基准测试线程性能

Question

I am experimenting stuff and trying to calculate the time taken to create N threads and doing some simple operation in the thread.

I have a variable initialized to 0 and waiting till it reaches the value N, where N is the number of threads. That way, I can ensure that all the threads are done executing atleast till the part before the return statement(last line of that function).

I am guessing that numbers [0,N] where N is the number of threads we want to create should be output in some random order but printing some stuff that doesnt make sense(addresses, may be). My question is why are the number not being printed in a random fashion from 0 to N

Code here:

void* increment(void *);

int main(int argc , char *argv[])
{
   if(argc != 2) {
    cout<<"Please provide the number of threads to create" <<endl;
    return -1;
}

int nthreads = atoi(argv[1]);
pthread_t thread_ids[nthreads];
int count = 0;

struct timeval timeStart, timeEnd;
gettimeofday(&timeStart, NULL);

int i = 0;
while(i < nthreads)
{
    pthread_t thread_id;
    if( pthread_create( &thread_ids[i] , NULL , increment , (void*) &count) < 0)
    {
        error("could not create thread");
        return 1;
    }
    ++i;
}

while(count < nthreads)
{
    cout<<"Waiting !"<<endl;
    //repeat
}

gettimeofday(&timeEnd, NULL);

for(i = 0; i < nthreads; i++)
    pthread_join(thread_ids[i], NULL);

std::cout << "This piece of code took "
<< ((timeEnd.tv_sec - timeStart.tv_sec) * 1000000L + timeEnd.tv_usec - timeStart.tv_usec) << " us to execute."
<< std::endl;

 return 0;
}

void* increment(void *i)
{
  int *p = (int*)i;
  cout<<*p<<endl<<std::flush;
  fflush(stdout);
  *p = *p + 1;
  return 0;
 }

Not sure whats wrong in this piece of code. Any pointers?

Answer 1

POSIX标准非常清楚，对象可能无法在一个线程中被访问，而在另一个线程中或可能会被修改。