[转]并行计算简介和多核CPU编程Demo

[sapzj1984]

2006年是双核的普及年，双核处理器出货量开始超过单核处理器出货量；2006年的11月份Intel开始供货4核；AMD今年也将发布4核，并计划今年下半年发布8核；
按照Intel一个文档所说:"假定22纳米处理时帧上有一枚13毫米大小的处理器，其上有40亿个晶体管、48MB高速缓存，功耗为100W。利用如此数量的晶体管，我们可设计拥有12个较大内核、48个（多核）中型内核、或144个小型内核（许多个内核）的处理器。"
而且Intel已经开发完成了一款80核心处理器原型，速度达到每秒一万亿次浮点运算。

随着个人多核CPU的普及，充分利用多核CPU的性能优势摆在了众多开发人员的面前；
以前的CPU升级，很多时候软件性能都能够自动地获得相应提升，而面对多核CPU，免费的午餐没有了，开发人员必须手工的完成软件的并行化，以从爆炸性增长的CPU性能中获益；
(ps:我想，以后的CPU很可能会集成一些专门用途的核(很可能设计成比较通用的模式)，比如GPU的核、图象处理的核、向量运算的核、加解密编解码的核、FFT计算的核、物理计算的核、神经网络计算的核等等:D )

先来看一下单个CPU上的并行计算:
单CPU上常见的并行计算：多级流水线(提高CPU频率的利器)、超标量执行(多条流水线并同时发送多条指令)、乱序执行(指令重排)、单指令流多数据流SIMD、超长指令字处理器(依赖于编译器分析)等

并行计算简介
并行平台的通信模型: 共享数据(POSIX、windows线程、OpenMP)、消息交换(MPI、PVM)
并行算法模型: 数据并行模型、任务依赖图模型、工作池模型、管理者-工作者模型、消费者模型
对于并行计算一个任务可能涉及到的问题： 任务分解、任务依赖关系、任务粒度分配、并发度、任务交互
并行算法性能的常见度量值： 并行开销、加速比、效率(加速比/CPU数)、成本(并行运行时间*CPU数)

一个简单的多核计算Demo
演示中主要完成的工作是：(工作本身没有什么意义 主要是消耗一些时间来代表需要做的工作)
代码:
double Sum0(double* data,long data_count)
{
double result=0;
for (long i=0;i<data_count;++i)
{
data[i]=sqrt(1-(data[i]*data[i]));
result+=data[i];
}
return result;
}

然后用OpenMP工具(vc和icc编译器支持)(函数SumOpenMP)和一个自己手工写的线程工具来并行化该函数(函数SumWTP)，并求出加速比；
(在多核CPU上执行Demo才可以看到多CPU并行的优势)
OpenMP是基于编译器命令的并行编程标准，使用的共享数据模型，现在可以用在C/C++、Fortan中；OpenMP命令提供了对并发、同步、数据读写的支持；

//我测试用的编译器vc2005
//需要在项目属性中打开多线程和OpenMP支持
//TestWTP.cpp
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <vector>
#include <math.h>

#define _IS_TEST_OpenMP
//要测试OpenMP需要编译器支持OpenMP,并在编译设置里面启用OpenMP
#ifdef _IS_TEST_OpenMP
#include <omp.h>
#endif

//使用CWorkThreadPool在多个CPU上完成计算的简单Demo
#include "WorkThreadPool.h"


double Sum0(double* data,long data_count); //单线程执行
double SumWTP(double* data,long data_count); //根据CPU数动态多线程并行执行
#ifdef _IS_TEST_OpenMP
double SumOpenMP(double* data,long data_count); //使用OpenMP来并行执行
#endif


const long g_data_count=200000;
double g_data[g_data_count];

int main()
{
long i;
double start0, start1, start2;
const long test_count=200*2;
double sumresult;

//inti
for (i=0;i<g_data_count;++i)
g_data[i]=rand()*(1.0/RAND_MAX);

//
start0=(double)clock();
sumresult=0;
for( i=0; i<test_count; i++ )
{
sumresult+=Sum0(g_data,g_data_count);
}
start0=((double)clock()-start0)/CLOCKS_PER_SEC;
printf ("<Single thread> ");
printf (" result = %10.7f ",sumresult);
printf (" Seconds = %10.7f ",start0 );


#ifdef _IS_TEST_OpenMP
start1=clock();
sumresult=0;
for( i=0; i<test_count; i++ )
{
sumresult+=SumOpenMP(g_data,g_data_count);
}
start1=((double)clock()-start1)/CLOCKS_PER_SEC;
printf (" <OpenMP> ");
printf (" result = %10.7f ",sumresult);
printf (" Seconds = %10.7f ",start1);

printf (" ");
printf ("%10.7f/%10.7f = %2.4f ",start0,start1,start0/start1);
#endif

//
start2=clock();
sumresult=0;
for( i=0; i<test_count; i++ )
{
sumresult+=SumWTP(g_data,g_data_count);
}
start2=((double)clock()-start2)/CLOCKS_PER_SEC;
printf (" <CWorkThreadPool with %d thread> ",CWorkThreadPool::best_work_count());
printf (" result = %10.7f ",sumresult);
printf (" Seconds = %10.7f ",start2);

printf (" ");
printf ("%10.7f/%10.7f = %2.4f ",start0,start2,start0/start2);


printf (" --------- ok ! ---------");
getchar();

return 0;
}


double Sum0(double* data,long data_count)
{
double result=0;
for (long i=0;i<data_count;++i)
{
data[i]=sqrt(1-(data[i]*data[i]));
result+=data[i];
}
return result;
}

#ifdef _IS_TEST_OpenMP
double SumOpenMP(double* data,long data_count)
{
double result=0;
#pragma omp parallel for schedule(static) reduction(+: result)
for (long i=0;i<data_count;++i)
{
data[i]=sqrt(1-(data[i]*data[i]));
result+=data[i];
}
return result;
}
#endif

struct TWorkData
{
long ibegin;
long iend;
double* data;
double result;
};

void sum_callback(TWorkData* wd)
{
wd->result=Sum0( &wd->data[wd->ibegin],(wd->iend-wd->ibegin) );
}

double SumWTP(double* data,long data_count)
{
static long work_count=CWorkThreadPool::best_work_count();
static std::vector<TWorkData> work_list(work_count);
static std::vector<TWorkData*> pwork_list(work_count);
long i;

static bool IS_inti=false;
if (!IS_inti)//分配任务
{
for (i=0;i<work_count;++i)
{
work_list[i].data=data;
if (0==i) work_list[i].ibegin=0;
else work_list[i].ibegin=work_list[i-1].iend;
work_list[i].iend=data_count*(i+1)/work_count;
}
for (i=0;i<work_count;++i)
pwork_list[i]=&work_list[i];
IS_inti=true;
}

//执行任务
CWorkThreadPool::work_execute((TThreadCallBack)sum_callback,(void**)&pwork_list[0],pwork_list.size());

double result=0;
for (i=0;i<work_count;++i)
result+=work_list[i].result;

return result;
}

[zhsj64]

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[sapzj1984]

//CWorkThreadPool的声明文件 WorkThreadPool.h
/////////////////////////////////////////////////////////////
//工作线程池 CWorkThreadPool
//用于把一个任务拆分成多个线程任务,从而可以使用多个CPU
//[e-mail]
////////////////////////////
//todo:改成任务领取模式
//todo:修改辅助线程优先级，继承自主线程
//要求：1.任务分割时分割的任务量比较接近
// 2.任务也不要太小，否则线程的开销可能会大于并行的收益
// 3.任务数最好是CPU数的倍数
// 4.主线程不能以过高优先级运行，否则其他辅助线程可能得不到时间片

#ifndef _WorkThreadPool_H_
#define _WorkThreadPool_H_

typedef void (*TThreadCallBack)(void * pData);

class CWorkThreadPool
{
public:
static long best_work_count(); //返回最佳工作分割数,现在的实现为返回CPU个数
static void work_execute(const TThreadCallBack work_proc,void** word_data_list,int work_count); //并行执行工作，并等待所有工作完成
static void work_execute(const TThreadCallBack* work_proc_list,void** word_data_list,int work_count); //同上，但不同的work调用不同的函数
static void work_execute_single_thread(const TThreadCallBack work_proc,void** word_data_list,int work_count) //单线程执行工作，并等待所有工作完成;用于调试等
{
for (long i=0;i<work_count;++i)
work_proc(word_data_list[i]);
}
static void work_execute_single_thread(const TThreadCallBack* work_proc_list,void** word_data_list,int work_count) //单线程执行工作，并等待所有工作完成;用于调试等
{
for (long i=0;i<work_count;++i)
work_proc_list[i](word_data_list[i]);
}
};


#endif //_WorkThreadPool_H_

[sapzj1984]

//CWorkThreadPool的实现文件 WorkThreadPool.cpp
/////////////////////////////////////////////////////////////
//工作线程池 TWorkThreadPool

#include <process.h>
#include <vector>
#include "windows.h"
#include "WorkThreadPool.h"

//#define _IS_SetThreadAffinity_
//定义该标志则执行不同的线程绑定到不同的CPU，减少线程切换开销； 不鼓励


class TWorkThreadPool;

//线程状态
enum TThreadState{ thrStartup=0, thrReady, thrBusy, thrTerminate, thrDeath };

class TWorkThread
{
public:
volatile HANDLE thread_handle;
volatile enum TThreadState state;
volatile TThreadCallBack func;
volatile void * pdata; //work data
volatile HANDLE waitfor_event;
TWorkThreadPool* pool;
volatile DWORD thread_ThreadAffinityMask;

TWorkThread() { memset(this,0,sizeof(TWorkThread)); }
};

void do_work_end(TWorkThread* thread_data);


void __cdecl thread_dowork(TWorkThread* thread_data) //void __stdcall thread_dowork(TWorkThread* thread_data)
{
volatile TThreadState& state=thread_data->state;
#ifdef _IS_SetThreadAffinity_
SetThreadAffinityMask(GetCurrentThread(),thread_data->thread_ThreadAffinityMask);
#endif
state = thrStartup;

while(true)
{
WaitForSingleObject(thread_data->waitfor_event, -1);
if(state == thrTerminate)
break;

state = thrBusy;
volatile TThreadCallBack& func=thread_data->func;
if (func!=0)
func((void *)thread_data->pdata);
do_work_end(thread_data);
}
state = thrDeath;
_endthread();
//ExitThread(0);
}

class TWorkThreadPool
{
private:
volatile HANDLE thread_event;
volatile HANDLE new_thread_event;
std::vector<TWorkThread> work_threads;
mutable long cpu_count;
inline long get_cpu_count() const {
if (cpu_count>0) return cpu_count;

SYSTEM_INFO SystemInfo;
GetSystemInfo(&SystemInfo);
cpu_count=SystemInfo.dwNumberOfProcessors;
return cpu_count;
}
inline long passel_count() const { return (long)work_threads.size()+1; }
void inti_threads() {
long best_count =get_cpu_count();

long newthrcount=best_count - 1;
work_threads.resize(newthrcount);
thread_event = CreateSemaphore(0, 0,newthrcount , 0);
new_thread_event = CreateSemaphore(0, 0,newthrcount , 0);
long i;
for( i= 0; i < newthrcount; ++i)
{
work_threads[i].waitfor_event=thread_event;
work_threads[i].state = thrTerminate;
work_threads[i].pool=this;
work_threads[i].thread_ThreadAffinityMask=1<<(i+1);
work_threads[i].thread_handle =(HANDLE)_beginthread((void (__cdecl *)(void *))thread_dowork, 0, (void*)&work_threads[i]);
//CreateThread(0, 0, (LPTHREAD_START_ROUTINE)thread_dowork,(void*) &work_threads[i], 0, &thr_id);
//todo: _beginthread 的错误处理
}
#ifdef _IS_SetThreadAffinity_
SetThreadAffinityMask(GetCurrentThread(),0x01);
#endif
for(i = 0; i < newthrcount; ++i)
{
while(true) {
if (work_threads[i].state == thrStartup) break;
else Sleep(0);
}
work_threads[i].state = thrReady;
}
}
void free_threads(void)
{
long thr_count=(long)work_threads.size();
long i;
for(i = 0; i <thr_count; ++i)
{
while(true) {
if (work_threads[i].state == thrReady) break;
else Sleep(0);
}
work_threads[i].state=thrTerminate;
}
if (thr_count>0)
ReleaseSemaphore(thread_event,thr_count, 0);
for(i = 0; i <thr_count; ++i)
{
while(true) {
if (work_threads[i].state == thrDeath) break;
else Sleep(0);
}
}
CloseHandle(thread_event);
CloseHandle(new_thread_event);
work_threads.clear();
}
void passel_work(const TThreadCallBack* work_proc,int work_proc_inc,void** word_data_list,int work_count) {
if (work_count==1)
{
(*work_proc)(word_data_list[0]);
}
else
{
const TThreadCallBack* pthwork_proc=work_proc;
pthwork_proc+=work_proc_inc;

long i;
long thr_count=(long)work_threads.size();
for(i = 0; i < work_count-1; ++i)
{
work_threads[i].func = *pthwork_proc;
work_threads[i].pdata =word_data_list[i+1];
work_threads[i].state = thrBusy;
pthwork_proc+=work_proc_inc;
}
for(i = work_count-1; i < thr_count; ++i)
{
work_threads[i].func = 0;
work_threads[i].pdata =0;
work_threads[i].state = thrBusy;
}
if (thr_count>0)
ReleaseSemaphore(thread_event,thr_count, 0);

//current thread do a work
(*work_proc)(word_data_list[0]);


//wait for work finish
for(i = 0; i <thr_count; ++i)
{
while(true) {
if (work_threads[i].state == thrReady) break;
else Sleep(0);
}
}
std::swap(thread_event,new_thread_event);
}
}
void private_work_execute(TThreadCallBack* pwork_proc,int work_proc_inc,void** word_data_list,int work_count) {
while (work_count>0)
{
long passel_work_count;
if (work_count>=passel_count())
passel_work_count=passel_count();
else
passel_work_count=work_count;

passel_work(pwork_proc,work_proc_inc,word_data_list,passel_work_count);

pwork_proc+=(work_proc_inc*passel_work_count);
word_data_list=&word_data_list[passel_work_count];
work_count-=passel_work_count;
}
}
public:
explicit TWorkThreadPool():thread_event(0),work_threads(),cpu_count(0) { inti_threads(); }
~TWorkThreadPool() { free_threads(); }
inline long best_work_count() const { return passel_count(); }
inline void DoWorkEnd(TWorkThread* thread_data){
thread_data->waitfor_event=new_thread_event;
thread_data->func=0;
thread_data->state = thrReady;
}

inline void work_execute(TThreadCallBack* pwork_proc,void** word_data_list,int work_count) {
private_work_execute(pwork_proc,1,word_data_list,work_count);
}
inline void work_execute(TThreadCallBack work_proc,void** word_data_list,int work_count) {
private_work_execute(&work_proc,0,word_data_list,work_count);
}
};
void do_work_end(TWorkThread* thread_data)
{
thread_data->pool->DoWorkEnd(thread_data);
}

//TWorkThreadPool end;
////////////////////////////////////////

TWorkThreadPool g_work_thread_pool;//工作线程池

long CWorkThreadPool::best_work_count() { return g_work_thread_pool.best_work_count(); }

void CWorkThreadPool::work_execute(const TThreadCallBack work_proc,void** word_data_list,int work_count)
{
g_work_thread_pool.work_execute(work_proc,word_data_list,work_count);
}

void CWorkThreadPool::work_execute(const TThreadCallBack* work_proc_list,void** word_data_list,int work_count)
{
g_work_thread_pool.work_execute((TThreadCallBack*)work_proc_list,word_data_list,work_count);
}

[大侠肖]

现在再来看你这个帖子，里面有些观点非常赞同的！