Communication Cost

• Three major components of the communication architecture that affect performance
  • Latency: time to do an operation (e.g., load/store or send/ recv .)
  • Bandwidth: rate of performing an operation
  • Overhead or occupancy: how long is the communication layer occupied doing an operation
• Latency
  • Already a big problem for microprocessors
  • Even bigger problem for multiprocessors due to remote operations
  • Must optimize application or hardware to hide or lower latency (algorithmic optimizations or prefetching or overlapping computation with communication)

• Bandwidth
  • How many ops in unit time e.g. how many bytes transferred per second
  • Local BW is provided by heavily banked memory or faster and wider system bus
  • Communication BW has two components: 1. node-to-network BW (also called network link BW) measures how fast bytes can be pushed into the router from the CA, 2. within-network bandwidth: affected by scalability of the network and architecture of the switch or router
• Linear cost model: Transfer time = T0 + n/B where T0 is start-up overhead, n is number of bytes transferred and B is BW
  • Not sufficient since overlap of comp. and comm. is not considered; also does not count how the transfer is done (pipelined or not)