1. A Case for Delay-conscious Caching of Web Documents Peter Scheuermann, Junho Shim, Radek Vingralek Department of Electrical and Computer Engineering Northwestern University, Evanston, IL 60208 Oracle Corporation 400 Oracle Parkway, Box 659413, Redwood Shores, CA 94065 元智大學系統實驗室 宮春富 1999/12/1

2. Outline ⊙ Introduction ⊙ Design ⊙ Experimental Evaluation ⊙ Conclusion

3. Introduction ⊙ The World Wide Web has become the predominant client/server architecture. ⊙ The high response time perceived by Web clients is caused primarily by the long communication delays, although other factors (such as slow service times) also contribute. ⊙ The communication delay to a certain extent can be reduced by buying links with a higher bandwidth and improving the efficiency of communication protocols. ⊙ One of the most effective ways of reducing the communication delay is by employing caching.

4. ⊙ The documents can be cached at clients as is done currently by most Web browsers, or by the Web servers themselves, which is most useful when a server contains many pointers to other servers. ⊙ In an attempt to tune the performance of caching proxies several techniques have been used, such as avoiding caching of documents that originate at nearby servers, and hierarchies of caches. ⊙ Cache replacement algorithms usually maximize the cache hit ratio by attempting to cache the data items which are most likely to be referenced in the future ⊙ We maximizing the cache hit ratio alone does not guarantee the best client response time in the Web environment. Cache Replacement

5. ⊙ We define a new performance metric called delay-savings ratio which generalizes the hit ratio metric by explicitly considering cache miss costs. ⊙ We describe a new cache replacement algorithm LNC-R which maximizes the delay-savings ratio. ⊙ The LNC-R cache replacement algorithm approximates the optimal cache replacement algorithm, ⊙ The design of LNC-R relies on a solid theoretical foundation. New Algorithm

6. 。 is the average delay to fetch document to cache 。 is the total number of references to 。 is the number of references to which were satisfied from the cache 。 average rate of reference to document 。 size of document ---- (1) ---- (2) ---- (3) Parameters and Functions

7. LNC-R Algorithm ⊙ LNC-R ( Least Normalized Cost Replacement ) selects for replacement the least profitable documents. ⊙ LNC-R sorts all documents held in the cache in ascending order of profit and selects the candidates for eviction in the sort order. ⊙ LNC-R simply tries to maximize the delay savings ratio by maximizing the profit from each unit of storage.

8. LNC-R-W3 Algorithm ⊙ Several studies of Web reference patterns show that Web clients exhibit a strong preference for accessing small documents --- (4) --- (5) -- (6)

9. Experimental Setup ⊙ We evaluated the performance of LNC-R-W3 on a client trace collected at Northwestern University. ⊙ We concentrate on two aspects: the dependence of reference rate on document size the correlation between the delay to fetch a document to cache and the size of the document ⊙ Previously published trace analyses show that small files are much more frequently referenced than large files. ⊙ The correlation between the document size and the delay to fetch the document is defined as :

10. Experimental Setup(con’t) ⊙ Some parameters: is the covariance between size and delay is the variance of size is the variance of delay show whether the delay to fetch a document to cache varies across documents of similar size ⊙ We measured the value on our trace as 0.2145, which is relatively low.Therefore, delay-conscious caching is indeed necessary.

11. Experimental Setup(con’t)

12. Performance Metrics ⊙ We also use cache hit ratio (HR) as a secondary metric, which is defined as: is the number of references to document which were satisfied from cache is the total number of references to document ⊙ LRU-MIN exploits the preference of Web clients for accessing small documents. LRU-MIN does not consider the delay to fetch documents to the cache and estimates reference rate to each document using only the time of last reference.

13. Parameter K ⊙ Increasing the value of K improves the reliability of reference rate estimates. ⊙ Large value of K also result in higher spatial overhead to store the reference samples. ⊙ The transition from K=1 to K=2 is particularly sharp, since LNC-R-W3 with K=1 does not need to retain any reference samples after eviction of corresponding documents. ⊙ We conjecture that K should be set to 2 or 3 to obtain the best performance.

14. Parameter K (con’t)

15. Parameter b ⊙ Parameter b determines the skew of dependence of reference rate on document size. ⊙ The higher the value of b, the stronger the preference of clients to access small documents. ⊙ We conjecture that for best performance on most web workloads b should be set between 1 and 2.

16. Parameter b (con’t)

17. Performance Comparison

18. Conclusion ⊙ The main contribution of this paper is to show the importance of delay- conscious cache replacement algorithms for Web document cache management. ⊙ To address the need for design of delay-conscious cache replacement algorithms, we developed a new cache replacement algorithm LNC-R-W3. ⊙ The experimental results indicate that LNC-R-W3 provides consistently better performance than LRU and LRU-MIN.