CS 838: Pervasive Parallelism

Introduction to pthreads

Copyright 2005 Mark D. HillUniversity of Wisconsin-Madison

Slides are derived from online references fromLawrence Livermore National Laboratory as well as CS

757 notes created by Mark Hill and Min XuThanks!

CS 838 2(C) 2005

Outline

• Programming Model– Threaded Model

– Threads Overview

– pthreads

• Syntax by Example– Expressing parallelism

– Synchronization

CS 838 3(C) 2005

Parallel Programming (Review)

• Multiple instruction streams working cooperatively at the same time

• Components– Communication

– Synchronization

CS 838 4(C) 2005

MPI (Review)

• Per-processor private address space

• Communication– Explicit

– Pass messages

• Synchronization– Implicit

– Message receive

CS 838 5(C) 2005

Contrast with pthreads

• Shared memory: single, shared address space

• Communication– Implicit

– A write to a shared address by any thread is immediately visible to all threads

• Synchronization– Explicit

– Why?

CS 838 6(C) 2005

Races

• Example– Initial value of a = 0

– T1: a = 1

– T2: a = 2

– T3: printf(“%d”, a);

– Output: ???

• Strategies to combat (details soon)– Locks (mutexes)

– Condition variables (CVs)

– Barriers

CS 838 7(C) 2005

Outline

• Programming Model– Threaded Model

– Threads Overview

– pthreads

• Syntax by Example– Expressing parallelism

– Synchronization

CS 838 8(C) 2005

Thread

• Stream of instructions that OS can schedule to run independently

• Think of procedure that runs independently from / concurrently with main program

• Lets you run many procedures (even many incarnations of the same one) at the same time

• Each one has its own, independent control flow

CS 838 9(C) 2005

System View

• Unix process– Created by OS with a fair amount of overhead

– Contains info about program resources & execution state

» pid, gid, uid, environment, working dir, instructions, registers, stack, heap, file descriptor, signal actions, shared libraries, IPC tools…

• Threads– Multiple can belong to the same Unix process

– All share process resources

– Lightweight

» Only duplicate enough state to run independently:• Stack pointer, registers, scheduling properties, pending & blocked signals,

thread-specific data

CS 838 10(C) 2005

Process vs. Thread

Courtesy of Lawrence Livermore National Lab http://www.llnl.gov/computing/tutorials/pthreads/

CS 838 11(C) 2005

Outline

• Programming Model– Threaded Model

– Threads Overview

– pthreads

• Syntax by Example– Expressing parallelism

– Synchronization

CS 838 12(C) 2005

pthreads

• Posix threads

• IEEE POSIX 1003.1c standard

• Implemented via a library

• Portable to many systems

CS 838 13(C) 2005

Outline

• Programming Model– Threaded Model– Threads Overview– pthreads

• Syntax by Example– Expressing parallelism– Synchronization

CS 838 14(C) 2005

Barrier With Sense Reversal

BARRIER(bar_name, p) { /* toggle private state */ local_sense = !(local_sense); LOCK(bar_name.lock); bar_name.counter++; UNLOCK(bar_name.lock); if (bar_name.counter == p) { bar_name.counter = 0; bar_name.flag = local_sense;} else { /* busy wait */ while(bar_name.flag != local_sense) {};}

CS 838 15(C) 2005

Summary

• pthreads is a library for threaded programming

• Write programs with pthreads– #include <pthread.h>– man pthreads for a list of functions

» Thread creation

» Synchronization• Lock / mutex

• Condition variable

• Barrier

» Thread termination

– Compile: cc –mt –lpthread program.c

• Review example in http://www.cs.wisc.edu/~markhill/cs838/Fall2005/handouts/eg_pthread.tar.gz