized properly so that scheduling of these messages does not depend on anything
- Avoid buffer shortage (and deadlock) by keeping reserved buffer for the sink queue
Three-lane protocols
- Quite popular due to its simplicity
- Let the request network be R, reply network Y, intervention/invalidation network be RR
- Network dependence (aka lane dependence) graph looks something like this
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