Message reconstruction

ASIM team
LIP6 Laboratory
Paris, France

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Message reconstruction

All packets corresponding to a given message are directly written in the destination processor memory by PCI-DDC. PCI-DDC must detect the completion of a given message and notify the destination processor. According to the routing tables used in the RCUBE routers, the HSL-network can be adaptive or not. When the network is adaptive, packets can follow several paths from a given sender to a given receiver. In that case the received packet order can be different from the sent packet order. When the network is not adaptive, there is only one path between any two nodes. Therefore, the received packet order is the same as the sent packet order. Adaptivity is an advantage from hot spot and fault tolerance point of view but the arrival order is not guaranteed. There is two possible methods to detect message transfer completion: This is defined in a configuration register (see chapter ).

To detect the completion of a message transfer in adaptive networks, PCI-DDC must use a List of Received Messages (LRM) implemented in main memory. The LRM is used to keep the current packet number for each message currently being received. PCI-DDC uses the MI as an index in the LRM. The last packet sent of a message contains the number of packet sent which is the expected packet number from the receiver point of view. Therefore, a message is completed when the received packet number is equal to the expected packet number. The LRM use is mandatory in adaptive networks. The LRM is useful in non-adaptive networks, because lost packets are always detected. The LRM is only managed by PCI-DDC, and thus do not imply processor overhead.
For non-adaptive networks, it is possible to detect message completion without using the LRM. This can save memory (the LRM is a large table in physical memory) and decrease the traffic on the PCI bus. In this case, message completion is detected by the Last Packet bit contained in the last packet of a message. With this method, lost packets are not detected.

Next: FIFO queues Up: Definitions and Principles Previous: Packet