course/semester2/pprog/assignment1/mthread/nbody.c

   1 /**
   2  * @file nbody.c
   3  * @purpose Implentation of multi-threaded N-Body simulator using pthreads
   4  * @author Sam Moore (20503628) - 2012
   5  */
   6
   7 #include "nbody.h" // Declarations
   8 #include "../single-thread/nbody.c" // Include all functions from the single threaded version
   9
  10 #include "graphics.h" // For declaration of Graphics_Run only
  11
  12 // --- Variable declarations --- //
  13
  14 pthread_t compute_thread; // The thread responsible for computations; it spawns worker threads
  15
  16 pthread_t * worker_thread = NULL; //Array of worker threads responsible for Force and Position updates
  17 System * sub_system = NULL; //Array of Systems used to divide up the main "universe" System for worker threads
  18 pthread_mutex_t mutex_workers; // Mutex used for the barrier between Force and Position updates
  19 pthread_cond_t workers_done_cv; // Conditional used for the barrier between Force and Position updates
  20 unsigned workers_busy; // Number of workers currently doing something
  21
  22 pthread_mutex_t mutex_runstate; // Mutex around the runstate
  23
  24
  25
  26 /**
  27  * @function Compute_Thread
  28  * @purpose Thread - Continuously computes steps for a system of bodies. Seperate from graphics functions.
  29  *      Spawns worker threads to divide up computation.
  30  * @param arg - Can be cast to the System for which computations are performed (ie: &universe)
  31  */
  32 void * Compute_Thread(void * arg)
  33 {
  34
  35         System * s = (System*)(arg); //cast argument to a System*
  36
  37
  38         // If no number of threads provided, use the default value, unless someone changed that to a stupid value
  39         if (options.num_threads <= 0)
  40                 options.num_threads = (DEFAULT_WORKING_THREADS > 1) ? DEFAULT_WORKING_THREADS : 1;
  41
  42         // Do a sanity check; there is no point spawning more threads than bodies.
  43         if (options.num_threads > s->N)
  44         {
  45                 fprintf(stderr,
  46                         "Warning: Using %u threads instead of %u specified, because there are only %u bodies to simulate!\n",
  47                         s->N, options.num_threads, s->N);
  48                 options.num_threads = s->N;
  49         }
  50
  51         if (options.num_threads > 1) // Allocate worker threads and sub systems, as long as there would be more than 1
  52         {
  53                 worker_thread = (pthread_t*)(calloc(options.num_threads, sizeof(pthread_t)));
  54                 if (worker_thread == NULL)
  55                 {
  56                         perror("Couldn't allocate array of worker threads");
  57                         QuitProgram(true);
  58                         pthread_exit(NULL);
  59                 }
  60                 sub_system = (System*)(calloc(options.num_threads, sizeof(System)));
  61                 if (sub_system == NULL)
  62                 {
  63                         perror("Couldn't allocate array of systems for worker threads to use");
  64                         QuitProgram(true);
  65                         pthread_exit(NULL);
  66                 }
  67
  68                 // Divide up the Body array owned by s into options.num_threads arrays, one for each worker thread
  69                 unsigned bodies_per_system = (s->N) / options.num_threads;
  70                 unsigned remainder = (s->N) % options.num_threads;
  71                 for (unsigned i = 0; i < options.num_threads; ++i)
  72                 {
  73                         sub_system[i].body = (s->body)+(i*bodies_per_system);
  74                         sub_system[i].N = bodies_per_system;
  75                         sub_system[i].steps = 0;
  76                         if (i == options.num_threads - 1)
  77                                 sub_system[i].N += remainder; // The last thread gets the remainder
  78
  79                 }
  80         }
  81
  82
  83         pthread_attr_t attr; //thread attribute for the workers.
  84         pthread_attr_init(&attr);
  85         pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED); //Needs to be detached, so that memory can be reused.
  86
  87         // The main computation loop
  88         while (true)
  89         {
  90
  91                 if (runstate != RUN) pthread_exit(NULL); //Check whether the thread needs to exit
  92
  93
  94
  95                 //Check whether the program should quit due to steps being computed, or a timeout
  96                 if (ExitCondition())
  97                 {
  98                         QuitProgram(false);
  99                         continue; // The check at the start of the next loop will stop the thread
 100                 }
 101
 102                 if (options.draw_graphics == false && options.verbosity != 0
 103                         && universe.steps % options.verbosity == 1)
 104                 {
 105                         DisplayStatistics();
 106                 }
 107
 108
 109                 if (options.num_threads <= 1)
 110                 {
 111                         // Just do everything in this thread
 112                         System_Forces(s, s);
 113                         System_Positions(s);
 114                         continue;
 115                 }
 116
 117
 118
 119
 120                 workers_busy = options.num_threads; //All threads working
 121
 122                 //Compute forces
 123                 for (unsigned i = 0; i < options.num_threads; ++i)
 124                 {
 125                         if (pthread_create(worker_thread+i, &attr, Force_Thread, (void*)(sub_system+i)) != 0)
 126                         {
 127                                 perror("In compute thread, couldn't create worker thread (force)");
 128                                 QuitProgram(true);
 129                                 pthread_exit(NULL);
 130                         }
 131                 }
 132
 133
 134
 135                 //Barrier - Wait for forces to be computed
 136                 pthread_mutex_lock(&mutex_workers);
 137                 while (workers_busy > 0)
 138                         pthread_cond_wait(&workers_done_cv, &mutex_workers);
 139                 pthread_mutex_unlock(&mutex_workers);
 140
 141                 //All the forces are now computed
 142
 143                 workers_busy = options.num_threads; //All threads working
 144
 145                 //Compute positions
 146                 for (unsigned i = 0; i < options.num_threads; ++i)
 147                 {
 148                         if (pthread_create(worker_thread+i, &attr, Position_Thread, (void*)(sub_system+i)) != 0)
 149                         {
 150                                 perror("In compute thread, couldn't create worker thread (position)");
 151                                 QuitProgram(true);
 152                                 pthread_exit(NULL);
 153                         }
 154                 }
 155
 156                 //Wait for positions to be computed
 157                 pthread_mutex_lock(&mutex_workers);
 158                 while (workers_busy > 0)
 159                         pthread_cond_wait(&workers_done_cv, &mutex_workers);
 160                 pthread_mutex_unlock(&mutex_workers);
 161
 162                 //Update number of steps computed
 163                 universe.steps += 1;
 164
 165
 166
 167         }
 168 }
 169
 170 /**
 171  * @function Force_Thread
 172  * @purpose Thread - Calculates the forces on objects in a System
 173  * @param s - Can be cast to the System*
 174  */
 175 void * Force_Thread(void * s)
 176 {
 177
 178         System_Forces((System*)s, &universe); //Simple wrapper
 179
 180         pthread_mutex_lock(&mutex_workers);
 181         workers_busy -= 1;      // Count this thread as done
 182         if (workers_busy == 0)
 183         {
 184                 pthread_cond_signal(&workers_done_cv); // All threads done; wake up the compute_thread
 185         }
 186         pthread_mutex_unlock(&mutex_workers);
 187         return NULL;
 188 }
 189
 190 /**
 191  * @function Position_Thread
 192  * @purpose Thread - Calculates the positions of objects in a System
 193  * @param s - Can be cast to the System*
 194  */
 195 void * Position_Thread(void * s)
 196 {
 197
 198         System_Positions((System*)s); // Simple wrapper
 199
 200         pthread_mutex_lock(&mutex_workers);
 201         workers_busy -= 1; // Count this thread as done
 202         if (workers_busy == 0)
 203         {
 204                 pthread_cond_signal(&workers_done_cv); //All threads done; wake up the compute_thread
 205         }
 206         pthread_mutex_unlock(&mutex_workers);
 207         return NULL;
 208 }
 209
 210 /**
 211  * @function QuitProgram
 212  * @purpose This function can either be called by the main thread in order to signal other threads
 213  *              that it wants to exit. The main thread then calls pthread_join before exiting.
 214  *      It can also be called by a child thread to request the main thread to exit.
 215  *      It is only used this way if there is an unrecovarable error (ie: Can't allocate memory in a child thread)
 216  */
 217 void QuitProgram(bool error)
 218 {
 219         if (runstate == QUIT || runstate == QUIT_ERROR)
 220                 return; //Don't do anything if already quitting
 221         pthread_mutex_lock(&mutex_runstate); // aquire mutex
 222         if (error) // set the runstate
 223                 runstate = QUIT_ERROR;
 224         else
 225                 runstate = QUIT;
 226         pthread_mutex_unlock(&mutex_runstate); //release mutex
 227 }
 228
 229 /**
 230  * @function Thread_Cleanup
 231  * @purpose Will be called in the main thread when exit() is called
 232  *      Automatically tells all other threads to quit (if they haven't already been told)
 233  *      Then waits for them to finish.
 234  *      Also frees memory associated with the worker threads.
 235  */
 236 void Thread_Cleanup(void)
 237 {
 238         if (runstate == RUN) // If this is true, as far as child threads are concerned, the simulation is still running
 239                 QuitProgram(false); // So call QuitProgram which will set runstate, and cause child threads to exit
 240         pthread_join(compute_thread, NULL);
 241         free(worker_thread);
 242         free(sub_system);
 243 }
 244
 245
 246 /**
 247  * @function Simulation_Run
 248  * @purpose Initialise and start the simulation. Will be called in the main thread.
 249  *      Replaces the single-threaded macro that does nothing, and sets up the compute thread
 250  * @param argc - Number of arguments - Passed to Graphics_Run if needed
 251  * @param argv - Argument strings - Passed to Graphics_Run if needed
 252  */
 253 void Simulation_Run(int argc, char ** argv)
 254 {
 255         atexit(Thread_Cleanup);
 256
 257         if (options.draw_graphics)
 258         {
 259                 // The graphics are enabled, so create a thread to do computations
 260                 // Graphics are done in the main loop
 261                 if (pthread_create(&compute_thread, NULL, Compute_Thread, (void*)&universe) != 0)
 262                 {
 263                         perror("Error creating compute thread");
 264                         exit(EXIT_FAILURE);
 265                 }
 266                 Graphics_Run(argc, argv);
 267         }
 268         else
 269                 Compute_Thread((void*)(&universe)); // Graphics are disabled, so do computations in the main thread
 270 }