Estimating Context Switch Cost: A Practitioner's Approach

Robert Kaiser

Workshop on Operating Systems Platforms for Embedded Real-Time applications (OSPERT 2008)
Prague, Czech Republic, July 1 2008

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Summary

Context switch cost is a limiting factor for many real-time systems: In order to improve desired properties such as quick response or low jitter, a system should support high switch rates, but the resulting switch overheads become prohibitive at a certain point. In order to find a favourable tradeoff between real-time properties and the amount of CPU resources that have to be sacrificed to achieve them, it would be necessary to determine the expected context switch cost for a given application and system configuration. This problem, however, is intractable in most practical situations.

In this work, we attempt a practitioner's approach to quantifying context switch cost: Based on a simple model of system behaviour, we present methods to estimate the expected overheads. To validate our methods we compare the behaviour of real systems against that of our model. These experiments also yield parameters along the way which are then used to configure our methods so they reflect realistic scenarios. The overheads can be attributed to individual tasks in the system, so, if there are tasks with different timing requirements, each of them can use its own, specifically adapted estimation. As a demonstration, we integrate our method into a simulation of two different proportional share scheduling algorithms and use it to compute the estimated system overhead as a function of the scheduler's minimal time allocation.

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