c++ - linux CFS schedualar下C/C++多线程场景出现意外结果-6ren

c++ - linux CFS schedualar下C/C++多线程场景出现意外结果

转载作者：太空宇宙更新时间：2023-11-04 04:02:36

我在主线程内创建了多个线程(4个线程)。虽然每个线程执行相同的功能，线程的调度与预期的不一样。根据我对操作系统的理解，linux CFS 调度程序将分配“t”虚拟运行时间量，并且在该时间量到期时，CPU 被当前线程抢占，分配给下一个线程。通过这种方式，每个线程都将获得公平的 CPU 份额。我得到的并不符合预期。

我期望所有线程(线程 1-4、主线程)都会在同一线程(任何)下次获得 CPU 之前获得 CPU。

预期输出为

foo3-->1--->Time Now : 00:17:45.346225000

foo3-->1--->Time Now : 00:17:45.348818000

foo4-->1--->Time Now : 00:17:45.350216000

foo4-->1--->Time Now : 00:17:45.352800000

main is running ---> 1--->Time Now : 00:17:45.355803000

main is running ---> 1--->Time Now : 00:17:45.360606000

foo2-->1--->Time Now : 00:17:45.345305000

foo2-->1--->Time Now : 00:17:45.361666000

foo1-->1--->Time Now : 00:17:45.354203000

foo1-->1--->Time Now : 00:17:45.362696000

foo1-->2--->Time Now : 00:17:45.362716000 // foo1 thread got CPU 2nd time as expected

foo1-->2--->Time Now : 00:17:45.365306000

但是我得到

foo3-->1--->Time Now : 00:17:45.346225000

foo3-->1--->Time Now : 00:17:45.348818000

foo4-->1--->Time Now : 00:17:45.350216000

foo4-->1--->Time Now : 00:17:45.352800000

main is running ---> 1--->Time Now : 00:17:45.355803000

main is running ---> 1--->Time Now : 00:17:45.360606000

foo3-->2--->Time Now : 00:17:45.345305000 // // foo3 thread got CPU 2nd time UNEXPECTEDLY before scheduling other threads as per CFS

foo3-->2--->Time Now : 00:17:45.361666000

foo1-->1--->Time Now : 00:17:45.354203000

foo1-->1--->Time Now : 00:17:45.362696000

foo1-->2--->现在时间:00:17:45.362716000

foo1-->2--->现在时间:00:17:45.365306000

这是我的程序(thread_multi.cpp)
#include <pthread.h>
#include <stdio.h>
#include "boost/date_time/posix_time/posix_time.hpp"
#include <iostream>
#include <cstdlib>
#include <fstream>
#define NUM_THREADS  4

using namespace std;

std::string now_str()
{
    // Get current time from the clock, using microseconds resolution
    const boost::posix_time::ptime now = 
        boost::posix_time::microsec_clock::local_time();

    // Get the time offset in current day
    const boost::posix_time::time_duration td = now.time_of_day();
    const long hours        = td.hours();
    const long minutes      = td.minutes();
    const long seconds      = td.seconds();


    const long nanoseconds = td.total_nanoseconds() - ((hours * 3600 + minutes * 60 + seconds) * 1000000000);
    char buf[40];

    sprintf(buf, "Time Now : %02ld:%02ld:%02ld.%03ld", hours, minutes, seconds, nanoseconds);
    return buf;
}


/* This is our thread function.  It is like main(), but for a thread*/
void *threadFunc(void *arg)
{
    char *str;
    int i = 0;

    str=(char*)arg;

    while(i < 100 )
    {

        ++i;
        ofstream myfile ("example.txt", ios::out | ios::app | ios::binary);

        if (myfile.is_open())
          {
            myfile << str <<"-->"<<i<<"--->" <<now_str() <<" \n";

          }
          else cout << "Unable to open file";
        // generate delay 
        for(volatile int k=0;k<1000000;k++);



        if (myfile.is_open())
          {
            myfile << str <<"-->"<<i<<"--->" <<now_str() <<"\n\n";
            myfile.close();
          }
          else cout << "Unable to open file";
    }


}

int main(void)
{
    pthread_t pth[NUM_THREADS]; // this is our thread identifier
    int i = 0;

    pthread_create(&pth[0],NULL, threadFunc,  (void *) "foo1");
    pthread_create(&pth[1],NULL,  threadFunc, (void *) "foo2");
    pthread_create(&pth[2],NULL, threadFunc,  (void *) "foo3");
    pthread_create(&pth[3],NULL,  threadFunc, (void *) "foo4");



std::cout <<".............\n" <<now_str() << '\n';      

    while(i < 100)
    {

        for(int k=0;k<1000000;k++);

        ofstream myfile ("example.txt", ios::out | ios::app | ios::binary);
          if (myfile.is_open())
          {
            myfile << "main is running ---> "<< i <<"--->"<<now_str() <<'\n';
            myfile.close();
          }
          else cout << "Unable to open file";


        ++i;
    }

//  printf("main waiting for thread to terminate...\n");
    for(int k=0;k<4;k++)
    pthread_join(pth[k],NULL);

std::cout <<".............\n" <<now_str() << '\n';      
    return 0;
}
这是完全公平的调度程序详细信息

kernel.sched_min_granularity_ns = 100000 kernel.sched_wakeup_granularity_ns = 25000 kernel.sched_latency_ns = 1000000

根据 sched_min_capsularity_ns 值，任何任务都将在该最短时间内执行，如果任务需要超过该最短时间，则计算时间片，并且每个任务都将在该时间片内执行。

这里时间片的计算使用公式，

time slice = ( weight of each task / total weight of all tasks under that CFS run-queue ) x sched_latency_ns

谁能解释一下为什么我会得到这些调度结果？？？任何有助于理解输出的帮助都将受到高度赞赏。预先感谢您。

我在linux下使用gcc。

编辑1:

如果我改变这个循环

for(int k=0;k<100000;k++);

进入

for(int k=0;k<10000;k++);

然后有时线程1连续10次获得CPU，线程2连续5次获得CPU，线程3连续5次获得CPU，主线程连续2次获得CPU，线程4连续7次获得CPU。看起来不同的线程在随机时间被抢占。

关于这些随机的连续 CPU 分配给不同线程的次数有什么线索吗？？