SIMULATION MODELING AND ANALYSIS WITH ARENA T Altiok

SIMULATION MODELING AND ANALYSIS WITH ARENA T. Altiok and B. Melamed Chapter 13 Modeling Computer Information Systems Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 1

Computer Networks • Computer networks consist of • computer nodes, called hosts • transmission lines, called communications links • System attributes • usability is the ease with which a user can learn to operate, prepare inputs for, and interpret outputs from a system or component • flexibility is the ease with which a system or component can be modified for use in applications or environments other than those for which it was specifically designed • interoperability is the ability of two or more systems or components to exchange information and use it • scalability is the ease with which a system or component can be modified to fit larger problems Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 2

Client/Server Networks • A common computer-network architecture is the Client/Server configuration • a client is a host playing the role of a requestor of services • a server is a host providing the requisite service • the two interact over a communications network • Client/Server architecture is versatile, message-based and modular • Example: web services implemented in the Client/Server architecture • the Internet/Web infrastructure has placed increasingly larger demands on servers as well as the networks connecting them • a crowded Internet can slow the delivery of responses (latency) to customer queries and transactions, resulting in poor quality of service (Qo. S) and disappointed users and loss of business • thus, performance evaluation of distributed Client/Server and web-based applications has become extremely important Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 3

Capacity of Web-Based Services • As the Web keeps growing, applications will have to be designed with capacity issues in mind • For example, businesses must often formulate and answer “what-if” questions such as the following: • what is the impact of a 20% increase in customer transactions on the response time of the system? • how would the response time be changed if part of the workload were shifted from one hard disk to another? • what good would it do to move a database to a remote host? Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 4

Client/Server System Architectures • In a Client/Server system, • a client computer sends requests for specific services • a server computer listens to client requests, processes them, and sends the response back to the client • Client/Server system architectures: • In a two-tier Client/Server architecture, the client side utilizes a user interface that permits direct communication with the server host • In a three-tier Client/Server architecture, a middle tier is added between the client side and the server side (typically for transaction processing, or monitoring) Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 5

Three-Tier Client/Server Systems • A schematic representation of a three-tier Client/Server system is shown below • The transaction processing (TP) monitor receives the transactions, queues them up for service, manages their path to completion, and finally, sends the reply back to the client Client Nodes Server Nodes TP Monitor Transmission Network Request Reply Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 6

Message-Based Communications • Clients and servers communicate via messages • a client request is a message requesting a specific service from a server (e. g. , a database service) • a server reply is packaged as another message, and sent back to the client (e. g. , a database result set) • each message has origination and destination information, as well as a message body • thus, there as many request types as there are services • Example: Online banking • users query their account status at an ATM (Automatic Teller Machine) or access a secure Web site for the same • example: “show me the balance of my account” • example: “show me the last 15 cashed checks” • the reply is printed on paper or displayed on a computer monitor Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 7

Client Hosts • Client hosts are computers that issue service requests initiated by computer programs or by customers connected to the host • A schematic representation of a client host is shown below • a generated request undergoes local preprocessing at the client host • it is then is transmitted to a server host usually over a communications network (unless the client and server are on the same host) • on service completion, the reply is transmitted back to the client host and undergoes local post-processing, thereby completing the request/reply cycle • The most critical performance measure is response time • the total time elapsed from the moment of submitting a request (just before local preprocessing) up until the reply becomes available (just after local post-processing) Client Process Local CPU Message Traffic CPU Queue Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 8

Server Hosts • Server hosts perform the bulk of request processing • A schematic representation of a server host is shown below • in its simplest form, a server node consists of one or more processors (CPUs) and a number of server processes that actually execute the code for the services requested by clients • the server processes and the CPU have each a message queue in front of them • each server process is enabled to perform a specific set of services Transaction Monitor CPU Message Traffic Server Processes Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 9

Communications Networks • Nodes in the transmission network • consist of hardware and software • interoperate and interact via transmission protocols • A schematic representation of an abstraction of a single-server queue of a transmission node is shown below • a finite buffer precedes a transmission server operating at a given transmission speed (rate), known as the bandwidth capacity (BWC) • as not all of this capacity is available to messages, the message transfer efficiency (MTE) is the ratio of available BWC to total BWC (typically in the range 60%-80%) • message transmission time is proportional to message size, for example, a BWC of 10 Mbps (equivalent to 1250 bytes/millisecond) at 70% MTE transmits a 1024 -byte message in 1024 / (1250 * 0. 7) = 1. 17 milliseconds Requests BWC = 1250 bytes/ms Replies Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 10

Example: 2 -Tier Client/Server System • Consider a Human Resources (HR) application, configured as a two-tier Client/Server system, consisting of • 4 client nodes • 1 server node • traffic flows as shown in the schematic below Server Node Client Nodes Monitor Network Requests Replies Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 Server Processes 11

HR Transactions Attributes • The HR system supports a number of services (request types) • services relate to company employee records • a database server maintains an HR database (HRDB) of employee-related information • the table below lists the attributes of supported HR services Request Type No. Type Name Service Requested Request Size (in bytes) Reply Size (in bytes) 1 Add employee 1024 256 2 Delete employee 1024 512 3 Find employee 256 512 4 Search for employees 512 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 12

HR Transactions Attributes (Cont. ) • Service requests in the table belong to the following types: 1. a request of type Add adds a new employee with all his/her information (name, address, phone, expertise, etc. ) to the HRDB in a message size of 1024 bytes, and the system returns a confirmation message of size 256 bytes. 2. a request of type Delete deletes an employee entry (with all related information) from the HRDB, and the system returns a confirmation message of size 512 bytes 3. a transaction of type Find finds the complete employee information in the HRDB, based on partial data (e. g. , the name alone), and the system returns a reply message of size 512 bytes 4. a transaction of type Search searches the HRDB for all employees with given characteristics (e. g. , same expertise, same department, etc. ), and the system returns a reply of random size, whose (discrete) distribution is given in the preceding table Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 13

HR Client Request Arrival Profiles • To characterize the traffic patterns in the system, we specify the arrival processes of all request types at each client node • the table below specifies client-side service request arrival profiles Client Node Request Inter-arrival Distribution (in messages / millisecond) Request Type Distribution 1 2 3 4 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 14

HR Server Profiles • To characterize the service in the system, we specify the servers and the time it takes to execute a service on them (elapsed times) • there are two server processes, called and , where the former provides services of types Add and Delete, and the latter provides services of types Find and Search. • the table below specifies server-side profiles of elapsed times Request Type No. Request Type Name Elapsed Time (in milliseconds) 1 Add 10 2 Delete 8 3 Find 15 4 Search Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 15

Arena Model of the 2 -Tier Client/Server HR System Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 16

Client Nodes Segment • The client nodes segment models the arrivals of requests • by client request type • by destination server Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 17

Client Nodes Segment Modules Dialog box of the Assign module Assign Service Requested_1 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 18

Network Segment • The network segment provides a simplified model of the entire communications network that performs message transmission • It consists of • a Process module to model transmission delay • a Decide module to model transaction routing • The Expression field specifies the delay time in the Process module (to be shown next) as the expression ((Type==1) * Request_Size(Service_Requested) + (Type==2) * Reply_Size(Service_Requested)) / ( 0. 7 * 200) where • Request_Size is used in the expression above to retrieve the associated request size as function of the service type • Reply_Size is used in the expression above to retrieve the associated reply size as function of the service type Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 19

Communications Network Modules Dialog box of the Process module Com_Network Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 20

Communications Network Modules (Cont. ) Dialog boxes of the Expression module specifying message by type (bottom) and their sizes by service type (top) Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 21

Server Node Segment • The server node segment models the server node • It uses a Decide module to dispatch requests to the appropriate server process Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 22

Server Node Modules Dialog box of the Process module Transaction Monitor Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 23

Server Node Modules (Cont. ) Dialog box of the Decide module Dispatch Requests for Services in Server Node Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 24

Server Node Modules (Cont. ) • The Decide module dispatches requests to a server process based on the outcome of an N-way condition as follows: • the Type attribute is checked first: if Type == 2, then this entity is a reply; otherwise, if Type == 1, then the entity is a request transaction • in the latter case (Type == 1), the Service_Requested attribute is checked next: if Service_Requested == 1 (Add request), then the transaction is dispatched to server process 1 if Service_Requested == 2 (Delete request), then the transaction is dispatched to server process 1 if Service_Requested == 3 (Find request), then the transaction is dispatched to server process 2 if Service_Requested == 4 (Search request), then the transaction is dispatched to server process 2 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 25

Server Node Modules (Cont. ) Dialog box of the Process module Server Process_1 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 26

HR System Simulation Results Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 27

HR System Simulation Results (Cont. ) Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 28

HR System Simulation Results (Cont. ) Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 29

Example: 3 -Tier Client/Server System • Consider a bookseller’s e-business network, configured as a three-tier Client/Server system, consisting of • a cluster of 2 server nodes (bold circles), each hosting 2 server processes with FIFO priority queues and providing multiple services • a transmission network that links the server nodes (yellow rectangle) • transaction processing (TP) middleware (purple circles) that dynamically balances the queue sizes of server processes in the cluster’s 4 client nodes • client nodes with traffic flows shown in the schematic below Client Nodes Server Node 1 Server Cluster Transmission Network Server Node 2 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 Client Nodes 30

Service Profiles • Server processes provide a number of services, each with a random elapsed (service) time • The table below displays • elapsed-time distributions by services • service priorities (lower priority numbers indicate higher priorities) Service No. Service Name Service Priority Elapsed Time at Server Node 1 (in milliseconds) Elapsed Time at Server Node 2 (in milliseconds) 1 2 3 4 Best Sellers (BS) New Releases (NR) Book Search (BE) View Cart (VC) Go to Cashier (GC) 5 4 3 2 1 Unif(10, 20) Unif(13, 23) Unif(4, 14) Unif(2, 12) Unif(1, 11) Unif(18, 28) Unif(21, 31) Unif(10, 20) Unif(7, 17) 5 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 Unif(5, 15) 31

Server Process Profiles • Services are allocated to server processes within server nodes • The table below displays this allocation Client Node / Server Process (SP) Service Name Node 1 / SP_11 BS, NR , BE Node 1 / SP_12 VC, GC Node 2 / SP_21 BS, NR Node 2 / SP_22 BE, VC, GC Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 32

TP Monitor Operation • A number of client nodes are connected to each server node • however, when a service request arrives at a server node, it is not necessarily processed there • rather, the TP monitor decides where it would be processed by selecting a server process (anywhere in the system) with the minimal queue size • In reality, dynamic load balancing aims to equalize the queue workload (the total service time needed to serve all transactions in the queue, usually excluding the one in service) • however, for the sake of modeling simplicity, this model will balance only queue sizes Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 33

Service Request Profiles • The table below displays • request inter-arrival time distributions • request mix distributions by server node Node 1 Distribution of Request Inter-Arrival Times (in milliseconds) Node 2 Expo(1/12) Expo(1/18) Distribution of Requests in Arrival Streams (Percentages in Mix) Best Sellers (BS) New Releases (NR) Book Search (BE) View Cart (VC) Go to Cashier (GC) 0. 20 0. 15 0. 30 0. 05 0. 30 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 0. 25 0. 07 0. 20 34

Reply Profiles • Once a service request completes processing, a reply is sent back to the client node • Reply messages have • a random size of 1024 bytes or 8096 bytes • the reply size distribution is Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 35

Performance Statistics • We wish to estimate • response times of service requests by type • delays in server process queues • resource utilization Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 36

Arena Model of Request Arrivals and the Transmission Network Segments Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 37

Request Arrival Modules Dialog box of the Create module Client Requests Node 1 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 38

Request Arrival Modules (Cont. ) • Generated request transaction entities proceed to a corresponding Assign module (Request Attributes 1 or Request Attributes 2) to assign values to attributes, as illustrated below Dialog box of the Assign module Request Attributes 2 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 39

Transmission Network Modules Dialog box of the Station module Com_Net Entrance Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 40

Transmission Network Modules (Cont. ) Dialog box of the Process module Network Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 41

Transmission Network Modules (Cont. ) • Since transmission times are size dependent, transmission delays are computed as an expression utilizing the message size attributes Req_Mes_Size and Rep_Mes_Size in the Expression field above • The expression is ((Type==Req)*Req_Mes_Size + (Type==Rep)*Rep_Mes_Size) / BWC / 0. 8 where • the message size is selected by the message type • the requisite service time is obtained by dividing the message size by the effective bandwidth capacity BWC*0. 80, where 0. 80 is the MTE (Message Transfer Efficiency) parameter Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 42

Transmission Network Modules (Cont. ) Dialog box of the Decide module Routing Map Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 43

Transmission Network Modules (Cont. ) Dialog box of the Route module Message Transmission Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 44

Transmission Network Modules (Cont. ) Dialog box of the Record module Response Time Tally Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 45

Transmission Network Modules (Cont. ) Dialog box of the spreadsheet view of the Set module (bottom) for the members of set Response Times (top) Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 46

Arena Model of Server Node 1 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 47

Server Node 1 Modules • The Decide module in server node 1 • examines incoming transactions • separates those arriving directly from client nodes from those dispatched from server node 2 by the TP monitor, since the logic sequences of transactions depend on the origination node Dialog box of the Decide module Is It a Dispatched Job_1 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 48

Node 1 TPM Modules Dialog box of the Process module TPM_1 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 49

Node 1 TPM Modules (Cont. ) • In the Search Condition field below, SP_Queue_Occupancy is an expression, to be defined next Dialog box of the Search module BB_1 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 50

Node 1 TPM Modules (Cont. ) • The expression SP_Queue_Occupancy below is a vector of 4 rows, each holding a separate expression which are functions of sever process queues • each row returns the queue size of a specific server process, provided the service request of a transaction belongs to a subset of services • otherwise, it returns a large number (1000), larger than any queue size Dialog box of the spreadsheet view of the Expression module (bottom) for the expression SP_Queue_Occupancy Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 51

Node 1 TPM Modules (Cont. ) Dialog box of the Assign module Assign Dest_1 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 52

Node 1 TPM Modules (Cont. ) Dialog box of the Decide module Process Here_1 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 53

Server Process 11 Service Modules Dialog box of the Process module S_Proc_11 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 54

Server Process Modules Dialog box of the spreadsheet view of the Queue module for server-process queues and the transmission network queue Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 55

Server Node Reply Modules Dialog box of the Assign module Assign Dest_1 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 56

Server Node Reply Modules (Cont. ) Dialog box of the Route module Route to Network_1 Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 57

Bookseller’s E-business Simulation Results • The Arena model of the bookseller’s e-business system was simulated for a 1 -hour period (3, 600, 000 milliseconds) of operation • Simulation results are shown next Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 58

Bookseller’s E-business Simulation Results (Cont. ) Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 59

Bookseller’s E-business Simulation Results (Cont. ) Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 60

Bookseller’s E-business Simulation Results (Cont. ) Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 61

Bookseller’s E-business Simulation Results (Cont. ) • Observe that the network resource Net_Server is busy some 85% off the time • average network delay of around 13 milliseconds across all transactions • these delays are quite insignificant, and entirely acceptable • Observe that the utilizations of server processes SP_11 through SP_22 vary widely • each first server process at each server node (SP_11 and SP_21) is much busier (over 58% utilization) than the second process in that node (SP_21 and SP_22) • this is due to the fact that services with longer elapsed times are allocated to the first server process at each server node Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 62

Bookseller’s E-business Simulation Results (Cont. ) • Observe that delays in server process queues are fairly proportional to the corresponding server process utilization • the average delay in the queue of server process SP_21 is over 21 milliseconds • the average delay in in the queue of server process SP_22 is a mere 2. 65 milliseconds • Observe that these delays directly affect the response times of customer requests • for example, BS and NR services have longer elapsed times than the other services • consequently, the server processes providing these services are the busiest, and the corresponding response times (about 73 and 63 milliseconds, respectively) are significantly larger than those of the other services Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 63

Bookseller’s E-business Simulation Results (Cont. ) • This bookseller’s e-business example clearly shows the importance of workload allocation in Client/Server systems, and especially in mission-critical applications • Delays in server-process queues and transmission network queues constitute major components of the response times • clearly, it is desirable to minimize these delays in order to reduce the overall response times • A more balanced allocation of workload to servers would help reduce server-process delays, and in turn reduce response times • On the other hand, to improve the transmission network performance, one may have to upgrade the network by adding additional equipment (routers, switches, etc. ) to reduce network delays • this is a far more expensive proposition than workload balancing Altiok / Melamed Simulation Modeling and Analysis with Arena Chapter 13 64