Clock synchronization is a fundamental requirement for any real-time distributed system operating with global schedules. The presence of a global clock permits co-scheduling, enhances the degree of synchronization between cooperating tasks of parallel programs, and permits implementation of quality of service (QoS) based communication. This paper describes the design and implementation of a high accuracy (/spl plusmn/5/spl mu/s) global clock on a Myrinet network of PC with low software overhead. The clock synchronization scheme uses a novel approach to reduce errors arising from variation of latency of transmitted clock values. The contribution of this work is that high-accuracy of clock synchronization is obtained without significant resource consumption. This makes the synchronization scheme equally suitable for high-performance and real-time distributed systems. The new approach achieves clock accuracy of the order of /spl plusmn/5/spl mu/s, compared to the other known clock synchronization algorithm on Myrinet that achieves accuracy of the order of milliseconds. Programmability of the Myrinet interface card and the presence of an on-board real time clock are critical to achieving this accuracy.

The Real-Time Message Passing Interface (MPI/RT) is a communication layer middleware standard that is aimed at providing guaranteed Quality of Service for data transfers on high performance networks. It poses "middleout" requirements both on applications and on the operating system. In this paper, we consider the "middledown" issues by modifying a POSIX compliant operating system in order to support hard real time scheduling, a feature needed for efficient MPI/RT on real-time Networks of Workstations (NOWs). This paper describes the evolution of Turtle, a variant of RT-Linux, that will later support a prototype implementation of time-driven MPI/RT. Analysis, design approach and results for Turtle are discussed.