FLITs

FLITs is an acronym for FLow control unITs (or FLow control digITs).

Large network packets are broken into small pieces called flits (flow control units). The first flit, called the header flit holds information about this packet's route (namely the destination address) and sets up the routing behavior for all subsequent flits associated with the packet. The head flit is followed by zero or more body flits, containing the actual payload of data. The final flit, called the tail flit, performs some book keeping to close the connection between the two nodes.

A virtual connection holds the state needed to coordinate the handling of the flits of a packet. At a minimum, this state identifies the output port of the current node for the next hop of the route and the state of the virtual connection (idle, waiting for resources, or active). The virtual connection may also include pointers to the flits of the packet that are buffered on the current node and the number of flit buffers available on the next node.

The growing need for performance from computing systems drove the industry into the multi-core and many-core arena. In this setup, the execution of a kernel( a program) is split across multiple processors and the computation happens in parallel, thus ensuring performance with respect to execution time. This however implies that the processors must now be able to communicate with each other and exchange data and control signals seamlessly. One straightforward approach is the bus based interconnect, a group of wires connecting all the processors. This approach is however not scalable as the number of processors in the system increase. Hence, a scalable high performance interconnect network lies at the core of parallel computer architecture.

The formal definition of an interconnection network

"An interconnection network I is represented by a strongly connected directed multigraph, I = G(N,C). The set of vertices of the multigraph N includes the set of processing element nodes P and the set of router nodes RT. The set of arcs C represents the set of unidirectional channels (possibly virtual) that connect either the processing elements to the routers or the routers to each other".

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