Marshall Kirk McKusick
Michael J. Karels
Computer Systems Research Group
Computer Science Division
Department of Electrical Engineering and Computer Science
University of California, Berkeley
Berkeley, California 94720
With the cost per byte of memory approaching that of the cost per byte for disks, and with file systems increasingly distant from the host machines, a new approach to the implementation of virtual memory is necessary. Rather than preallocating swap space which limits the maximum virtual memory that can be supported to the size of the swap area, the system should support virtual memory up to the sum of the sizes of physical memory plus swap space. For systems with a local swap disk, but remote file systems, it may be useful to use some of the memory to keep track of the contents of the swap space to avoid multiple fetches of the same data from the file system.
The new implementation should also add new functionality. Processes should be allowed to have large sparse address spaces, to map files into their address spaces, to map device memory into their address spaces, and to share memory with other processes. The shared address space may either be obtained by mapping a file into (possibly different) parts of their address space, or by arranging to share ``anonymous memory'' (that is, memory that is zero fill on demand, and whose contents are lost when the last process unmaps the memory) with another process as is done in System V.
One use of shared memory is to provide a high-speed Inter-Process Communication (IPC) mechanism between two or more cooperating processes. To insure the integrity of data structures in a shared region, processes must be able to use semaphores to coordinate their access to these shared structures. In System V, these semaphores are provided as a set of system calls. Unfortunately, the use of system calls reduces the throughput of the shared memory IPC to that of existing IPC mechanisms. We are proposing a scheme that places the semaphores in the shared memory segment, so that machines that have a test-and-set instruction can handle the usual uncontested lock and unlock without doing a system call. Only in the unusual case of trying to lock an already-locked lock or in releasing a wanted lock will a system call be required. The interface will allow a user-level implementation of the System V semaphore interface on most machines with a much lower runtime cost.