Unix. Threads & Concurrency. Erick Fredj Computer Science The Jerusalem College of Technology

Transcription

1 Unix Threads & Concurrency Erick Fredj Computer Science The Jerusalem College of Technology 1

2 Threads Processes have the following components: an address space a collection of operating system state a CPU context or thread of control On multiprocessor systems, with several CPUs, it would make sense for a process to have several CPU contexts (threads of control) Multiple threads of control could run in the same address space on a single CPU system too! thread of control and address space are orthogonal concepts 2

3 Threads Threads share a process address space with zero or more other threads Threads have their own PC, SP, register state etc stack A traditional process can be viewed as an address space with a single thread 3

4 Single thread state within a process 4

5 Multiple threads in an address space 5

6 What is a thread? A thread executes a stream of instructions an abstraction for control-flow Practically, it is a processor context and stack Allocated a CPU by a scheduler Executes in the context of a memory address space 6

7 Summary of private per-thread state Stack (local variables) Stack pointer Registers Scheduling properties (i.e., priority) Set of pending and blocked signals Other thread specific data 7

8 Shared state among threads Open files, sockets, locks User ID, group ID, process/task ID Address space Text Data (off-stack global variables) Heap (dynamic data) Changes made to shared state by one thread will be visible to the others Reading and writing memory locations requires synchronization! a major topic for later 8

9 How do you program using threads? Split program into routines to execute in parallel True or pseudo (interleaved) parallelism 9

10 Why program using threads? Utilize multiple CPU s concurrently Low cost communication via shared memory Overlap computation and blocking on a single CPU Blocking due to I/O Computation and communication Handle asynchronous events 10

11 Thread usage A word processor with three threads 11

12 Common thread programming models Manager/worker Peer Manager thread handles I/O and assigns work to worker threads Worker threads may be created dynamically, or allocated from a thread-pool Like manager worker, but manager participates in the work Pipeline Each thread handles a different stage of an assembly line Threads hand work off to each other in a producer-consumer relationship 12

13 What does a typical thread API look like? POSIX standard threads (Pthreads) First thread exists in main(), typically creates the others pthread_create (thread,attr,start_routine,arg) Returns new thread ID in thread Executes routine specified by start_routine with argument specified by arg Exits on return from routine or when told explicitly 13

14 Thread API (continued) pthread_exit (status) Terminates the thread and returns status to any joining thread pthread_join (threadid,status) Blocks the calling thread until thread specified by threadid terminates Return status from pthread_exit is passed in status One way of synchronizing between threads pthread_yield () Thread gives up the CPU and enters the run queue 14

15 Using create, join and exit primitives 15

16 An example Pthreads program #include <pthread.h> #include <stdio.h> #define NUM_THREADS 5 void *PrintHello(void *threadid) printf("\n%d: Hello World!\n", threadid); pthread_exit(null); int main (int argc, char *argv[]) pthread_t threads[num_threads]; int rc, t; for(i=0; i<num_threads; i++) printf("creating thread %d\n", t); rc = pthread_create(&threads[i], NULL, PrintHello, (void *)i); if (rc) printf("error; return code from pthread_create() is %d\n", rc); exit(-1); pthread_exit(null); Program Output Creating thread 0 Creating thread 1 0: Hello World! 1: Hello World! Creating thread 2 Creating thread 3 2: Hello World! 3: Hello World! Creating thread 4 4: Hello World! For more examples see: 16

17 Thread Argument Passing #include <pthread.h> #include <stdio.h> #include <stdlib.h> #define NUM_THREADS 3 char *messages[num_threads]; struct thread_data int thread_id; int sum; char *message; ; struct thread_data thread_data_array[num_threads]; 17

18 Thread Argument Passing void *PrintHello(void *threadarg) int taskid, sum; char *hello_msg; struct thread_data *my_data; sleep(1); my_data = (struct thread_data *) threadarg; taskid = my_data->thread_id; sum = my_data->sum; hello_msg = my_data->message; printf("thread %d: %s Sum=%d\n", taskid, hello_msg, sum); pthread_exit(null); 18

19 Thread Argument Passing int main(int argc, char *argv[]) pthread_t threads[num_threads]; int *taskids[num_threads]; int rc, t, sum; sum=0; messages[0] = "English: Hello World!"; messages[1] = "French: Bonjour, le monde!"; messages[2] = "Spanish: Hola al mundo"; for(t=0;t<num_threads;t++) sum = sum + t; thread_data_array[t].thread_id = t; thread_data_array[t].sum = sum; thread_data_array[t].message = messages[t]; printf("creating thread %d\n", t); rc = pthread_create(&threads[t], NULL, PrintHello, (void *)&thread_data_array[t]); if (rc) printf("error; return code from pthread_create() is %d\n", rc); exit(-1); pthread_exit(null); 19

20 Pthread Joining #include <pthread.h> #include <stdio.h> #include <stdlib.h> #define NUM_THREADS 3 void *BusyWork(void *null) int i; double result=0.0; for (i=0; i< ; i++) result = result + (double)random(); printf("thread result = %e\n",result); pthread_exit((void *) 0); 20

21 Pthread Joining int main(int argc, char *argv[]) pthread_t thread[num_threads]; pthread_attr_t attr; int rc, t, status; /* Initialize and set thread detached attribute */ pthread_attr_init(&attr); pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE); for(t=0;t<num_threads;t++) printf("creating thread %d\n", t); rc = pthread_create(&thread[t], &attr, BusyWork, NULL); if (rc) printf("error; return code from pthread_create() is %d\n", rc); exit(-1); 21

22 Pthread Joining /* Free attribute and wait for the other threads */ pthread_attr_destroy(&attr); for(t=0;t<num_threads;t++) rc = pthread_join(thread[t], (void **)&status); if (rc) printf("error return code from pthread_join() is %d\n", rc); exit(-1); printf("completed join with thread %d status= %d\n",t, status); pthread_exit(null); 22

23 Stack Management #include <pthread.h> #include <stdio.h> #define NTHREADS 4 #define N 1000 #define MEGEXTRA pthread_attr_t attr; void *dowork(void *threadid) double A[N][N]; int i,j; size_t mystacksize; pthread_attr_getstacksize (&attr, &mystacksize); printf("thread %d: stack size = %d bytes \n", threadid, mystacksize); for (i=0; i<n; i++) for (j=0; j<n; j++) A[i][j] = ((i*j)/3.452) + (N-i); pthread_exit(null); 23

24 Stack Management int main(int argc, char *argv[]) pthread_t threads[nthreads]; size_t stacksize; int rc, t; pthread_attr_init(&attr); pthread_attr_getstacksize (&attr, &stacksize); printf("default stack size = %d\n", stacksize); stacksize = sizeof(double)*n*n+megextra; printf("amount of stack needed per thread = %d\n",stacksize); pthread_attr_setstacksize (&attr, stacksize); printf("creating threads with stack size = %d bytes\n",stacksize); for(t=0; t<nthreads; t++) rc = pthread_create(&threads[t], &attr, dowork, (void *)t); if (rc) printf("error; return code from pthread_create() is %d\n", rc); exit(-1); printf("created %d threads.\n", t); pthread_exit(null); 24

25 Mutex Variable in Threads Program #include <pthread.h> #include <stdio.h> #include <stdlib.h> /* The following structure contains the necessary information to allow the function "dotprod" to access its input data and place its output into the structure. This structure is unchanged from the sequential version. */ typedef struct double *a; double *b; double sum; int veclen; DOTDATA; 25

26 Mutex Variable in Threads Program /* Define globally accessible variables and a mutex */ #define NUMTHRDS 4 #define VECLEN 100 DOTDATA dotstr; pthread_t callthd[numthrds]; pthread_mutex_t mutexsum; 26

27 Mutex Variable in Threads Program /* The function dotprod is activated when the thread is created. As before, all input to this routine is obtained from a structure of type DOTDATA and all output from this function is written into this structure. The benefit of this approach is apparent for the multi-threaded program: when a thread is created we pass a single argument to the activated function - typically this argument is a thread number. All the other information required by the function is accessed from the globally accessible structure. */ void *dotprod(void *arg) /* Define and use local variables for convenience */ int i, start, end, offset, len ; double mysum, *x, *y; offset = (int)arg; len = dotstr.veclen; start = offset*len; end = start + len; x = dotstr.a; y = dotstr.b; 27

28 Mutex Variable in Threads Program /* Perform the dot product and assign result to the appropriate variable in the structure. */ mysum = 0; for (i=start; i<end ; i++) mysum += (x[i] * y[i]); /* Lock a mutex prior to updating the value in the shared structure, and unlock it upon updating. */ pthread_mutex_lock (&mutexsum); dotstr.sum += mysum; pthread_mutex_unlock (&mutexsum); pthread_exit((void*) 0); 28

29 Mutex Variable in Threads Program /* The main program creates threads which do all the work and then print out result upon completion. Before creating the threads, The input data is created. Since all threads update a shared structure, we need a mutex for mutual exclusion. The main thread needs to wait for all threads to complete, it waits for each one of the threads. We specify a thread attribute value that allow the main thread to join with the threads it creates. Note also that we free up handles when they are no longer needed. */ int main (int argc, char *argv[]) int i; double *a, *b; int status; pthread_attr_t attr; /* Assign storage and initialize values */ a = (double*) malloc (NUMTHRDS*VECLEN*sizeof(double)); b = (double*) malloc (NUMTHRDS*VECLEN*sizeof(double)); for (i=0; i<veclen*numthrds; i++) a[i]=1; b[i]=a[i]; 29

30 Mutex Variable in Threads Program dotstr.veclen = VECLEN; dotstr.a = a; dotstr.b = b; dotstr.sum=0; pthread_mutex_init(&mutexsum, NULL); /* Create threads to perform the dotproduct */ pthread_attr_init(&attr); pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE); for(i=0;i<numthrds;i++) /* Each thread works on a different set of data. * The offset is specified by 'i'. The size of * the data for each thread is indicated by VECLEN. */ pthread_create( &callthd[i], &attr, dotprod, (void *)i); pthread_attr_destroy(&attr); 30

31 Mutex Variable in Threads Program /* Wait on the other threads */ for(i=0;i<numthrds;i++) pthread_join( callthd[i], (void **)&status); /* After joining, print out the results and cleanup */ printf ("Sum = %f \n", dotstr.sum); free (a); free (b); pthread_mutex_destroy(&mutexsum); pthread_exit(null); 31

32 Pros & cons of threads Pros Overlap I/O with computation! Cheaper context switches Better mapping to shared memory multiprocessors Cons Potential thread interactions Complexity of debugging Complexity of multi-threaded programming Backwards compatibility with existing code 32