Some pthread code...why is it running slow? - c

Question

This code is supposed to coordinate people in a conference. Can anyone help me find out why its so slow? Thanks. It is written in C and uses POSIX threads.
This is a mulch-threaded program that helps synchronize a speaker and a hoard of talkative reporters.
#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

It is because you have thread lock starvation effect. When some thread doesn't own the resource, but constantly checks for it inside a critical section, thus preventing actual owner to obtain the lock and release the resource.
This occurs usually when the time of operation outside of lock is too short. You need to use different synchronization between threads, otherwise you get an effect as many people trying to get into the door to speak while the speaker has no chance to leave the place...