Single User versus Multiple Users

A second difference between mainstream computers and most massively parallel systems is the number of simultaneous users or applications that can be supported. Front-end/back-end massively parallel systems typically allow only a few users to be using the back end at one time. This style of resource scheduling is characterized by batch operation or "sign-up sheets." This is an adequate model for systems that will be dedicated to a single application but is a step backward in productivity for multiuser environments when compared with mainstream computers that support timesharing operating systems. As has been known for many years, timesharing provides a means to more highly utilize a computer system. Raw peak MFLOPS (i.e., millions of floating-point operations per second) are not as important as the number of actual FLOPS that are used in real programs; unused FLOPS are wasted FLOPS. The real measure of system effectiveness is the number of solutions per year that the user base can achieve.

Early in BBN's use of the Butterfly I computer, we realized that flexible multiuser access was required in order to get the most productivity out of the system. The ability to cluster together an arbitrary number of processors was added to the ChrysalisÔ operating system (and later carried forward into Mach 1000 and nX), providing a simple but powerful "space-sharing" mechanism to allow multiple users to share a system. However, in order to eliminate the front end and move general computing and software development activities onto the system, real time-sharing capabilities were needed to enable processors to be used by multiple users. The Mach 1000 and nX operating systems provide true time sharing.