1. Introduction
High performance computing has undergone rapid change in the last decades. Today, Linux clusters become more and more popular, and thousands of processors can be incorporated into one parallel machine. These processors may be connected by expensive high-speed interconnects such as Myrinet or QsNET to achieve high performance, or by cheap slow commodity Fast Ethernet or Gigabit Ethernet to lower the cost of a cluster. For practical purpose, most clusters use both kinds of interconnection. On such large scale clusters, collective communications involving large number of processors would become performance bottleneck, thus optimizing their performance makes great sense. Through experiment, we found that different algorithms can behave very differently on different networks, for example, an algorithm that is optimal for Myrinet may be inferior for Fast Ethernet. In this study, we carried out lots of experiments to investigate the best allgather algorithm for Fast Ethernet.