tailieunhanh - THE FRACTAL STRUCTURE OF DATA REFERENCE- P6

THE FRACTAL STRUCTURE OF DATA REFERENCE- P6:For purposes of understanding its performance, a computer system is traditionally viewed as a processor coupled to one or more disk storage devices, and driven by externally generated requests (typically called transactions). Over the past several decades, very powerful techniques have become available to the performance analyst attempting to understand, at a high level, the operational behavior of such systems. | Hierarchical Reuse Model 11 Figure . Distribution of page frame interarrival times. Each curve shows a user or system storage pool at one of 11 surveyed VM installations. Figure assumes that we are interested in a cache management scheme based upon track images. While a scheme of this type usually applies to storage control cache however the caching performed in host processor memory is normally based on smaller units of granularity. For example the minidisk cache facility mentioned earlier manages data in units of one page frame one block of4096 bytes . The unit of granularity used in managing the cache has an important effect upon its interarrival times and miss ratio. To illustrate the impact of cache granularity Figure presents the interarrival times observed at the page level of granularity based upon the same traces as those presented in Figure . Note that at the page level of granularity a reasonable guess for the value of 0 would be roughly half of that obtained assuming cache management based upon tracks thus 0page . 4. VISITS TO MEMORY So far we have observed that data item interarrivals tend to exhibit a fractal structure. The present section shows that this fact beyond being interesting in itself is extremely helpful to the performance analyst. The power of to solve practical problems comes from the simple and mathematically tractable statements that it yields about the time spent in memory during a cache visit. We shall examine closely the structure of such visits which the hierarchical reuse model predicts to be predominately transient. Based upon our analysis of 12 THE FRACTAL STRUCTURE OF DATA REFERENCE cache visits we then calculate the resulting memory requirements. Finally we illustrate these results by assessing the effectiveness of typical cache memories. VISIT DURATION In the previous section we examined the visit to cache ofa track referenced exactly once. We now extend the discussion to an arbitrary visit to cache