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Deadlock avoidance
- Multiple virtual channels per port
- Break the dependence cycle
- Allocate virtual channels according to source and destination of packets
- Up*-Down* routing
- Applies to indirect networks only
- Logically treat the topology as a spanning tree with processors at leaves
- Route up to common ancestor and then down
- No cycles involved
- Comes for free in trees, fat trees, Butterflies
- Turn model
- Attacks the fundamental problem: the turns
- In a 2D mesh there are eight possible turns and that form two different types of cycles (4 turns each)
- Restrict the use of turns
- Avoid one turn each from these two cycles: 16 possibilities
- Out of these 16, 12 are deadlock free
- West-first: avoids +y to –x and –y to –x turns i.e. cannot turn west
- North-last: avoids +y to –x and +y to +x turns i.e. cannot turn from north
- Negative-first: offers a set of choices
- Dimension order routing is inherently deadlock-free because it disallows all y to x turns (too restrictive)
Adaptive routing
- For fault tolerance and better network utilization
- Tremendous contention in 2D mesh with DOR for transpose traffic or for accessing locks allocated on a corner node
- Adaptive routing can make use of other available paths and introduce more concurrency
- Main idea: decide the next output port dynamically based on switch state (locally or globally gathered)
- Fully vs. partially adaptive: allow all or some paths
- Minimal vs. non-minimal adaptive: allow only shortest paths (i.e. hop count goes down monotonically) or allow arbitrary paths
- Hot-potato routing: example of non-minimal adaptive; misroute one of the contending packets
- Multipath routing
- A special case of adaptive routing where the decision is static
- Source-based multipath: source chooses the entire path from the legal set of paths
- Table-based multipath: routing table provides multiple possible output ports programmed at boot time
- Deadlock avoidance
- Non-minimal adaptive routing is prone to deadlocks
- Normal trick is to have a deadlock-free virtual network called the escape route
- Livelock is a bigger problem: with hot-potato routing a packet may keep on moving around the same path never getting to the destination
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