thread_pool.h

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/*
Copyright (c) 2013 Genome Research Ltd.
Author: James Bonfield <jkb@sanger.ac.uk>

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*/

/*
 * This file implements a thread pool for multi-threading applications.
 * It consists of two distinct interfaces: thread pools an results queues.
 *
 * The pool of threads is given a function pointer and void* data to pass in.
 * This means the pool can run jobs of multiple types, albeit first come
 * first served with no job scheduling.
 *
 * Upon completion, the return value from the function pointer is added to
 * a results queue. We may have multiple queues in use for the one pool.
 *
 * An example: reading from BAM and writing to CRAM with 10 threads. We'll
 * have a pool of 10 threads and two results queues holding decoded BAM blocks
 * and encoded CRAM blocks respectively.
 */

#ifndef _THREAD_POOL_H_
#define _THREAD_POOL_H_

#include <pthread.h>

struct t_pool;
struct t_results_queue;

typedef struct t_pool_job {
    void *(*func)(void *arg);
    void *arg;
    struct t_pool_job *next;

    struct t_pool *p;
    struct t_results_queue *q;
    int serial;
} t_pool_job;

typedef struct t_res {
    struct t_res *next;
    int serial; // sequential number for ordering
    void *data; // result itself
} t_pool_result;

struct t_pool;

typedef struct {
    struct t_pool *p;
    int idx;
    pthread_t tid;
    pthread_cond_t  pending_c;
    long long wait_time;
} t_pool_worker_t;

typedef struct t_pool {
    int qsize;    // size of queue
    int njobs;    // pending job count
    int nwaiting; // how many workers waiting for new jobs
    int shutdown; // true if pool is being destroyed

    // queue of pending jobs
    t_pool_job *head, *tail;

    // threads
    int tsize;    // maximum number of jobs
    t_pool_worker_t *t;

    // Mutexes
    pthread_mutex_t pool_m; // used when updating head/tail

    pthread_cond_t  empty_c;
    pthread_cond_t  pending_c; // not empty
    pthread_cond_t  full_c;

    // array of worker IDs free
    int *t_stack, t_stack_top;

    // Debugging to check wait time
    long long total_time, wait_time;
} t_pool;

typedef struct t_results_queue {
    t_pool_result *result_head;
    t_pool_result *result_tail;
    int next_serial;
    int curr_serial;
    int queue_len;  // number of items in queue
    int pending;    // number of pending items (in progress or in pool list)
    pthread_mutex_t result_m;
    pthread_cond_t result_avail_c;
} t_results_queue;


/*
 * Creates a worker pool of length qsize with tsize worker threads.
 *
 * Returns pool pointer on success;
 *         NULL on failure
 */
t_pool *t_pool_init(int qsize, int tsize);

/*
 * Adds an item to the work pool.
 *
 * FIXME: Maybe return 1,0,-1 and distinguish between job dispathed vs
 * result returned. Ie rather than blocking on full queue we're permitted
 * to return early on "result available" event too.
 * Caller would then have a while loop around t_pool_dispatch.
 * Or, return -1 and set errno to E_AGAIN to indicate job not yet submitted.
 *
 * Returns 0 on success
 *        -1 on failure
 */
int t_pool_dispatch(t_pool *p, t_results_queue *q,
        void *(*func)(void *arg), void *arg);
int t_pool_dispatch2(t_pool *p, t_results_queue *q,
         void *(*func)(void *arg), void *arg, int nonblock);

/*
 * Flushes the pool, but doesn't exit. This simply drains the queue and
 * ensures all worker threads have finished their current task.
 *
 * Returns 0 on success;
 *        -1 on failure
 */
int t_pool_flush(t_pool *p);

/*
 * Destroys a thread pool. If 'kill' is true the threads are terminated now,
 * otherwise they are joined into the main thread so they will finish their
 * current work load.
 *
 * Use t_pool_destroy(p,0) after a t_pool_flush(p) on a normal shutdown or
 * t_pool_destroy(p,1) to quickly exit after a fatal error.
 */
void t_pool_destroy(t_pool *p, int kill);

/*
 * Pulls a result off the head of the result queue. Caller should
 * free it (and any internals as appropriate) after use. This doesn't
 * wait for a result to be present.
 *
 * Results will be returned in strict order.
 * 
 * Returns t_pool_result pointer if a result is ready.
 *         NULL if not.
 */
t_pool_result *t_pool_next_result(t_results_queue *q);
t_pool_result *t_pool_next_result_wait(t_results_queue *q);

/*
 * Frees a result 'r' and if free_data is true also frees
 * the internal r->data result too.
 */
void t_pool_delete_result(t_pool_result *r, int free_data);

/*
 * Initialises a results queue.
 *
 * Results queue pointer on success;
 *         NULL on failure
 */
t_results_queue *t_results_queue_init(void);

/* Deallocates memory for a results queue */
void t_results_queue_destroy(t_results_queue *q);

/*
 * Returns true if there are no items on the finished results queue and
 * also none still pending.
 */
int t_pool_results_queue_empty(t_results_queue *q);

/*
 * Returns the number of completed jobs on the results queue.
 */
int t_pool_results_queue_len(t_results_queue *q);

/*
 * Returns the number of completed jobs plus the number queued up to run.
 */
int t_pool_results_queue_sz(t_results_queue *q);

#endif /* _THREAD_POOL_H_ */