Ó 1998 Menascé & Almeida. All Rights Reserved.1 Part VIII Web Performance Modeling (Book, Chapter 10)

Ó 1998 Menascé & Almeida. All Rights Reserved.2 Learning Objectives Characteristics of Web workloads: – burstiness – heavy-tailed distributions See some meaningful examples of how QN models apply to WEB server and client architectures: – Client-side Models –Server-side Models

Ó 1998 Menascé & Almeida. All Rights Reserved.3 New Phenomena in the Internet and WWW Self-similarity - a self-similar process looks bursty across several time scales. Heavy-tailed distributions in workload characteristics, that means a very large variability in the values of the workload parameters.

Ó 1998 Menascé & Almeida. All Rights Reserved.4 WWW Traffic Burst Bytes Chronological time (slots of 1000 sec)

Ó 1998 Menascé & Almeida. All Rights Reserved.5 Incorporating New Phenomena in the Workload Characterization Burstiness Modeling burstiness in a given period can be represented by a pair of parameters (a,b) –a is the ratio between the maximum observed request rate and the average request rate during the period. –b is the fraction of time during which the instantaneous arrival rate exceeds the average arrival rate. (a = 6, b = 5%) => Web server throughput degraded by 12 to 20%

Ó 1998 Menascé & Almeida. All Rights Reserved.6 Burstiness Modeling Consider an HTTP LOG composed of L requests to a Web server.  : time interval during which the requests arrive : average arrival rate, = L /  The time interval  is divided into n equal subintervals of duration  / n called epochs Arr(k): number of HTTP requests that arrive in epoch k k : arrival rate during epoch k

Ó 1998 Menascé & Almeida. All Rights Reserved.7 Burstiness Modeling Arr + : total number of HTTP requests that arrive in epochs in which k > b = (number of epochs for which k > ) / n above-average arrival rate, + = Arr + / (b*  ) a = + / = Arr + / (b*L)

Ó 1998 Menascé & Almeida. All Rights Reserved.8 Burstiness Modeling: an example Example: Consider that 19 requests are logged at a Web server at instants: What are the burstiness parameters?

Ó 1998 Menascé & Almeida. All Rights Reserved.9 Burstiness Modeling: an example Let us consider the number of epochs n=21 Each epoch has a duration of  / n = 31 /21 = 1.48 The average arrival rate = 19/31 = req./sec The number of arrivals in each of the 21 epochs are: 1, 0, 3, 0, 4, 0, 1, 0, 4, 0, 1, 0, 0, 1, 0, 0, 0, 0, 0, 0, 4 Thus, 1 = 1/1.48 = 0.676, that exceeds the avg. = In 8 of the 21 epochs, k exceeds b = 8 / 21 = a = Arr + / (b*L) = 19 / (0.381 * 19) = 2.625

Ó 1998 Menascé & Almeida. All Rights Reserved.10 The Impact of Burstiness As shown in some studies, the maximum throughput of a Web server decreases as the burstiness factors increase. How can we represent in performance models the effects of burstiness? We know that the maximum throughput is equal to the inverse of the maximum service demand or the service demand of the bottleneck resource.

Ó 1998 Menascé & Almeida. All Rights Reserved.11 The Impact of Burstiness To account for the burstiness effect, we write the service demand of the bottleneck resource as: –D = D f +   b –D f is the portion of the service demand that does not depend on burstiness –  is a factor used to inflate the service demand according to burstiness factor b. It is given by: –  = (U 1 /X U 2 /X 2 0 )/(b 1 -b 2 ) –The measurement interval is divided into 2 subintervals  1 and  2 to obtain U i, X i 0, and b i

Ó 1998 Menascé & Almeida. All Rights Reserved.12 The Impact of Burstiness: an example Consider the HTTP LOG of the previous slides. During 31 sec in which the 19 requests arrived, the CPU was found to be the bottleneck. What is the burstiness adjustment that should be applied to the CPU service demand to account for the burstiness effect on the performance of the Web server? Each subinterval lasts 31/2 = 15.5 sec The number of requests during each subinterval is 14 and 5, respectively. The measured CPU utilization in each interval was 0.18 and 0.06

Ó 1998 Menascé & Almeida. All Rights Reserved.13 The Impact of Burstiness: an example (2) The throughput in each interval is: –X 1 0 = 14/15.5 = –X 2 0 = 5/15.5 = Using the previous algorithm: –b 1 = 0.273, b 2 = –  = (0.18/ /0.323)/( ) = –the adjustment factor is:  × b = × = Assuming Df = 0.02 sec, we are able to calculate the maximum server throughput as a function of the burstiness factor (b).

Ó 1998 Menascé & Almeida. All Rights Reserved.14 The Impact of Burstiness: an example (2)

Ó 1998 Menascé & Almeida. All Rights Reserved.15 Incorporating New Phenomena in the Workload Characterization Accounting for Heavy Tails in the Model Due to the large variability of the size of documents, average results for the whole population would have very little statistical meaning. Categorizing the requests into a number of classes, defined by ranges of document sizes, improves the accuracy and significance of performance metrics. Multiclass queuing network models, with classes associated with requests for docs of different size.

Ó 1998 Menascé & Almeida. All Rights Reserved.16 Accounting for Heavy Tails: an example (1) The HTTP LOG of a Web server was analyzed during 1 hour. A total of 21,600 requests were successfully processed during the interval. Let us use a multiclass model to represent the server. There are 5 classes in the model, each corresponding to the 5 file size ranges.

Ó 1998 Menascé & Almeida. All Rights Reserved.17 Accounting for Heavy Tails: an example (2) File Size Distributions.

Ó 1998 Menascé & Almeida. All Rights Reserved.18 Accounting for Heavy Tails: an example (3) The arrival rate for each class r is a fraction of the overall arrival rate = 21,600/3,600 = 6 requests/sec. 1 = 6  0.25 = 1.5 req./sec 2 = 6  0.40 = 2.4 req./sec 3 = 6  0.20 = 1.2 req./sec 4 = 6  0.10 = 0.6 req./sec 5 = 6  0.05 = 0.3 req./sec

Ó 1998 Menascé & Almeida. All Rights Reserved.19 An Intranet Example with Proxy Cache Server Clients Proxy Server External Web Servers router (50  sec/packet) Internet LAN (10 Mbps Ethernet)......

Ó 1998 Menascé & Almeida. All Rights Reserved.20 An Intranet Example (cont’d) Cache Hit routerLAN cpu disk proxy cache server incoming link outgoing link ISP Internet web server clients

Ó 1998 Menascé & Almeida. All Rights Reserved.21 An Intranet Example (cont’d) Cache Miss routerLAN cpu disk proxy cache server incoming link outgoing link ISP Internet web server clients

Ó 1998 Menascé & Almeida. All Rights Reserved.22

Ó 1998 Menascé & Almeida. All Rights Reserved.23 largest service demand throughput in HTTP req/sec

Ó 1998 Menascé & Almeida. All Rights Reserved.24

Ó 1998 Menascé & Almeida. All Rights Reserved.25 Intranet Example Increasing the Bandwidth of the link to the ISP

Ó 1998 Menascé & Almeida. All Rights Reserved.26 A Complete Web Server Example Web server 10 Mbps Ethernet router (50  sec/packet) T1 link ISPInternet 6 HTTP req/sec

Ó 1998 Menascé & Almeida. All Rights Reserved.27 A Complete Web Server Example (cont’d) incoming link outgoing link routerLAN cpu disk Web server

Ó 1998 Menascé & Almeida. All Rights Reserved.28 A Complete Web Server Example (cont’d) Workload

Ó 1998 Menascé & Almeida. All Rights Reserved.29 A Web Server Example (cont’d)

Ó 1998 Menascé & Almeida. All Rights Reserved.30 A Web Server Example (cont’d)

Ó 1998 Menascé & Almeida. All Rights Reserved.31 A Web Server Example (cont’d)

Ó 1998 Menascé & Almeida. All Rights Reserved.32 A Web Server Example (cont’d)

Ó 1998 Menascé & Almeida. All Rights Reserved.33 A Web Server Example (cont’d)

Ó 1998 Menascé & Almeida. All Rights Reserved.34 major contribution to service demand A Web Server Example (cont’d)

Ó 1998 Menascé & Almeida. All Rights Reserved.35 Part VIII: Summary New Phenomena in the Internet and WWW Burstiness Heavy-tailed distributions Client-side Models no cache proxy server case using a cache proxy server Server-side Models Single Web server Mirrored Web servers