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Chapter: Embedded Systems

Evaluating Operating System Performance

The scheduling policy does not tell us all that we would like to know about the performance of a real system running processes.



The scheduling policy does not tell us all that we would like to know about the performance of a real system running processes. Our analysis of scheduling policies makes some simplifying assumptions:


■We have assumed that context switches require zero time.


We also assumed that processes don’t interact, but the cache causes the execution of one program to influence the execution time of other programs. The techniques for bounding the cache-based performance of a single program do not work when multiple programs are in the same cache. Many real-time systems have been designed based on the assumption that there is no cache present, even though one actually exists. This grossly conservative assumption is made because the system architects lack tools that permit them to analyze the effect of caching. Since they do not know where caching will cause problems, they are forced to retreat to the simplifying assumption that there is no cache. The result is extremely over designed hardware, which has much more computational power than is necessary. However, just as experience tells us that a well-designed cache provides significant performance benefits for a single program, a properly sized cache can allow a microprocessor to run a set of processes much more quickly. By analyzing the effects of the cache, we can make much better use of the available hardware.


Li and Wolf[Li99] developed a model for estimating the performance of multiple processes sharing a cache. In the model, some processes can be given reservations in the cache, such that only a particular process can inhabit a reserved section of the cache; other processes are left to share the cache. We generally want to use cache partitions only for performance-critical processes in cache reservations are wasteful of limited cache space. Performance is estimated by constructing a schedule, taking in to account not just execution time of the processes but also the state of the cache. Each process in the shared section of the cache is modeled by a binary variable: 1 if present in the cache and 0 if not. Each process is also characterized by three total execution times: assuming no caching, with typical caching, and with all code always resident in the cache. The always-resident time is unrealistically optimistic, but it can be used to find a lower bound on the required schedule time. During construction of the schedule, we can look at the current cache state to see whether the no-cache or typical-caching execution time should be used at this point in the schedule. We can also update the cache state if the cache is needed for another process. Although this model is simple, it provides much more realistic performance estimates than assuming the cache either is none x is tent or is perfect. Example 6.9 shows how cache management can improve CPU utilization. Going in to alow-power mode takes time; generally, the more that is shutoff, the longer the delay incurred during restart. Because power-down and power-up are not free, modes should be changed carefully. Determining when to switch into and out of a power-up mode requires an analysis of the overall system activity.


■Avoiding a power-down mode can cost un necessary power.


■Powering down too soon can cause severe performance penalties.


Re-entering run mode typically costs a considerable amount of time.


A straight forward method is to power up the system when a request is received. This works as long as the delay in handling the request is acceptable. A more sophisticated technique is predictive shutdown. The goal is to predict when the next request will be made and to start the system just before that time, saving their quest or the start-up time. In general, predictive shutdown techniques are probabilistic—they make guesses about activity patterns based on a proba- bilistic model of expected behavior. Because they rely on statistics, they may not always correctly guess the time of the next activity.


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