Virtual Memory

Virtual memory originated as a technique to extend the apparent size of physical memory. By moving pages of memory to and from backing storage (disk or drum) and adjusting virtual to physical memory mappings, a system could allow applications to make use of more memory than existed in the hardware. As applications executed, page-in and page-out traffic would change the portion of virtual memory that was actually resident in physical memory. The ability to change the mapping of virtual to physical addresses insulated applications from the effects of

not having all of their data in memory all the time and allowed their data to occupy different physical pages as needed.

Current operating systems emphasize the use of virtual memory for flexible mapping and sharing of data. Among the facilities that depend on this are mapped files, shared memory, and shared libraries. These features provide enhanced functionality and increased performance to applications. Paging is also provided by these operating systems, but it is less important than the advanced mapping and sharing features supported by virtual memory. Among the operating systems that provide such features are Mach, OSF/1,^[*] System V Release 4,^[**] and SunOS.^[***] These features are an important part of the systems environment into which supercomputers must fit, now and in the future. The use of standard operating systems is important for interoperability and commonality of application development with other hardware (both supercomputers and other systems).

This shift in the use of virtual memory changes the design tradeoffs surrounding its use in supercomputers. For the original paging-oriented use, it was hard to justify incorporating virtual memory mapping hardware. This was because the cycle time of a supercomputer was so short compared with disk access time that paging made little sense. This is still largely the case, as advances in processor speed have not been matched by corresponding advances in disk bandwidth. The need for virtual memory to support common operating systems features changes this tradeoff. Systems without hardware support for virtual memory are unable to support operating systems features that depend on virtual memory. In turn, loss of these features removes support for applications that depend on them and deprives both applications and the system as a whole of the performance improvements gained from these features. This makes it more difficult for such systems to operate smoothly with other systems in the distributed supercomputing environment of the future. The next generation of operating systems assumes the existence of virtual memory; as a result, hardware that does not support it will be at a disadvantage.

[*] OSF/1 is a trademark of the Open Software Foundation.

[**] System V is a trademark of UNIX Systems Laboratories, Inc.

[***] SunOS is a trademark of Sun Microsystems, Inc.