Shielded Processors: Guaranteeing Sub-millisecond Response in Standard Linux

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Steve Brosky (Nov. 12, 2002)

The following technical paper will be presented at the Fourth Real-Time Linux Workshop, which is being held December 6-7, 2002 in Boston. A link for downloading the full paper appears below this excerpt from the beginning of the paper.

Abstract

There has been much good work in making standard Linux into a more responsive system for real-time applications. The low latency patches and the preemption patches have allowed guarantees on worst case interrupt response time at slightly above a millisecond. These guarantees today are only met when there is no networking or graphics activity in the system. The shielded processor concept dedicates selected processors in a symmetric multiprocessing system for the real-time components of an application. This paper will describe the implementation of shielded processors in RedHawk Linux and the benefits of shielded processors. It will also present the results of benchmarks for both interrupt response and program execution determinism. Interrupt response time guarantees are significantly below one millisecond and can be guaranteed even in the presence of networking and graphics activity.

Introduction

Concurrent Computer Corporation has had more than a decade of experience in utilizing the shielded CPU model for attaining real-time performance under a real-time version of an SVR4 UNIX-based operating system. The key benefit of the shielded CPU approach is that it allows a commodity operating system to be used for applications that have hard real-time deadlines. Commodity operating systems like UNIX or Linux provide a benefit for these applications because they have large numbers of programmers that are familiar with the programming API, and there is a rich set of development tools and other application software available for these operating systems.

Shielded CPUs can provide more deterministic performance because the overhead of the operating system is essentially offloaded onto a subset of CPUs in the system. A shielded CPU is therefore able to provide a more deterministic execution environment. In applying the shielded processor model to Linux, several nuances were found which affected the expected behavior of processes running on shielded CPUs . . .

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(92K PDF)

Steve Brosky is Chief Scientist at Concurrent Computer Systems.

About the Fourth Real-Time Linux Workshop

Further information about the Fourth Real-Time Linux Workshop, including abstracts of many of the other presentations, is available here. The complete set of papers is expected to be made available at LinuxDevices.com for viewing/download following the conference.

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