Threads

Progressing With Parallel Processing eWeek (09/18/06) Vol. 23, No. 37, P. D5 Multithreading skills are becoming essential as parallel processing hardware proliferates, and developers ignore at their own peril indications of this trend such as Intel's investments in college curriculum and resources for multithread development training. The Java programming language supports the expression of concurrency in the same language developers are already employing for application logic, while powerful abstractions for C++ are also offered by concurrency toolkits and frameworks. Being able to count the threads developers are using on the fingers of one hand is folly, according to principal author of "Java Concurrency in Practice" Brian Goetz. He and his five co-authors note that "The need for thread safety is contagious," because "frameworks may create threads on your behalf, and code called from these threads must be thread-safe." It is the authors' contention that developers should never lose sight of the application state and avoid becoming overwhelmed by threading mechanisms. Developers must also keep in mind that careless habits that are acceptable in single-thread environments may be exposed in multithread environments.

2.2 Threads Process: address space + code execution There is no law that states that a process cannot have more than one “line” of execution. Threads: single address space + many threads of execution Shared global variables, stack address space, open files, signals, child processes, etc.

Threads Process – used to group resources together Threads – entities scheduled for execution on CPU Sometimes called lightweight processes Multithreading – multiple threads in the same process

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Thread functions Create threads: #include <pthread.h> int pthread_create ( pthread_t* thread, pthread_attr_t* attr, void* (*start_routine)(void *), void* arg );

Thread termination void pthread_exit ( void* retval ); int pthread_cancel ( pthread_t thread );

Why threads? Easier to create a thread than a process Share resources Compute and I/O within a single process on a single processor can be overlapped Compute and compute within a single process on a multiprocessor can be overlapped

Example: word processor Multiple threads: User interaction Document reformatting Automatic saving/backups Alternative is everything else stops when (2) or (3) occurs.

Example: web server Dispatcher – handles incoming requests Workers – performs request Checks cache Reads from disk if necessary Adds to cache

Threads and alternatives Threads retain the simple sequential, blocking model while allowing for parallelism. The single threaded server retains the simple sequential, block model but performance suffers (no parallelism). Finite state machine = each computation has a saved state and there exists some set of events that can occur to change the state High performance through parallelism Uses nonblocking calls and interrupts (not simple) May be in user space or kernel.

Threads Thread types: (also popup) Implementations: Detached Joinable User space Kernel Hybrid

Pitfall: global variables #include <stdio.h> bool err = false; void* t1 ( void* p ) { … err = true; if (err) puts( “hello” ); //may never get here! } void* t2 ( void* p ) { err = false; int main ( int argc, char* argv[] ) { if (err) puts( “something bad happened.” ); Pitfall: global variables

Pitfalls: global variables

Pitfall: global variables Solution: don’t use ‘em. Solution: create thread-wide globals (using your own libraries) Solution: (other mechanisms that we will explore in the future)

Other pitfalls Libraries may not be reentrant Solution: rewrite the library Solution: wrap each library call with a wrapper Signals, alarms, and I/O may not be thread specific.

Other pthread functions int thread_yield ( ); int pthread_join ( pthread_t th, void** thread_return ); Many, many others

Multithreaded example 1

/* file: pt1.cpp date: 23-sep-2005 author: george j. grevera, ph.d. compile: g++ -o pt1.exe pt1.cpp -lpthread -lm desc.: shell of a multithreaded app */ #include <math.h> #include <pthread.h> #include <stdio.h> //global variables const int N = 10; //max number of threads pthread_t thread[N]; //for thread id storage //----------------------------------------------------------------------

//---------------------------------------------------------------------- //program execution begins here. int main ( const int argc, const char* const argv[] ) { //create N threads for (int i=0; i<N; i++) { pthread_create( &thread[i], 0, start_routine, (void*)i ); printf( "main: thread %d created. \n", thread[i] ); } //wait for the N threads to finish void* v; printf( "main: waiting \n" ); pthread_join( thread[i], &v ); printf( "main: returning \n" ); return 0;

//---------------------------------------------------------------------- //example worker thread function. this function does a lot of "work" // (i.e., computation). double doWork ( const int whoAmI ) { double sum = 0.0; for (int i=0; i<10000000; i++) { sum += sin( i ) * cos( i ); } return sum; //main thread function void* start_routine ( void* p ) { int whoAmI = (int)p; printf( "%d processing \n", whoAmI ); doWork( whoAmI ); printf( "%d exit \n", whoAmI ); return 0;

/* file: pt1.cpp date: 23-sep-2005 author: george j. grevera, ph.d. compile: g++ -o pt1.exe pt1.cpp -lpthread -lm desc.: shell of a multithreaded app */ #include <math.h> #include <pthread.h> #include <stdio.h> //global variables const int N = 10; //max number of threads pthread_t thread[N]; //for thread id storage //---------------------------------------------------------------------- //example worker thread function. this function does a lot of "work" // (i.e., computation). double doWork ( const int whoAmI ) { double sum = 0.0; for (int i=0; i<10000000; i++) { sum += sin( i ) * cos( i ); } return sum; //main thread function void* start_routine ( void* p ) { int whoAmI = (int)p; printf( "%d processing \n", whoAmI ); doWork( whoAmI ); printf( "%d exit \n", whoAmI ); return 0; //program execution begins here. int main ( const int argc, const char* const argv[] ) { //create N threads for (int i=0; i<N; i++) { pthread_create( &thread[i], 0, start_routine, (void*)i ); printf( "main: thread %d created. \n", thread[i] ); //wait for the N threads to finish void* v; printf( "main: waiting \n" ); pthread_join( thread[i], &v ); printf( "main: returning \n" );

Multithreading discussion pthread_create only allows a single parameter to be passed to the thread pthread_join only allows a single parameter to be returned from a thread How can we pass and return many parameters?

Multithreading: advanced topic We know that process memory is shared among all threads. We know that the stack is part of the process memory. Therefore the stack is part of the memory that is shared among the threads. How can we demonstrate that the stack is shared among threads?

From where are local variables allocated? /* This program demonstrates that, although stack variables are not shared among threads, stack memory (_all_ process memory) is indeed shared by threads. g++ -o sharedStack.exe sharedStack.cpp -lpthread -lm -lrt */ #include <iostream> #include <math.h> #include <pthread.h> #include <sched.h> #include <unistd.h> using namespace std; const int N = 2; //max number of threads //this will be a pointer to a local variable in thread 0. static int* whoAmIPointer = NULL; //---------------------------------------------------------------------- From where are local variables allocated?

//---------------------------------------------------------------------- int main ( const int argc, const char* const argv[] ) { pthread_t thread[::N]; //for thread id storage //create N threads for (int i=0; i< ::N; i++) { pthread_create( &thread[i], 0, start_routine, (void*)i ); cout << "main: thread " << i << " created with id=" << thread[i] << endl; } //wait for the N threads to finish void* v; cout << "main: wait" << endl; pthread_join( thread[i], &v ); cout << "main: returning" << endl; return 0; Nothing new here.

//---------------------------------------------------------------------- void* start_routine ( void* p ) { int whoAmI = (int)p; int whoAmICopy = whoAmI; cout << whoAmI << " processing" << endl; if (whoAmI==0) { //is this thread 0? //make the global var point to my local var ::whoAmIPointer = &whoAmI; sched_yield(); sleep( 5 ); } else { //this is not thread 0 so wait until thread 0 sets the global var // that points to thread 0's local var. while (::whoAmIPointer==NULL) { } //change thread 0's local var *::whoAmIPointer = 92; if (whoAmI!=whoAmICopy) { cout << "Hey! Wait a minute! Somebody changed who I am from " << whoAmICopy << " to " << whoAmI << "!" << endl; cout << whoAmI << " done" << endl << whoAmI << " exit" << endl; return 0; What does :: mean in C++?

//---------------------------------------------------------------------- void* start_routine ( void* p ) { int whoAmI = (int)p; int whoAmICopy = whoAmI; cout << whoAmI << " processing" << endl; if (whoAmI==0) { //is this thread 0? //make the global var point to my local var ::whoAmIPointer = &whoAmI; sched_yield(); sleep( 5 ); } else { //this is not thread 0 so wait until thread 0 sets the global var // that points to thread 0's local var. while (::whoAmIPointer==NULL) { } //change thread 0's local var *::whoAmIPointer = 92; if (whoAmI!=whoAmICopy) { cout << "Hey! Wait a minute! Somebody changed who I am from " << whoAmICopy << " to " << whoAmI << "!" << endl; cout << whoAmI << " done" << endl << whoAmI << " exit" << endl; return 0; What does :: mean in C++? :: is the C++ scope resolution operator. In this case, it refers to a global variable.

//---------------------------------------------------------------------- void* start_routine ( void* p ) { int whoAmI = (int)p; int whoAmICopy = whoAmI; cout << whoAmI << " processing" << endl; if (whoAmI==0) { //is this thread 0? //make the global var point to my local var ::whoAmIPointer = &whoAmI; sched_yield(); sleep( 5 ); } else { //this is not thread 0 so wait until thread 0 sets the global var // that points to thread 0's local var. while (::whoAmIPointer==NULL) { } //change thread 0's local var *::whoAmIPointer = 92; if (whoAmI!=whoAmICopy) { cout << "Hey! Wait a minute! Somebody changed who I am from " << whoAmICopy << " to " << whoAmI << "!" << endl; cout << whoAmI << " done" << endl << whoAmI << " exit" << endl; return 0;

//---------------------------------------------------------------------- void* start_routine ( void* p ) { int whoAmI = (int)p; int whoAmICopy = whoAmI; cout << whoAmI << " processing" << endl; if (whoAmI==0) { //is this thread 0? //make the global var point to my local var ::whoAmIPointer = &whoAmI; sched_yield(); sleep( 5 ); } else { //this is not thread 0 so wait until thread 0 sets the global var // that points to thread 0's local var. while (::whoAmIPointer==NULL) { } //change thread 0's local var *::whoAmIPointer = 92; if (whoAmI!=whoAmICopy) { cout << "Hey! Wait a minute! Somebody changed who I am from " << whoAmICopy << " to " << whoAmI << "!" << endl; cout << whoAmI << " done" << endl << whoAmI << " exit" << endl; return 0;

//---------------------------------------------------------------------- void* start_routine ( void* p ) { int whoAmI = (int)p; int whoAmICopy = whoAmI; cout << whoAmI << " processing" << endl; if (whoAmI==0) { //is this thread 0? //make the global var point to my local var ::whoAmIPointer = &whoAmI; sched_yield(); sleep( 5 ); } else { //this is not thread 0 so wait until thread 0 sets the global var // that points to thread 0's local var. while (::whoAmIPointer==NULL) { } //change thread 0's local var *::whoAmIPointer = 92; if (whoAmI!=whoAmICopy) { cout << "Hey! Wait a minute! Somebody changed who I am from " << whoAmICopy << " to " << whoAmI << "!" << endl; cout << whoAmI << " done" << endl << whoAmI << " exit" << endl; return 0;

//---------------------------------------------------------------------- void* start_routine ( void* p ) { int whoAmI = (int)p; int whoAmICopy = whoAmI; cout << whoAmI << " processing" << endl; if (whoAmI==0) { //is this thread 0? //make the global var point to my local var ::whoAmIPointer = &whoAmI; sched_yield(); sleep( 5 ); } else { //this is not thread 0 so wait until thread 0 sets the global var // that points to thread 0's local var. while (::whoAmIPointer==NULL) { } //change thread 0's local var *::whoAmIPointer = 92; if (whoAmI!=whoAmICopy) { cout << "Hey! Wait a minute! Somebody changed who I am from " << whoAmICopy << " to " << whoAmI << "!" << endl; cout << whoAmI << " done" << endl << whoAmI << " exit" << endl; return 0;

How could this ever be true (if threads didn’t share stack memory)? //---------------------------------------------------------------------- void* start_routine ( void* p ) { int whoAmI = (int)p; int whoAmICopy = whoAmI; cout << whoAmI << " processing" << endl; if (whoAmI==0) { //is this thread 0? //make the global var point to my local var ::whoAmIPointer = &whoAmI; sched_yield(); sleep( 5 ); } else { //this is not thread 0 so wait until thread 0 sets the global var // that points to thread 0's local var. while (::whoAmIPointer==NULL) { } //change thread 0's local var *::whoAmIPointer = 92; if (whoAmI!=whoAmICopy) { cout << "Hey! Wait a minute! Somebody changed who I am from " << whoAmICopy << " to " << whoAmI << "!" << endl; cout << whoAmI << " done" << endl << whoAmI << " exit" << endl; return 0; How could this ever be true (if threads didn’t share stack memory)?

/* This program demonstrates that, although stack variables are not shared among threads, stack memory (_all_ process memory) is indeed shared by threads. g++ -o sharedStack.exe sharedStack.cpp -lpthread -lm -lrt */ #include <iostream> #include <math.h> #include <pthread.h> #include <sched.h> #include <unistd.h> using namespace std; const int N = 2; //max number of threads //this will be a pointer to a local variable in thread 0. static int* whoAmIPointer = NULL; //---------------------------------------------------------------------- void* start_routine ( void* p ) { int whoAmI = (int)p; int whoAmICopy = whoAmI; cout << whoAmI << " processing" << endl; if (whoAmI==0) { //is this thread 0? //make the global var point to my local var ::whoAmIPointer = &whoAmI; sched_yield(); sleep( 5 ); } else { //this is not thread 0 so wait until thread 0 sets the global var // that points to thread 0's local var. while (::whoAmIPointer==NULL) { } //change thread 0's local var *::whoAmIPointer = 92; if (whoAmI!=whoAmICopy) { cout << "Hey! Wait a minute! Somebody changed who I am from " << whoAmICopy << " to " << whoAmI << "!" << endl; cout << whoAmI << " done" << endl << whoAmI << " exit" << endl; return 0; int main ( const int argc, const char* const argv[] ) { pthread_t thread[::N]; //for thread id storage //create N threads for (int i=0; i< ::N; i++) { pthread_create( &thread[i], 0, start_routine, (void*)i ); cout << "main: thread " << i << " created with id=" << thread[i] << endl; //wait for the N threads to finish void* v; cout << "main: wait" << endl; pthread_join( thread[i], &v ); cout << "main: returning" << endl;

Win32 thread functions CreateThread ExitThread TerminateThread WaitForSingleObject GetExitCodeThread

Win32 threads & fibers Fibers (in win32 – not available in Linux) A lightweight thread Owned by thread Threads are preemptively scheduled Fibers are not preemptively scheduled When thread is preempted, so is fiber When thread is resumed, so is fiber May be scheduled by owning thread