MultiThreaded ApplicationsMultiThreaded Applications

What is Multithreaded What is Multithreaded Programming?Programming?

• Having your software appear to perform multiple tasks in parallel– Individual paths of execution– Threads live in 1 process– Increases parallelization (blocking I/O)– Simplifies design– Better use of CPU– Bad if thread work is small (context

switch)

• Necessary for building servers

Thread BasicsThread BasicsA re-hash of 2601A re-hash of 2601

• Thread scheduling– Round-robin– Time slicing (each process gets a

turn)– Context-switch– Thread states

•Running•Ready•Blocked

Thread BasicsThread Basics

• Thread Priorities– REALTIME– HIGH– NORMAL– IDLE

• The CPU will try and run the higher priorities

• Can lead to starvation

Multiprogramming ConceptsMultiprogramming Concepts

• Atomic operations– Needed for data integrity– Having multiple ASM statements as one– Can't be interrupted by CPU switch

• Mutual Exclusion– Allows programmer to perform a series of

instructions in an atomic manner.

• Race Conditions– Having two or more processes compete for

the same resources

Multiprogramming ConceptsMultiprogramming Concepts

• Starvation– When one thread never gets service

(I.e. low priority)– Usually because another process is a

hog

Multiprogramming ConceptsMultiprogramming Concepts

• Semaphores– Used when there are multiple

resources that are the same.– When thread wants resource, takes it

and decrements semaphore.– When finished, returns resource and

increments semaphore– If at 0, fall asleep.

• Is woken up when a resource is free•THEN decrements back to 0

DeadlockDeadlock

• When threads can't make progress• "I have the bat and want the ball;

you have the ball and want the bat"• Most operating systems simply

ignore deadlock

Priority InversionPriority Inversion

• Assume A higher priority than B– B has mutex that A needs

• Assume A has highest, B middle and C lowest.– C acquires mutex that is shared with A– Thread B (higher than C) becomes

ready to run– Thread B begins to run– A becomes ready…

Win32 MultithreadingWin32 Multithreading

• To create a thread in Win32:– Determine security attributes (NULL for

default)– Determine it's stack size (0 for default)– Have a function(s) that can be used by a

thread– The option to pass 1 parameter to that

function– Special flags if you want it suspended (or 0

for default)– A pointer to a DWORD for the thread ID

Threaded Function DefinitionThreaded Function Definition

• Must look similar to below:

DWORD WINAPI threadFunc (LPVOID par);

Useful functionsUseful functions

• DWORD tID = GetCurrentThreadId ( );– Determines the thread ID for the

current thread you are in– Good for inside the threaded function– Good for outside the threaded

function to determine primary thread's ID

Suspend and ResumeSuspend and Resume

• To pause or resume a thread, you only need its handle:DWORD SuspendThread (HANDLE hThread);

DWORD ResumeThread (HANDLE hThread);

#include <windows.h>#include <stdio.h>

DWORD WINAPI WorkerThreadProc (LPVOID lpThreadParameter);

int main (void) {HANDLE h1, h2, h3;DWORD id1, id2, id3;h1 = CreateThread (NULL, 0, WorkerThreadProc, (LPVOID)5000, 0, &id1);h2 = CreateThread (NULL, 0, WorkerThreadProc, (LPVOID)5000, 0, &id2);h3 = CreateThread (NULL, 0, WorkerThreadProc, (LPVOID)5000, 0, &id3);

if (h1 != NULL) {Sleep (6000);printf ("Done");CloseHandle(h1); CloseHandle (h2); CloseHandle (h3);

}else {

printf ("Failed to create worker thread: error 0x%x\n", GetLastError());}return 0;

}

DWORD WINAPI WorkerThreadProc (LPVOID lpThreadParameter, LPVOID par2) {DWORD dwNumSeconds = (DWORD)lpThreadParameter;DWORD dwThreadId = GetCurrentThreadId();for (int i = 0; i < (int)dwNumSeconds; i++) {

printf ("[%08x] Hello, multithreaded world! = %d\n", dwThreadId, i);}return (dwThreadId);

}

Now, in JavaNow, in Java

• Java is pure OOP• Class has a run( ) method• Instances are the threads• 2 ways to create Threaded class

– extend Thread– implement Runnable

class MyThread extends Thread {int threadID;MyThread(int x) {

threadID = x;this.start( ); // Start calls run

}public void run ( ) {

for (int i = 0; i < 5000; i++) {System.out.println (threadID+":"+i);

}// First to 5000 kills the programSystem.exit(1);

}public static void main (String args[ ]) {

MyThread t1 = new MyThread(1);MyThread t2 = new MyThread(2);MyThread t3 = new MyThread(3);

}}

class MyThread implements Runnable {int threadID;Thread t;MyThread(int x) {

threadID = x;t = new Thread(this);t.start( );

} public void run ( ) {

for (int i = 0; i < 5000; i++) {System.out.println (threadID+":"+i);

}System.exit(1);

}public static void main (String args[ ]) {

MyThread t1 = new MyThread(1);MyThread t2 = new MyThread(2);MyThread t3 = new MyThread(3);

}}