We discuss the translation lookaside buffer (TLB) consistency problem for multiprocessors, and introduce the Mach shootdown algorithm for maintaining TLB consistency in software. This algorithm has been implemented on several multiprocessors, and is in regular production use. Performance evaluations establish the basic costs of the algorithm and show that it has minimal impact on application performance. As a result, TLB consistency does not pose an insurmountable obstacle to multiprocessors with several hundred processors. We also discuss hardware support options for TLB consistency ranging from a minor interrupt structure modification to complete hardware implementations. Features are identified in current hardware that compound the TLB consistency problem; removal or correction of these features can simplify and/or reduce the overhead of maintaining TLB consistency in software.

The authors describe the design, implementation, and evaluation of the Mach virtual-memory management system. The Mach virtual-memory system exhibits architecture independence, multiprocessor and distributed system support, and advanced functionality. The performance of this virtual-memory system is shown to often exceed that of commercially developed memory management systems targeted at specific hardware architectures.>

This paper describes the design and implementation of virtual memory management within the CMU Mach Operating System and the experiences gained by the Mach kernel group in porting that system to a variety of architectures. As of this writing, Mach runs on more than half a dozen uniprocessors and multiprocessors including the VAX family of uniprocessors and multiprocessors, the IBM RT PC, the SUN 3, the Encore MultiMax, the Sequent Balance 21000 and several experimental computers. Although these systems vary considerably in the kind of hardware support for memory management they provide, the machine-dependent portion of Mach virtual memory consists of a single code module and its related header file. This separation of software memory management from hardware support has been accomplished without sacrificing system performance. In addition to improving portability, it makes possible a relatively unbiased examination of the pros and cons of various hardware memory management schemes, especially as they apply to the support of multiprocessors.

Mach is a multiprocessor operating system being implemented at Carnegie-Mellon University. An important component of the Mach design is the use of memory objects which can be managed either by the kernel or by user programs through a message interface. This feature allows applications such as transaction management systems to participate in decisions regarding secondary storage management and page replacement.