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Visual basic 2005Visual basic 2005IX. MULTITHREADING

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MultithreadingMultithreadingObjectives

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Visual Basic 2005

In this chapter you will learn:•What threads are and why they are useful.•How threads enable you to implement and manage concurrent activities.•The life cycle of a thread.•Thread priorities and scheduling.•To create and execute Threads.•Thread synchronization.•What producer/consumer relationships are and how they are implemented with multithreading.•To ensure that the User Interface thread performs all GUI modifications that are initiated by other threads in a Windows application.

OverviewOverview

It would be nice if we could perform one action at a time and perform it well

The human body performs a great variety of operations in parallel or concurrently

Ironically, most programming languages do not enable programmers to specify concurrent activities

Rather, programming languages generally provide only a simple set of control statements that enable programmers to perform one action at a time

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OverviewOverview

The FCL provides concurrency primitivesMultithreading is the ability of an

application to contain "threads of execution," each of which designates a portion of a program that may execute concurrently with other threads

Multithreading is available to all .NET programming languages

The FCL's multithreading capabilities are located in the System.Threading namespace

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Life Cycle of a ThreadLife Cycle of a Thread

At any time, a thread is said to be in one of several thread states

Two classes critical for multithreaded applications are Thread and Monitor, both of the System.Threading namespace

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Thread life cycle in UML state diagram:

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Unstarted

Running

SuspendedWaitSleepJoin Stopped BlockedStart

Pulse, PulseAll, Interrupt

Sleep interval expires

Wait, Sleep, JoinRe

sume

Susp

end

Abort

CompleteLock available,

I/O completion

Lock unavailable,

Issue I/O request


A Thread object begins its life cycle in the Unstarted state

The thread remains in the Unstarted state until the Thread's Start method is called, which places the thread in the Running state

A Running thread enters the Stopped (or Aborted) state when its ThreadStart delegate terminates

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A thread is considered to be Blocked if it is unable to use a processor, even if one is available

There are three ways in which a Running thread enters the WaitSleepJoin state:◦The thread can call Monitor method Wait◦The thread can call Thread method Sleep ◦The thread can call Thread method Join

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If a Running Thread's Suspend method is called, the Running thread enters the Suspended state

A Suspended thread returns to the Running state when another thread in the program invokes the Suspended Thread's Resume method

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Thread Priorities and Thread Thread Priorities and Thread SchedulingScheduling

Every thread has a priority in the range between ThreadPriority.Lowest and ThreadPriority.Highest

These values come from the ThreadPriority enumeration (namespace System.Threading):◦Lowest◦BelowNormal◦Normal◦AboveNormal ◦Highest

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The Windows operating system supports a concept called timeslicing that enables threads of equal priority to share a processor

With timeslicing, each thread receives a brief burst of processor time, called a quantum, during which the thread can execute

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The job of the thread scheduler is to keep the highest-priority thread running at all times

And also, to ensure that all such threads execute for a quantum in round-robin fashion

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Visual Basic 2005

Multithreading


Thread-priority scheduling, mulitlevel priority queue

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Priority Highest

Priority AboveNormal

Priority Normal

Priority BelowNormal

Priority Lowest

A B

C

D E F

G

Ready threads

Visual Basic 2005

Multithreading

Creating and Executing ThreadsCreating and Executing Threads

To create a thread, we can use the following declaration:

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Dim thread1 As New Thread(New ThreadStart(AddressOf Class1.method))

thread1 object variable

Thread class ThreadStart delegate

AddressOf keyword

Method to execute in the thread

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Threads sleeping and printing

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Imports System.Threading

Module ModThreadTester

Sub Main() ' Create and name each thread. Use MessagePrinter's ' Print method as argument to ThreadStart delegate. Dim printer1 As New MessagePrinter Dim thread1 As New Thread(New ThreadStart(AddressOf printer1.Print)) thread1.Name = "thread1"

Dim printer2 As New MessagePrinter Dim thread2 As New Thread(New ThreadStart(AddressOf printer2.Print)) thread2.Name = "thread2“

Dim printer3 As New MessagePrinter Dim thread3 As New Thread(New ThreadStart(AddressOf printer3.Print)) thread3.Name = "thread3“

. . .continued

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Console.WriteLine("Starting threads in Main")

' call each thread's Start method to place each ' thread in Running state thread1.Start() thread2.Start() thread3.Start()

Console.WriteLine("Threads started, Main ends" & vbCrLf) Console.Read() End Sub

. . .continued

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Class MessagePrinter ' Print method of this class used to control threads Private sleepTime As Integer Private Shared random As New Random

' constructor to initialize a MessagePrinter object Sub New() ' pick random sleep time between 0 and 5 seconds sleepTime = random.Next(5001) End Sub ' New

' method Print controls thread that prints messages Public Sub Print() ' obtain reference to currently executing thread Dim current As Thread = Thread.CurrentThread

Console.WriteLine(current.Name & " going to sleep for " & _ sleepTime & " milliseconds") Thread.Sleep(sleepTime) ' sleep for sleepTime milliseconds

' display the thread's name Console.WriteLine(current.Name + " done sleeping") End Sub ' Print End Class ' MessagePrinterEnd Module

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Thread Synchronization and Class Thread Synchronization and Class MonitorMonitor

Often, multiple threads of execution manipulate shared data

If threads with access to shared data simply read that data, then any number of threads can access the data simultaneously without problems

However, when multiple threads share data and the data is modified by one or more of the threads, then indeterminate results may occur

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Multithreading


The problem can be solved by giving one thread at a time exclusive access to code that manipulates the shared data

During this time, other threads wishing to manipulate the data are kept waiting

When the thread with exclusive access to the data completes its data manipulations, one of the waiting threads is allowed to proceed

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Each thread accessing the shared data excludes all other threads from doing so simultaneously, this is called mutual exclusion or thread synchronization

Visual Basic programmers use the .NET Framework's monitors to perform synchronization

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Class Monitor provides the methods for locking objects to implement synchronized access to shared data

Locking an object means that only one thread can access that object at a time

When a thread wishes to acquire exclusive control over an object, the thread invokes the Monitor method Enter to acquire the lock on the data object

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Visual Basic provides another means of manipulating an object's lock, the SyncLock statement

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SyncLock objectReference ' code that requires synchronization goes here End SyncLock

The objectReference is the same reference that normally would be

passed to Monitor methods Enter, Exit, Pulse and PulseAll

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Producer/Consumer Relationship Producer/Consumer Relationship without Thread Synchronizationwithout Thread Synchronization

The producer portion of an application generates data

the consumer portion of the application uses that data

A producer thread calls a produce method to generate data and place it in a shared region of memory called a buffer

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A consumer thread calls a consume method to read the data

If the producer wishes to put the next data in the buffer but determines that the consumer has not yet read the previous data from the buffer, the producer thread should call Wait

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Producer

Consumer

Buffer

data

data

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Producer produces 1 and stores to buffer 1

Consumer consumes 1 and retrieves from buffer

Producer produces 2 and stores to buffer

Producer produces 3 and stores to buffer

2

3





Consumer still consumes 5 and retrieves from buffer 5

Buffer

Visual Basic 2005

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Producer/Consumer Relationship with Producer/Consumer Relationship with Thread SynchronizationThread Synchronization

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Producer tries to write. Buffer full, Producer waits 1


Buffer

1

Occupied Count

1

Producer produces 2 and stores to buffer 2 1

1 0

Consumer consumes 2 and retrieves from buffer 2 0

Producer produces 3 and stores to buffer 3 1

Producer tries to write. Buffer full, Producer waits 3 1

Consumer consumes 3 and retrieves from buffer 3 0

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Producer/Consumer Relationship: Producer/Consumer Relationship: Circular BufferCircular Buffer

There are too many interactions that occur with the operating system, the network, the user and other components, which can cause the threads to operate at different speeds

When threads wait, programs can become less productive, user-interactive programs can become less responsive and network applications can suffer longer delays because the processor is not used efficiently

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To minimize the waiting for threads that share resources and operate at the same relative speeds, we can implement a circular buffer

This provides extra locations in which the producer can place values (if it "gets ahead" of the consumer) and from which the consumer can retrieve those values (while it is "catching up" to the producer)

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All buffers empty, Consumer waits -1

Buffers

0

Occupied Count

-1 -1

Producer produces 1 and stores to buffer 1 1-1 -1

Consumer consumes 1 and retrieves from buffer 1 0-1 -1

Producer produces 2 and stores to buffer 1 12 -1

Consumer consumes 2 and retrieves from buffer 1 02 -1

Producer produces 3 and stores to buffer 1 12 3



Consumer consumes 3 and retrieves from buffer 4 25 6

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End of MultithreadingEnd of MultithreadingMULTITHREADING 32

Visual Basic 2005

Multithreading

Technology

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