详解clone函数

我们都知道linux中创建新进程是系统调用fork，但实际上fork是clone功能的一部分，clone和fork的主要区别是传递了几个参数。clone隶属于libc，它的意义就是实现线程。

看一下clone函数：

int clone(int (*fn)(void * arg), void *stack, int flags, void * arg);

fn就是即将创建的线程要执行的函数，stack是线程使用的堆栈。

再来看一下clone和pthread_create的区别：linux中的pthread_create最终调用clone。

我们的目的不是为了介绍clone，而是探究clone中的上下文切换问题。

（1）进程切换：把运行的进程的CPU寄存器中的数据取出存放到内核态堆栈中，同时把要载入的进程的数据放入到寄存器中（硬件上下文），还会把所有一切的状态信息进行切换。

（2）时间片轮转的方式使多个任务在同一颗CPU上执行变成了可能，但同时也带来了保存现场和加载现场的直接消耗（上下文切换会带来直接和间接两种因素影响程序性能的消耗。直接消耗包括：CPU寄存器需要保存和加载，系统调度器的代码需要执行，TLB实例需要重新加载，CPU 的pipeline需要刷掉；间接消耗指的是多核的cache之间得共享数据，间接消耗对于程序的影响要看线程工作区操作数据的大小）。

（3）clone任务[1]：

• Allocate data structures for thread representation
  
• Initialize structures according to clone parameters
  
• Set up kernel and user stack as well as argument for the thread function
  
• Put the thread on the corresponding CPU core’s run queue
  
• Notify target core via an interrupt so that the new thread will be scheduled
  

（4）我们在clone出线程时指定高的优先级，或许会减少因抢占而造成的上下文切花开销。

#include <pthread.h>#include <stdio.h>#include <stdlib.h>#include <errno.h>#include <assert.h>#define N 4#define M 30000#define THREAD_NUM      4#define POLICY          SCHED_RRint nwait = 0;volatile long long sum;long loops = 6e3;pthread_mutex_t mutex;void set_affinity(int core_id) {cpu_set_t cpuset;CPU_ZERO(&cpuset);CPU_SET(core_id, &cpuset);assert(pthread_setaffinity_np(pthread_self(), sizeof(cpu_set_t), &cpuset) == 0);}void* thread_func(void *arg) {//set_affinity((int)(long)arg);for (int j = 0; j < M; j++) {pthread_mutex_lock(&mutex);nwait++;for (long i = 0; i < loops; i++) // This is the key of speedup for parrot: the mutex needs to be a little bit congested.sum += i;pthread_mutex_unlock(&mutex);for (long i = 0; i < loops; i++)sum += i*i*i*i*i*i;//fprintf(stderr, "compute thread %u %d\n", (unsigned)pthread_self(), sched_getcpu());  }}int main() {    //set_affinity(23);    pthread_t             threads[THREAD_NUM], id;    pthread_attr_t        attrs[THREAD_NUM];    struct sched_param    scheds[THREAD_NUM], sched;    int                   idxs[THREAD_NUM];    int                   policy, i, ret;    id = pthread_self();    ret = pthread_getschedparam(id, &policy, &sched);    assert(!ret && "main pthread_getschedparam failed!");    sched.sched_priority = sched_get_priority_max(POLICY);    ret = pthread_setschedparam(id, POLICY, &sched); //set policy and corresponding priority    assert(!ret && "main pthread_setschedparam failed!");    for (i = 0; i < THREAD_NUM; i++) {        idxs[i] = i;        ret = pthread_attr_init(&attrs[i]);assert(!ret && "pthread_attr_init failed!");               ret = pthread_attr_getschedparam(&attrs[i], &scheds[i]);assert(!ret && "pthread_attr_getschedparam failed!");           ret = pthread_attr_setschedpolicy(&attrs[i], POLICY);assert(!ret && "pthread_attr_setschedpolicy failed!");          scheds[i].sched_priority = sched_get_priority_max(POLICY);              ret = pthread_attr_setschedparam(&attrs[i], &scheds[i]);assert(!ret && "pthread_attr_setschedparam failed!");          ret = pthread_attr_setinheritsched(&attrs[i], PTHREAD_EXPLICIT_SCHED);assert(!ret && "pthread_attr_setinheritsched failed!");    }    for (i = 0; i < THREAD_NUM; i++) {        ret = pthread_create(&threads[i], &attrs[i], thread_func, &idxs[i]);assert(!ret && "pthread_create() failed!");    }    for (i = 0; i < THREAD_NUM; i++)        ret = pthread_join(threads[i], NULL);    return 0;}

我们让四个子线程和主线程都采取RR调度，并设置最高优先级，我们用VTune观察Preemption Context Switches是否会因此减少。

VTune现象：

现在设置最低优先级：

原来设置最低优先级可以减少Preemption Context Switches，但是增加了Synchronization Context Switches。显然最高优先级运行用时少（4.470s，而最低优先级用时7.280s）。

REFERENCES:

[1] Balazs Gerofi, etc, Clone n(): Parallel Thread Creation for Upcoming Many-Core Architectures, 2012, IEEE International Conference on Cluster Computing.