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Национальный исследовательский университет «МИЭТ»

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Операционные системы

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Введение в параллельное программирование

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		Our running Example: The PI program
			Numerical Integration
				Mathematically, we know that:
	4.0			1
				1
				4.0	dx =
				(1+x2)	dx =
				(1+x2)
)				0
)
2
x				We can approximate the
+				We can approximate the
4.0/(1	2.0			integral as a sum of
	2.0

=				rectangles:
=
F(x)				N
				N
				F(xi) x
				i = 0
	0.0	X	1.0	Where each rectangle has
	0.0

				width x and height F(xi) at
				the middle of interval i.

PI Program:

The sequential program

static long num_steps = 100000; double step;

void main ()

{ int i; double x, pi, sum = 0.0;

step = 1.0/(double) num_steps;

for (i=1;i<= num_steps; i++){ x = (i-0.5)*step;

sum = sum + 4.0/(1.0+x*x);

}

pi = step * sum;

}

Outline

Parallel programming, wetware, and software

Parallel programming APIs

– Thread Libraries

– Win32 API

–POSIX threads

–Compiler Directives

–OpenMP*

–Message Passing

–MPI

More complicated examples

Choosing which API is for you

*Other names and brands may be claimed as the property of others.

Threads and Parallel Programming

Operating Systems 101:

–Process: A unit of work managed by an OS with its own address space (the heap) and OS managed resources.

–Threads: resources within a process that execute the instructions in a program. They have their own program counter and a private memory region (a stack) but share the other resources within the process … including the heap.

Threads are the natural unit of execution for parallel programs on shared memory hardware.

The threads share memory so data structures don’t have to be torn apart into distinct pieces.

Programming with thread libraries

The OS provides an API for creating, managing, and destroying threads.

–Windows* threading API

–Posix threads (on Linux)

Advantages of threads libraries:

–The thread library gives you detailed control over the threads.

Disadvantage of thread libraries:

–The thread library REQUIRES that you take detailed control over the threads.

*Other names and brands may be claimed as the property of others.

Win32 API

#include <windows.h> #define NUM_THREADS 2

HANDLE thread_handles[NUM_THREADS]; CRITICAL_SECTION hUpdateMutex;

static long num_steps = 100000; double step;

double global_sum = 0.0;

void Pi (void *arg)

{

int i, start;

double x, sum = 0.0;

start = *(int *) arg;

step = 1.0/(double) num_steps;

for (i=start;i<= num_steps; i=i+NUM_THREADS){

x = (i-0.5)*step;

sum = sum + 4.0/(1.0+x*x);

}

EnterCriticalSection(&hUpdateMutex); global_sum += sum; LeaveCriticalSection(&hUpdateMutex);

}

void main ()

{

double pi; int i; DWORD threadID;

int threadArg[NUM_THREADS];

for(i=0; i<NUM_THREADS; i++) threadArg[i] = i+1;

InitializeCriticalSection(&hUpdateMutex);

for (i=0; i<NUM_THREADS; i++){ thread_handles[i] = CreateThread(0, 0,

(LPTHREAD_START_ROUTINE) Pi, &threadArg[i], 0, &threadID);

}

WaitForMultipleObjects(NUM_THREADS,

thread_handles, TRUE,INFINITE);

pi = global_sum * step;

printf(" pi is %f \n",pi);

}

Win32 thread library: It’s not as bad as it looks

Setup multi-threading support	void main ()
Setup multi-threading support	{
		double pi; int i;

static long num_steps = 100000;
double step;

] = i+1;

Setup arguments, book); keeping, and launch the threads

Define the work each thread	0,
		Pi,
will do and pack it into a
		0, &threadID);
function
	}

Update the		,
final answer		,
final answer		TRUE,INFINITE);
one thread at		TRUE,INFINITE);
one thread at		Wait for all the threads to finish
a time		pi = global_sum * step;
		printf(" pi is %f \n",pi);
	}	Compute
		Compute
		and print
		final answer