1.7 Topology, Routing and Virtual Channel Selection

Topology, Routing strategies and input and output virtual channel selection strategies need to be decided for any inter-connection network. Once we have all of these in place we can simulate an inter-connection network.

1.7.1 Topology

For every architecture one wants to design, a topology file has to written which defines a few basic functions for that particular topology. These are:

void getNeighbours(int nodeid, int numP);
This is called initially for every switch and this populates the data structure next in a switch which contains the connectivity of that switch. The switch specified by switch has numP ports.

int getNext(int portid, int nodeid, int numP)
Returns the index of the switch/node that is connected to the switch nodeid, at portid. The number of ports this node has is numP.

int getNextChannel(int portid, int nodeid, int numP)
Returns the index of the channel that is connected to the switch nodeid, at portid. The number of ports this node has is numP.

int getStartPort(int nodeid, int numP, int dest)
Return the index of the port that is connected to this compute node from a switch

int getStartVc()
Returns the index of the first virtual channel (mostly 0).

int getStartSwitch(int nodeid)
Returns the index of the node/switch that is connected to the first port

int getStartNode()
Returns the index of the first node. Each poser has a separate index, irrespective of the type of the poser.

int getEndNode()
Returns the index of the last node.

1.7.2 Routing

Routing strategy needs to be specified for every interconnection network. There is usually at least one routing strategy that needs to be defined for every topology, Usually we have many more. The following functions need to be defined for every routing strategy.

int selectRoute(int current, int dest, int numP, Topology* top, Packet *p, map<int,int> &bufsize, unsigned short *xsubi)
Returns the portid that should be taken on switch current if the destination is dest. The number of ports on a switch is numP. We also pass the pointer to the topology and to the Packet.

int selectRoute(int current, int dest, int numP, Topology* top, Packet *p, map<int,int> &bufsize, map<int,int> &portContention, unsigned short *xsubi)
Returns the portid that should be taken on switch current if the destination is dest. The number of ports on a switch is numP. We also pass the pointer to the topology and to the Packet. Bufsize is the state of the ports in a switch, i.e. how many buffers on each port are full, while portContention is used to give priority to certain ports, when more options are available.

int expectedTime(int src, int dest, POSE_TimeType ovt, POSE_TimeType origOvt, int length, int *numHops)
Returns the expected time for a packet to travel from src to dest, when the number of hops it will need to travel is numHops.

int convertOutputToInputPort(int id, Packet *p, int numP, int *next)
Translate this output port to input port on the switch this port is connected to.

1.7.3 Input Virtual Channel Selection

For every switch, we need to know the mechanism it uses to choose input virtual channel. There are a few different input virtual channel selection strategies, and a switch can choose among them. Each should implement the following function.

int selectInputVc(map<int,int> &availBuffer, map<int,int> &request, map<int,vector<Header> > &inBuffer, int globalVc, int curSwitch)
Returns the input virtual channel to be used depending on the strategy and the input parameters.

1.7.4 Output Virtual Channel Selection

For every switch, we need to know the mechanism it uses to choose output virtual channel. There are a few different output virtual channel selection strategies, and a switch can choose among them. Each should implement the following function.

int selectOutputVc(map<int,int> &bufsize, Packet *p, int unused)
Returns the output virtual channel to be used depending on the strategy and the input parameters.