具体解释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的主要差别是传递了几个參数。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运行的函数，stack是线程使用的堆栈。

再来看一下clone和pthread_create的差别：linux中的pthread_create终于调用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：把执行的进程的CPU寄存器中的数据取出存放到内核态堆栈中，同一时候把要加载的进程的数据放入到寄存器中（硬件上下文）。还会把全部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上运行变成了可能，但同一时候也带来了保存现场和载入现场的直接消耗（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。直接消耗包含：CPU寄存器须要保存和载入。系统调度器的代码须要运行，TLB实例须要又一次载入，CPU 的pipeline须要刷掉；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，也许会降低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_RR

int 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
", (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採取RR调度，并设置最高优先级，我们用VTune观察Preemption Context Switches是否会因此降低。

VTune现象：

如今设置最低优先级：

原来设置最低优先级能够降低Preemption Context Switches，可是添加了Synchronization Context Switches。

显然最高优先级执行用时少（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.