If the number of processors exceed the number of jobs in the system, some processors stand idle and overall system efficiency degrades unless jobs are multi-tasked. In an operational environment, if a forecast is required every 4 hours but takes 8 hours on a single processor, then multi-tasking will be useful. Other examples include long running experiments that take too long to complete or when it's impossible to exhaust a monthly computer time allocation using one processor. In environments where multiple processors are dedicated to a single job, it is essential to attain high parallel efficiency otherwise system efficiency and overall throughput suffers. In the past, multi-tasking has not been used in any significant way at GFDL. The reason is that there have always been enough jobs in the system to keep all processors busy. Typically, in the middle of 1997, there were about 30 batch jobs in GFDL's Cray T90 at all times and the system efficiency averaged over 24 hours was about 93%. At the point where a system becomes under saturated, overall system efficiency will drop significantly unless multi-tasking is used.