一些 pthread 代码...为什么它运行慢？

Question

此代码用于协调会议中的人员。谁能帮我找出为什么这么慢？谢谢。它是用 C 编写的，使用 POSIX 个线程。

这是一个 mulch-threaded 程序，可以帮助同步演讲者和一群健谈的记者。

#include<stdio.h> 
#include<stdlib.h>
#include<pthread.h>

typedef struct
{
    int ID;
}Args;

pthread_t spkr; 
pthread_t * rptr;
pthread_mutex_t mutex;

void speaker();
void * speak();
int answerStart();
int answerDone();

int reporter(int);
void * report();
void enterConferenceRoom();
void leaveConferenceRoom();
void questionStart();
void questionDone();

int repNum = 100;
int capacity = 8;
int current = 0;    

int canAsk(int);
int reporterIndex = -1;

int signal = -1;
void sendSignal(int);
int getSignal();

main(int argc, char *argv[])
{
    if(argc != 3)
        printf("Usage : <number of reporters : capacity of conference room>\n");
    else
    {
        repNum = atoi(argv[1]);
        capacity = atoi(argv[2]);   
    }

    rptr = (pthread_t *) malloc(repNum * sizeof(pthread_t));

    speaker();

    int i = 0;
    for(; i < repNum; i++)
        reporter(i);

    for(i = 0; i < repNum; i++)
        pthread_join(rptr[i], NULL);

    pthread_join(spkr, NULL);
}

void speaker()
{
    if(pthread_create( &spkr, NULL, speak, (void *) NULL ))
        { printf("\n\nSOMETHING WENT TERRIBLLY WRONG!!!\n\n"); exit(1); }
}

void * speak()
{
    while(repNum > 0)
        answerStart();

    pthread_exit(0);

    return NULL;
}

int answerStart()
{
    while(getSignal() != 1)
        if(repNum < 1)
            return;

    printf("Speaker starts to answer questions for reporter %d.\n", reporterIndex);

    answerDone();
}

int answerDone()
{
    printf("Speaker is done with answer for reporter %d.\n", reporterIndex);

    sendSignal(0);
}

int reporter(int id)
{
    Args * argum = (Args * )malloc(sizeof(Args));
    argum->ID = id;

    if(pthread_create( &rptr[id], NULL, report, (void *) argum ))
        { printf("\n\nSOMETHING WENT TERRIBLLY WRONG!!!\n\n"); exit(1); }
}

void * report(void * argum)
{
    Args * a = (Args *) argum;

    int id = a->ID;

    while(1)
        if(canEnter())
        {
            enterConferenceRoom(id);
            break;
        }

    int numQs = (id % 4 ) + 2;
    int i = 0;
    for(; i < numQs; i++)
    {
        questionStart(id);
        questionDone(id);
    }

    leaveConferenceRoom(id);

    pthread_exit(0);

    return NULL;
}

int canEnter()
{
    int ret = 0;

    pthread_mutex_lock( &mutex );

    if(current < capacity) 
    {
        current++; 
        ret = 1;
    }

    pthread_mutex_unlock( &mutex );

    return ret;
}

void enterConferenceRoom(int id)
{
    printf("Reporter %d enters the conference room.\n", id);
}

void leaveConferenceRoom(int id)
{
    printf("Reporter %d leaves the conference room.\n", id);

    pthread_mutex_lock( &mutex );

    current--;

    repNum--;

    pthread_mutex_unlock( &mutex );
}

void questionStart(int id)
{
    while(!canAsk(id));

    printf("Reporter %d asks a question.\n", id);

    sendSignal(1);
}

int canAsk(int id)
{
    int ret = 0;

    pthread_mutex_lock( &mutex );

    if(reporterIndex < 0) 
    {
        reporterIndex = id;
        ret = 1;
    }

    pthread_mutex_unlock( &mutex );

    return ret;
}

void questionDone(int id)
{
    while(getSignal() != 0);

    printf("Reporter %d is satisfied.\n", id);

    pthread_mutex_lock( &mutex );

    reporterIndex = -1;

    pthread_mutex_unlock( &mutex );
}

void sendSignal(int sig)
{
    pthread_mutex_lock( &mutex );

    signal = sig;

    pthread_mutex_unlock( &mutex );
}

int getSignal()
{
    pthread_mutex_lock( &mutex );

    int sig = signal;

    pthread_mutex_unlock( &mutex );

    return sig;
}

Answer 1

是因为你有线程锁饥饿效应。当某些线程不拥有资源，但不断在临界区内检查它，从而阻止实际所有者获得锁并释放资源时。

这种情况一般发生在锁外操作时间过短的情况下。你需要在线程之间使用不同的同步，否则你会得到很多人试图进入门说话而演讲者没有机会离开这个地方的效果...

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