Code to implement network protocols can be either inside the kernel of an operating system or in user-level processes. Kernel-resident code is hard to develop, debug, and maintain, but user-level implementations typically incur significant overhead and perform poorly. The performance of user-level network code depends on the mechanism used to demultiplex received packets. Demultiplexing in a user-level process increases the rate of context switches and system calls, resulting in poor performance. Demultiplexing in the kernel eliminates unnecessary overhead. This paper describes the packet filter, a kernelresident, protocol-independent packet demultiplexer.Individual user processes have great flexibility in selecting which packets they will receive. Protocol implementations using the packet filter perform quite well, and have been in production use for several years.

Abstract:"lights" is a system under development for the inter pretation of simple moving light displays of jointed objects against a stationary background. The displays being studied differ from those examined by previous researchers in that (1) objects are represented by a relatively small number of points, (2) objects are not rigid, and (3) the viewing geometry is such that highly varying degrees of perspective distortion occur. An algorithm is presented which segments the points of an MLD of a wire-frame man into body parts. The relationship of this algorithm to preVious theories of MLD perception and actual human performance ;s discussed.

We discuss the translation lookaside buffer (TLB) consistency problem for multiprocessors, and introduce the Mach shootdown algorithm for maintaining TLJ3 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.

We have improved the performance of the Mach 3.0 operating system by redesigning its internal thread and interprocess communication facilities to use continuations as the basis for control transfer. Compared to previous versions of Mach 3.0, our new system consumes 85% less space per thread. Cross-address space remote procedure calls execute 14% faster. Exception handling runs over 60% faster.In addition to improving system performance, we have used continuations to generalize many control transfer optimizations that are common to operating systems, and have recast those optimizations in terms of a single implementation methodology. This paper describes our experiences with using continuations in the Mach operating system.

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. This paper explores the goals, design and implementation of Mach and its external memory management facility. The relationship between memory and communication in Mach is examined as it relates to overall performance, applicability of Mach to new multiprocessor architectures, and the structure of application programs.