Altiok / Melamed Simulation Modeling and Analysis with Arena

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SIMULATION MODELING AND ANALYSIS WITH ARENA T.
Altiok and B. Melamed Chapter 13 Modeling
Computer Information Systems
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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

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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 (QoS)
• and disappointed users and loss of business
• thus, performance evaluation of distributed
  Client/Server and web-based applications has
  become extremely important

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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?

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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)

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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

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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 are 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

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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)

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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

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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

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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

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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

 
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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

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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

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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

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Arena Model of the 2-Tier Client/Server HR System
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Client Nodes Segment

• The client nodes segment models the arrivals of
  requests
• by client request type
• by destination server

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Client Nodes Segment Modules
Dialog box of the Assign module Assign Service
Requested_1
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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
• ((Type1) Request_Size(Service_Requested)
  (Type2) 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

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Communications Network Modules
Dialog box of the Process module Com_Network
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Communications Network Modules (Cont.)
Dialog boxes of the Expression module specifying
message by type (bottom) and their sizes by
service type (top)
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Server Node Segment

• The server node segment models the server node
• It uses a Decide module to dispatch requests to
  the appropriate server process

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Server Node Modules


Dialog box of the Process module Transaction
Monitor
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Server Node Modules (Cont.)

Dialog box of the Decide module Dispatch
Requests for Services in Server Node
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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

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Server Node Modules (Cont.)

Dialog box of the Process module Server Process_1
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HR System Simulation Results

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HR System Simulation Results (Cont.)

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HR System Simulation Results (Cont.)


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Example 3-Tier Client/Server System

• Consider a booksellers 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 clusters 4
  client nodes
• client nodes with traffic flows shown in the
  schematic below

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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)

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Server Process Profiles

• Services are allocated to server processes
  within server nodes
• The table below displays this allocation

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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

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Service Request Profiles

• The table below displays
• request inter-arrival time distributions
• request mix distributions by server node

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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

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Performance Statistics

• We wish to estimate
• response times of service requests by type
• delays in server process queues
• resource utilization

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Arena Model of Request Arrivals and the
Transmission Network Segments


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Request Arrival Modules


Dialog box of the Create module Client Requests
Node 1
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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
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Transmission Network Modules


Dialog box of the Station module Com_Net Entrance
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Transmission Network Modules (Cont.)
Dialog box of the Process module Network
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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
• ((TypeReq)Req_Mes_Size (TypeRep)Rep_Mes_Si
  ze) / 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 BWC0.80, where 0.80 is the
  MTE (Message Transfer Efficiency) parameter

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Transmission Network Modules (Cont.)


Dialog box of the Decide module Routing Map
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Transmission Network Modules (Cont.)

Dialog box of the Route module Message
Transmission
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Transmission Network Modules (Cont.)

Dialog box of the Record module Response Time
Tally
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Transmission Network Modules (Cont.)
Dialog box of the spreadsheet view of the Set
module (bottom) for the members of set Response
Times (top)
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Arena Model of Server Node 1


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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
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Node 1 TPM Modules

Dialog box of the Process module TPM_1
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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
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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
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Node 1 TPM Modules (Cont.)
Dialog box of the Assign module Assign Dest_1
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Node 1 TPM Modules (Cont.)
Dialog box of the Decide module Process Here_1
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Server Process 11 Service Modules

Dialog box of the Process module S_Proc_11
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Server Process Modules


Dialog box of the spreadsheet view of the Queue
module for server-process queues and the
transmission network queue
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Server Node Reply Modules

Dialog box of the Assign module Assign Dest_1
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Server Node Reply Modules (Cont.)

Dialog box of the Route module Route to Network_1
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Booksellers E-business Simulation Results

• The Arena model of the booksellers e-business
  system was simulated for a 1-hour period
  (3,600,000 milliseconds) of operation
• Simulation results are shown next

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Booksellers E-business Simulation Results
(Cont.)

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Booksellers E-business Simulation Results
(Cont.)

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Booksellers E-business Simulation Results
(Cont.)

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Booksellers 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

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Booksellers 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

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Booksellers E-business Simulation Results
(Cont.)

• This booksellers 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