Cost/performance perspectives of paging with electronic and electromechanical backing stores

Abstract

An analysis is made of the cost/performance effectiveness of virtual memory systems employing paging disks or drums and those employing electronic backing stores such as charge coupled devices (CCD), MOS shift registers, or bubbles. The analysis is based on a multiprogrammed job stream and a simplified queuing model employing page-fault rates suitable for a variety of multiprogrammed environments. Smaller memory systems and single job programming are also considered. It is shown that memory access plus page transfer time is a critical parameter, in addition to cost. Curves are generated for a variety of job environments in which the cost/performance effectiveness of two-level memory hierarchies employing electronic backing stores is compared to that of systems employing paging drums. Competitive prices per bit are determined for a number of system environments. The analysis clearly demonstrates that in many problem environments, the electronic backing store will replace the traditional paging drum or disk if the cost per bit is significanfly less than the main memory cost per bit as projected. Disk and tape memories can be expected to continue as the dominant memories for bulk storage in the near future because of their already small and diminishing costs per bit.