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Web Caching Location Client Caching Server Caching Proxy Caching ---- widely used form There are many benefit of proxy caching. It reduces network traffic, average latency of fetching Web documents, and the load on busy Web servers.
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An Overview of Proxy An Overview of Proxy Caching AlgorithmsCaching Algorithms
Haifeng WangHaifeng Wang
Web CachingWeb Caching
• Internet traffic• Load on web servers• Access delay
web caching provides an efficient remedy to thelatency problem and network traffic by bringingdocuments closer to clients.
Web Caching LocationWeb Caching Location
• Client Caching• Server Caching• Proxy Caching ---- widely used form
There are many benefit of proxy caching. It reduces network traffic, average latency of fetching Web documents, and the load on busy Web servers.
Web Caching LocationWeb Caching Location
Web Caching Replacement AlgorithmWeb Caching Replacement Algorithm
• effective use of caching, an informative decision has to be made to evict document from the cache in case of cache saturation.
• key to the effectiveness of proxy caches that can yield high hit ratio.
• differ to page replacement. Why?
Characteristic of Web CachingCharacteristic of Web Caching
• Web caching is variable-size caching • The cost of retrieving missed Web
documents from their original servers depends on many factors.
• Web documents are frequently updated • Zipf-like popularity of web documents
Key ParametersKey Parameters
• Frequency Information• Recency Information• Document size• Network cost
There are four key parameters that most proxy replacement policies considering in design
Frequency informationRecency information
Hyper-G
CERALRV
Hybrid
SIZE
SLRU GDSLog2(SIZE)LRU-MINLRU-threshold
LRULFU
Classification of caching policies according traffic information consideration
Replacement Algorithm(1)Replacement Algorithm(1)
1) LRU (Least-Recently-Used) LRU evicts the least recently accessed document first
2) LRU-Threshold It works the same way as LRU except that documents
that are larger than a given threshold are never cached.
3) LRU-MIN LRU-MIN gives preference to small-size documents to
stay in the cache.
Replacement Algorithm(2)Replacement Algorithm(2)
4) LFU (Lease-Frequently-Used) LFU evicts the least frequently accessed document first 5) Hyper-G Hyper-G is an extension of the LFU policy, where ties
are broken according to the last access time. 6) LLF LLF considers the document download time as its
primary and the document with the lowest download time is evicted first
Replacement Algorithm(3)Replacement Algorithm(3)
7) Size Size evicts the largest documents first
8) Log2-Size Log2-Size consider document size as the primary key
according to [log2(size)], large documents are evicted first, using the last access time as a secondary key.
Replacement Algorithm(4)Replacement Algorithm(4)
9) GDS (GreedyDual-Size) The GDS algorithm associates a value H with
each cached page p. H is set to cost/size upon an access to a document. When a replacement needs to be made, the page with the lowest H value, minH, is replaced, and then all pages reduce their H values by minH. If a page is accessed, its H value is restored to cost/size upon an access to a document.
Replacement Algorithm(5)Replacement Algorithm(5)
10) CERA (Cost-Effective-Replacement-Algorithm) CERA use a benefit value (BV) which is assigned to each
object to represent its importance in the cache. When the cache is full, the object with the lowest BV is replaced. BV = (Cost / Size) * Pr + Age
11) Hybrid Hybrid is aimed at reducing the total latency. A function is
computed for each document which is designed to capture the utility of retaining a given document in the cache. The document with the smallest function value is then evicted.
Replacement Algorithm(6)Replacement Algorithm(6)
12) LRV (Lowest-Relative-Value) LRV includes the cost and size in the calculation of a
value that estimates the utility of keeping a document in the cache. It evicts the document with the lowest value. The calculation of the value is based on extensive empirical analysis of trace data.
13) SLRU (Size-Adjust LRU) Document is ordered according to ratio calculated
according frequency, cost and size, it evicts the document with the lowest ratio first.
Performance IssuePerformance Issue
• No conclusion on which algorithm a proxy should use.
• Document size is significance and need to incorporate it in the design of replacement policy.
• Good algorithm adjusts dynamically to changes in the workload characteristics.
