threadpool.c

/*
 * See Licensing and Copyright notice in threadpool.h
 */
/*
 * @brief A simple threadpool implementation using a single queue.
 *
 * The queue is inspired by this paper (look for the queue with two locks):
 *
 * Maged M. Michael and Michael L. Scott. 1998. Nonblocking algorithms and
 * preemption-safe locking on multiprogrammed shared memory multiprocessors. J.
 * Parallel Distrib. Comput. 51, 1 (May 1998), 1-26. DOI=10.1006/jpdc.1998.1446
 * http://dx.doi.org/10.1006/jpdc.1998.1446
 *
 * @ARTICLE{Michael98non-blockingalgorithms,
 *    author = {Maged M. Michael and Michael L. Scott},
 *    title = {Non-Blocking Algorithms and Preemption-Safe Locking on Multiprogrammed Shared Memory Multiprocessors},
 *    journal = {Journal of Parallel and Distributed Computing},
 *    year = {1998},
 *    volume = {51},
 *    pages = {1--26},
 * }
 *
 * @note The algorithm/strategy for killing idle workers should be moved into
 *       the threadhandler and it should also be improved (the current strategy
 *       is probably not very good).
 */
 
#include <stdlib.h>
#include "SDL.h"
#include "SDL_error.h"
#include "SDL_thread.h"
#include "threadpool.h"
 
#include "log.h"
 
#define THREADPOOL_TIMEOUT (5 * 100) /* The time a worker thread waits in ms. */
#define THREADSIG_STOP     (1) /* The signal to stop a worker thread */
#define THREADSIG_RUN      (0) /* The signal to indicate the worker thread is running */
 
static int MAXTHREADS = 8; /* Bit overkill, but oh well. */
 
typedef struct Node_ {
   void *data;          /* The element in the list */
   struct Node_ *next;  /* The next node in the list */
} Node;
 
struct ThreadQueue_ {
   Node *first;         /* The first node */
   Node *last;          /* The second node */
   /* A semaphore to ensure reads only happen when the queue is not empty */
   SDL_sem *semaphore;
   SDL_mutex *t_lock;   /* Tail lock. Lock when reading/updating tail */
   SDL_mutex *h_lock;   /* Same as tail lock, except it's head lock */
};
 
typedef struct ThreadQueueData_ {
   int (*function)(void *);  /* The function to be called */
   void *data;               /* And its arguments */
} ThreadQueueData;
 
typedef struct ThreadData_ {
   int (*function)(void *); /* The function to be called */
   void *data;             /* Arguments to the above function */
   int signal;             /* Signals to the thread */
   SDL_sem *semaphore;     /* The semaphore to signal new jobs or new signal in the
                              'signal' variable */
   ThreadQueue *idle;      /* The queue with idle threads */
   ThreadQueue *stopped;   /* The queue with stopped threads */
} ThreadData;
 
typedef struct vpoolThreadData_ {
   SDL_cond *cond;         /* Condition variable for signalling all jobs in the vpool
                              are done */
   SDL_mutex *mutex;       /* The mutex to use with the above condition variable */
   int *count;             /* Variable to count number of finished jobs in the vpool */
   ThreadQueueData *node;  /* The job to be done */
} vpoolThreadData;
 
/* The global threadpool queue */
static ThreadQueue *global_queue = NULL;
 
/*
 * Prototypes.
 */
static ThreadQueue* tq_create (void);
static void tq_enqueue( ThreadQueue *q, void *data );
static void* tq_dequeue( ThreadQueue *q );
static void tq_destroy( ThreadQueue *q );
static int threadpool_worker( void *data );
static int threadpool_handler( void *data );
static int vpool_worker( void *data );
 
static ThreadQueue* tq_create (void)
{
   ThreadQueue *q;
   Node *n;
 
   /* Queue memory allocation. */
   q        = calloc( 1, sizeof(ThreadQueue) );
 
   /* Allocate and insert the dummy node */
   n        = calloc( 1, sizeof(Node) );
   n->next  = NULL;
   q->first = n;
   q->last  = n;
 
   /* Create locks. */
   q->t_lock      = SDL_CreateMutex();
   q->h_lock      = SDL_CreateMutex();
   q->semaphore   = SDL_CreateSemaphore( 0 );
 
   return q;
}
 
static void tq_enqueue( ThreadQueue *q, void *data )
{
   /* Allocate new struct. */
   Node *n  = calloc( 1, sizeof(Node) );
   n->data  = data;
   n->next  = NULL;
 
   /* Lock */
   SDL_mutexP( q->t_lock );
 
   /* Enqueue. */
   q->last->next  = n;
   q->last        = n;
 
   /* Signal and unlock. This wil break if someone tries to enqueue 2^32+1
    * elements or something. */
   SDL_SemPost( q->semaphore );
   SDL_mutexV( q->t_lock );
}
 
static void* tq_dequeue( ThreadQueue *q )
{
   void *d;
   Node *newhead, *node;
 
   /* Lock the head. */
   SDL_mutexP( q->h_lock );
 
   /* Start running. */
   node     = q->first;
   newhead  = node->next;
 
   /* Head not consistent. */
   if (newhead == NULL) {
      WARN(_("Tried to dequeue while the queue was empty!"));
      /* Ugly fix :/ */
      /*
      SDL_mutexV(q->h_lock);
      return NULL;
      */
      /* We prefer to wait until the cache updates :/ */
      do {
         node     = q->first;
         newhead  = node->next;
      } while (newhead == NULL);
   }
 
   /* Remember the value and assign newhead as the new dummy element. */
   d        = newhead->data;
   q->first = newhead;
 
   /* Unlock */
   SDL_mutexV( q->h_lock );
 
   free( node );
   return d;
}
 
static void tq_destroy( ThreadQueue *q )
{
   /* Iterate through the list and free the nodes */
   while (q->first->next != NULL)
      free( tq_dequeue(q) ); /* Locks q->t_lock, so we must destroy mutex after. */
 
   /* Clean up threading structures. */
   SDL_DestroySemaphore( q->semaphore );
   SDL_DestroyMutex( q->h_lock );
   SDL_DestroyMutex( q->t_lock );
 
   free( q->first );
   free( q );
}
 
int threadpool_newJob( int (*function)(void *), void *data )
{
   ThreadQueueData *node;
 
   if (global_queue == NULL) {
      WARN(_("Threadpool has not been initialized yet!"));
      return -2;
   }
 
   /* Allocate and set parameters. */
   node           = calloc( 1, sizeof(ThreadQueueData) );
   node->data     = data;
   node->function = function;
 
   /* Actually enque. */
   tq_enqueue( global_queue, node );
 
   return 0;
}
 
static int threadpool_worker( void *data )
{
   ThreadData *work;
 
   work = (ThreadData*) data;
 
   /* Work loop */
   while (1) {
      /* Wait for new signal */
      while (SDL_SemWait( work->semaphore ) == -1) {
          /* Putting this in a while-loop is probably a really bad idea, but I
           * don't have any better ideas. */
          WARN(_("SDL_SemWait failed! Error: %s"), SDL_GetError());
      }
      /* Break if received signal to stop */
      if (work->signal == THREADSIG_STOP)
         break;
 
      /* Do work :-) */
      work->function( work->data );
 
      /* Enqueue itself in the idle worker threads queue */
      tq_enqueue( work->idle, work );
   }
   /* Enqueue itself in the stopped worker threads queue when stopped */
   tq_enqueue( work->stopped, work );
 
   return 0;
}
 
static int threadpool_handler( void *data )
{
   (void) data;
   int nrunning, newthread;
   ThreadData *threadargs, *threadarg;
   /* Queues for idle workers and stopped workers */
   ThreadQueue *idle, *stopped;
   ThreadQueueData *node;
 
   /* Initialize the idle and stopped queues. */
   idle     = tq_create();
   stopped  = tq_create();
 
   /* Allocate threadargs to communicate with workers */
   threadargs = calloc( MAXTHREADS, sizeof(ThreadData) );
 
   /* Initialize threadargs */
   for (int i=0; i<MAXTHREADS; i++) {
      threadargs[i].function  = NULL;
      threadargs[i].data      = NULL;
      threadargs[i].semaphore = SDL_CreateSemaphore( 0 ); /* Used to give orders. */
      threadargs[i].idle      = idle;
      threadargs[i].stopped   = stopped;
      threadargs[i].signal    = THREADSIG_RUN;
      /* 'Workers' that do not have a thread are considered stopped */
      tq_enqueue( stopped, &threadargs[i] );
   }
 
   /* Set the number of running threads to 0 */
   nrunning = 0;
 
   /*
    * Thread handler main loop.
    */
   while (1) {
      /*
       * We must now wait, this shall be done on each active thread. However they will
       * be put to sleep as time passes. When we receive a command we'll proceed to process
       * it.
       */
      if (nrunning > 0) {
         /*
          * Here we'll wait until thread gets work to do. If it doesn't it will
          * just stop a worker thread and wait until it gets something to do.
          */
         if (SDL_SemWaitTimeout( global_queue->semaphore, THREADPOOL_TIMEOUT ) != 0) {
            /* There weren't any new jobs so we'll start killing threads ;) */
            if (SDL_SemTryWait( idle->semaphore ) == 0) {
               threadarg         = tq_dequeue( idle );
               /* Set signal to stop worker thread */
               threadarg->signal = THREADSIG_STOP;
               /* Signal thread and decrement running threads counter */
               SDL_SemPost( threadarg->semaphore );
               nrunning -= 1;
            }
 
            /* We just go back to waiting on a thread. */
            continue;
         }
 
         /* We got work. Continue to handle work. */
      }
      else {
         /*
          * Here we wait for a new job. No threads are alive at this point and the
          * threadpool is just patiently waiting for work to arrive.
          */
         if (SDL_SemWait( global_queue->semaphore ) == -1) {
             WARN(_("SDL_SemWait failed! Error: %s"), SDL_GetError());
             continue;
         }
 
         /* We got work. Continue to handle work. */
      }
 
      /*
       * Get a new job from the queue. This should be safe as we have received
       * a permission from the global_queue->semaphore.
       */
      node        = tq_dequeue( global_queue );
      newthread   = 0;
 
      /*
       * Choose where to get the thread. Either idle, revive stopped or block until
       * another thread becomes idle.
       */
      /* Idle thread available */
      if (SDL_SemTryWait(idle->semaphore) == 0)
         threadarg         = tq_dequeue( idle );
      /* Make a new thread */
      else if (SDL_SemTryWait(stopped->semaphore) == 0) {
         threadarg         = tq_dequeue( stopped );
         threadarg->signal = THREADSIG_RUN;
         newthread         = 1;
      }
      /* Wait for idle thread */
      else {
         while (SDL_SemWait(idle->semaphore) == -1) {
             /* Bad idea */
             WARN(_("SDL_SemWait failed! Error: %s"), SDL_GetError());
         }
         threadarg         = tq_dequeue( idle );
      }
 
      /* Assign arguments for the thread */
      threadarg->function  = node->function;
      threadarg->data      = node->data;
      /* Signal the thread that there's a new job */
      SDL_SemPost( threadarg->semaphore );
 
      /* Start a new thread and increment the thread counter */
      if (newthread) {
         SDL_CreateThread( threadpool_worker,
               "threadpool_worker",
               threadarg );
         nrunning += 1;
      }
 
      /* Free the now unused job from the global_queue */
      free(node);
   }
   /* Clean up. */
   tq_destroy( idle );
   tq_destroy( stopped );
   free( threadargs );
 
   return 0;
}
 
int threadpool_init (void)
{
   MAXTHREADS = SDL_GetCPUCount() + 1; /* SDL 1.3 is pretty cool. */
 
   /* There's already a queue */
   if (global_queue != NULL) {
      WARN(_("Threadpool has already been initialized!"));
      return -1;
   }
 
   /* Create the global queue queue */
   global_queue = tq_create();
 
   /* Initialize the threadpool handler. */
   if ( SDL_CreateThread( threadpool_handler, "threadpool_handler", NULL ) == NULL ) {
      ERR( _( "Threadpool init failed: %s" ), SDL_GetError() );
      return -1;
   }
 
   return 0;
}
 
ThreadQueue* vpool_create (void)
{
   return tq_create();
}
 
void vpool_enqueue( ThreadQueue *queue, int (*function)(void *), void *data )
{
   ThreadQueueData *node;
 
   /* Allocate and set up data. */
   node           = calloc( 1, sizeof(ThreadQueueData) );
   node->data     = data;
   node->function = function;
 
   /* Add to vpool. */
   tq_enqueue( queue, node );
}
 
static int vpool_worker( void *data )
{
   int cnt;
   vpoolThreadData *work = (vpoolThreadData*) data;
   ThreadQueueData *node = work->node;
 
   /* Do work */
   node->function( node->data );
 
   /* Decrement the counter and signal vpool_wait if all threads are done */
   SDL_mutexP( work->mutex );
   cnt   = *(work->count) - 1;
   if (cnt <= 0)                    /* All jobs done. */
      SDL_CondSignal( work->cond );  /* Signal waiting thread */
   *(work->count) = cnt;
   SDL_mutexV( work->mutex );
 
   /* 'work' may not be valid memory anymore. */
   free( node );
 
   return 0;
}
 
/* @brief Run every job in the vpool queue and block until every job in the
 *        queue is done.
 *
 * @note It destroys the queue when it's done.
 */
void vpool_wait( ThreadQueue *queue )
{
   int cnt;
   SDL_cond *cond;
   SDL_mutex *mutex;
   vpoolThreadData *arg;
   ThreadQueueData *node;
 
   /* Create temporary threading structures. */
   cond  = SDL_CreateCond();
   mutex = SDL_CreateMutex();
   /* This might be a little ugly (and inefficient?) */
   cnt   = SDL_SemValue( queue->semaphore );
 
   /* Allocate all vpoolThreadData objects */
   arg = calloc( cnt, sizeof(vpoolThreadData) );
 
   SDL_mutexP( mutex );
   /* Initialize the vpoolThreadData */
   for (int i=0; i<cnt; i++) {
      /* This is needed to keep the invariants of the queue */
      while (SDL_SemWait( queue->semaphore ) == -1) {
          /* Again, a really bad idea */
          WARN(_("SDL_SemWait failed! Error: %s"), SDL_GetError());
      }
      node = tq_dequeue( queue );
 
      /* Set up arguments. */
      arg[i].node    = node;
      arg[i].cond    = cond;
      arg[i].mutex   = mutex;
      arg[i].count   = &cnt;
 
      /* Launch new job. */
      threadpool_newJob( vpool_worker, &arg[i] );
   }
 
   /* Wait for the threads to finish */
   SDL_CondWait( cond, mutex );
   SDL_mutexV( mutex );
 
   /* Clean up */
   SDL_DestroyMutex( mutex );
   SDL_DestroyCond( cond );
   tq_destroy( queue );
   free( arg );
}