Proxy Caching the Estimates Page Load Delays Roland P. Wooster and Marc Abrams Network Research Group, Computer Science Department, Virginia Tech 元智大學 資訊工程所 系統實驗室 陳桂慧

Outline Related results Removal algorithms –Latency Estimation Algorithm (LAT) –Hybird Algorithm (HYB) Results of experiment

A removal algorithm can optimize one of several measures: –minimize the number of bytes that travel over the Internet –minimize the number of hits to popular Web servers; or –minimize the time that end users wait for a document to load.

Related Results The evidence in Williams et al [WILL96] is that proxy caches can –minimize server loads by using SIZE as a removal algorithm, –minimize wasted network bandwidth by using LFU, and achieve hit rates in the 20-50% range.

Latency Estimation Algorithm (LAT) clat j = (1-ALPHA) clat j + ALPHA s clat cbw j = (1-ALPHA) cbw j + ALPHA s cbw. –Clat j : estimated latency (time) to open a connection to the server –cbw j : estimated bandwidth of the connection –s clat and s cbw : the connection establishment latency and bandwidth for that document are measured di = clat ser(i) + si/cbw ser(i) –ser(i) : the server on which document i resides –si : the document's size –di : LAT selects for replacement the document i with the smallest download time estimate

Hybrid Algorithm (HYB) Motivated by Bolot and Joschka’s algorithm W 1 rtt i + W 2 s i + (W 3 + W 4 s i )/t i –t i : the time since the document was last referenced –rtt i : the time it took to retrieve the document (clat ser(i) + W B /cbw ser(i) )(nref i ** W N )/ s i –nref i : the number of references to document i since it last entered the cache –si : the size in bytes of document i –WB and WN : constants that set the relative importance of the variables cbw ser(i) and nref j

Comparison to Bolot and Hoschka Estimate the quantities on a per-server not a per- document basis. Estimation algorithm uses history of references, rather than just the last reference. HYB use the number of accesses to a document, rather than the last access time

WorkloadModeReplacement Policies Parameter Values 1.VT-CS1onlineLAT,LFU,LRU,SIZE N/A 2.VT-CS2,VT-CS3 online HYB,LFU,LRU,SIZE WB=8Kb, WN=0.9Kb,CONN=2Kb 3.VT-CS2,BU,VT-LIB replay HYB,LFU,LRU,SIZE WB=8Kb, WN=0.9Kb,CONN=2Kb 4. VT-CS2,BU,VT-LIB replay HYB See Table Table 1 : Experiments performed Trial WB WN CONN 18 Kbyte Kbyte 216 Kbyte Kbyte 38 Kbyte Kbyte 416 Kbyte Kbyte Table 2: Weight settings for Exeriment 4 in Table 1.

Variation of download time, workload VT-CS1, Experiment 1.

Variation of download time, workload VT-CS2, Experiment 2

Experiment 3 HRWHRTime BetterWorse Better Worse Better Worse VT-CS2 SIZE*LFU*LRU*SIZE*HYB* LFU* HYB* LRU* LFU* HYB* SIZE LRU BU SIZE *LFU *LRUSIZE *SIZE *LRU * HYB LRU HYB LFU HYB LFU VT-LIB SIZE *LFU* LRULFU HYBLFU HYB* LRU HYB SIZE* SIZE LRU

Experiment 4 HR is always sensitive to weight W B, Time is rarely sensitive to the weights, and WHR is sensitive to W B and sometimes W N.

Conclusions LAT, which estimates download time and replaces documents with shortest download time, HYB, which combines estimates of the connection time of a server and the network bandwidth that would be achieved to the server with document size and number of document accesses since the document entered the cache. Suggest : replacement only on estimated download time (algorithm LAT) is a bad idea.