Case: What does hit-ratio mean? Hierarchical Caching

Caching can be used to reduce the access overhead. Hierarchical caching takes the advantage of the topology of the Internet and dep开发者_如何学编程loys the cache at different ISPs. Suppose the hierarchical topology of the Internet is composed of three-level ISPs, national, regional and local. The hit ratios of searching a webpage at different level of the cache of the ISP are as follows:

ISP Level | Hit Ratio

National      0.2

Regional     0.4

Local           0.6

The Internet delays for retrieving the web pages (a round trip time) from the remote trans-continental Internet website is 200msec, the Internet delay for retrieving the web pages from a National ISP's cache is 10msec, and the Internet delay for other caches are all negligible.

1. What does hit-ratio mean? The bigger the number, the faster to retrieve from cache?

2. What is the probability that a requested website is not found in the hierarchical caching?

3. What is the average delay for retrieving a web page?

Here we assume the web page is very small and the transmission time is negligible.

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1. hit ratio = (# requests satisfied from cache)/ (# requests) more hit from local cache would result in faster loading

2. probability not found in any of cache = (1-0.6) * (1-0.4) * (1-0.2) [not found in local cache * not found in regional cache * not found in national cache

3. average delay= 0.6x0 ms + (1-0.6)x0.4x0 ms + (1-0.6)x(1-0.4)x0.2x10 ms + (1-0.6)x(1-0.4)x(1-0.2)x100 ms

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The hit ratio is the proportion of requests that were satisfied from the cache. That should give you enough information to work out the answers to your other questions.