面试题：C语言线程池空闲线程资源管理

Question

在一个使用C语言实现的Linux线程池项目中，假设线程池中的空闲线程长时间不被使用，可能会导致系统资源浪费。请设计一种机制，当空闲线程达到一定时间阈值时，能够合理释放这些线程资源，并在需要时能够重新创建合适数量的线程，阐述具体实现思路及关键代码片段。

Answer 1

实现思路

1. 记录线程空闲时间：每个线程在进入空闲状态时，记录当前时间。
2. 定时检查：使用一个管理线程，定时检查所有线程的空闲时间。
3. 释放空闲线程：如果某个线程的空闲时间超过设定的阈值，将其释放。
4. 动态创建线程：当任务队列中有新任务且当前活动线程数小于设定的最大线程数时，创建新线程。

关键代码片段

1. 线程结构体定义

typedef struct {
    pthread_t tid;
    int is_busy;
    time_t idle_start_time;
} Thread;

2. 管理线程检查空闲线程

void* monitor_thread(void* arg) {
    ThreadPool* pool = (ThreadPool*)arg;
    while (1) {
        sleep(CHECK_INTERVAL);
        pthread_mutex_lock(&pool->mutex);
        for (int i = 0; i < pool->max_threads; i++) {
            if (!pool->threads[i].is_busy && (time(NULL) - pool->threads[i].idle_start_time > IDLE_THRESHOLD)) {
                pool->threads[i].is_busy = 1;
                pthread_cancel(pool->threads[i].tid);
                pool->active_threads--;
            }
        }
        pthread_mutex_unlock(&pool->mutex);
    }
    return NULL;
}

3. 任务处理线程

void* worker_thread(void* arg) {
    ThreadPool* pool = (ThreadPool*)arg;
    while (1) {
        Task task;
        pthread_mutex_lock(&pool->mutex);
        while (pool->task_queue.size == 0 &&!pool->shutdown) {
            pthread_cond_wait(&pool->cond, &pool->mutex);
        }
        if (pool->shutdown && pool->task_queue.size == 0) {
            pthread_mutex_unlock(&pool->mutex);
            pthread_exit(NULL);
        }
        task = pool->task_queue.tasks[pool->task_queue.head];
        pool->task_queue.head = (pool->task_queue.head + 1) % pool->task_queue.capacity;
        pool->task_queue.size--;
        pool->threads[pool->active_threads].is_busy = 1;
        pool->active_threads++;
        pthread_mutex_unlock(&pool->mutex);

        (task.func)(task.arg);

        pthread_mutex_lock(&pool->mutex);
        pool->threads[pool->active_threads - 1].is_busy = 0;
        pool->threads[pool->active_threads - 1].idle_start_time = time(NULL);
        pool->active_threads--;
        pthread_mutex_unlock(&pool->mutex);
    }
    return NULL;
}

4. 添加任务时动态创建线程

void add_task(ThreadPool* pool, void* (*func)(void*), void* arg) {
    pthread_mutex_lock(&pool->mutex);
    while (pool->task_queue.size == pool->task_queue.capacity &&!pool->shutdown) {
        pthread_cond_wait(&pool->not_full, &pool->mutex);
    }
    if (pool->shutdown) {
        pthread_mutex_unlock(&pool->mutex);
        return;
    }
    pool->task_queue.tasks[pool->task_queue.tail] = (Task){func, arg};
    pool->task_queue.tail = (pool->task_queue.tail + 1) % pool->task_queue.capacity;
    pool->task_queue.size++;
    if (pool->active_threads < pool->max_threads && pool->task_queue.size > pool->min_idle_threshold) {
        int i;
        for (i = 0; i < pool->max_threads; i++) {
            if (!pool->threads[i].is_busy) {
                break;
            }
        }
        if (i == pool->max_threads) {
            if (pthread_create(&pool->threads[pool->max_threads].tid, NULL, worker_thread, pool) != 0) {
                // 处理创建线程失败
            }
            pool->max_threads++;
        }
    }
    pthread_cond_signal(&pool->not_empty);
    pthread_mutex_unlock(&pool->mutex);
}

以上代码为简化示意，实际使用中可能需要更多错误处理和完善。