Synchronization errors in concurrent programs are notoriously difficult to find and correct. Deadlock, partial deadlock, and unsafeness are conditions that constitute such errors.A model of concurrent semaphore programs based on multidimensional, solid geometry is presented. While previously reported geometric models are restricted to two-process mutual exclusion problems, the model described here applies to a broader class of synchronization problems. The model is shown to be exact for systems composed of an arbitrary, yet fixed number of concurrent processes, each consisting of a straight line sequence of arbitrarily ordered semaphore operations.

The MARUTI operating system is designed to support real-time applications on a variety of hardware systems. The kernel supports objects as primitive entities, and provides a communication mechanism that allows transparent distribution in networked systems. Fault tolerance is provided through replication and consistency-control mechanisms. Most importantly, MARUTI supports guaranteed-service scheduling, in which jobs that are accepted by the system are verified to satisfy general time constraints.Guaranteed-service scheduling means that, given a job with a set of service requirements and time constraints, the system automatically verifies the schedulability of each component of the job with respect to the job's constraints and those of other jobs in the system. These time constraints include those that govern interrupt processing, which allows the MARUTI approach to succeed where tess rigorous approaches do not. The result is that MARUTI applications can be executed in a predictable, deterministic fashion.

The speed of mass storage devices has a significant impact on the performance of computer systems. The speed that is realized on a particular mass storage device, however, depends heavily on how that device is used. Operating systems, such as the UNIX time‐sharing system, use layout policies and head‐scheduling disciplines that are designed to work well on average. Numerous studies have shown that disk access patterns exhibit a high degree of locality. Further, studies have shown that these access patterns do not necessarily correspond to the usage patterns anticipated by the system's designers, and that head scheduling is used infrequently enough that it has limited effect. This paper describes the design, implementation, and use of a disk subsystem that adaptively corrects the disparity between expected access patterns and actual access patterns by reorganizing disk data. A representative experiment that demonstrates the resulting performance improvement is presented.

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