OpenMP help on code

Question

this is my first time posting so I will appologise for my novice mistakes. Please also excuse the fact that not all variable names are in english. My problem is the following: i've written this code using openMP in both Visual Studio 2010 and in eclipse for c/c++ using the cygwin gcc compiler toolchain. In visual I get a speed-up but in eclipse I get a slow down twice the amount of the serial version. Can someone please explain what I have done wrong please? In short i'm just simulating the speed-up from when I copy from an array of 3D vectors into a double array in order to send over MPI.

#include <omp.h>
#include <time.h>
#include <stdio.h>
#include <vector>
const int NUMAR_FORME=10;
  const int NUMAR_SECUNDE_SIMULATE=60; //number of buffers
  const int dimensiuni_forme[10]={100,200,300,400,500,600,700,800,900,10000}; //size of each buffer
  //-------- the buffers, cuurently only worker_buffer and buff is used
     std::vector<std::vector<std::vector<double> > > worker_buffer;
  std::vector<std::vector<double> > send_buffer,corect;
  double **buff;
   double **worker_buffer1;
  long i,j,k,l;
int flag=0;
int numarator=0; //number of tests runed
clock_t start;
start=clock();
worker_buffer.resize(1);
buff = new double* [2];
int de_scris=0; //this tells me in which buffer to store, nou I alternate buff[0], buff[1], buff[0], buff[1]
worker_buffer[0].resize(NUMAR_SECUNDE_SIMULATE);
for(i=0;i<NUMAR_SECUNDE_SIMULATE;i++)
    worker_buffer[0][i].resize(dimensiuni_forme[9]);



while(numarator<60)
{

        if(numarator!=0)
            delete [] buff[de_scris];

    if(numarator!=0)
        de_scris=(de_scris+1)%2;
    long limita;
    limita=NUMAR_SECUNDE_SIMULATE*dimensiuni_forme[9]*3; //3-comes from the fact that I will have a 3D vector structure
    buff[de_scris]= new double [limita];
    for(i=0;i<NUMAR_SECUNDE_SIMULATE;i++)
    {   for(j=0;j<dimensiuni_forme[9];j++)
        {
            worker_buffer[0][i][j]=(i*dimensiuni_forme[9]+j)*3;
            buff[de_scris][(i*dimensiuni_forme[9]+j)*3]=worker_buffer[0][i][j];
            buff[de_scris][(i*dimensiuni_forme[9]+j)*3+1]=worker_buffer[0][i][j]+0.5;
            buff[de_scris][(i*dimensiuni_forme[9]+j)*3+2]=worker_buffer[0][i][j]+0.75;
        }
    }
    numarator++;

}

start=clock()-start;
printf("TICKS TOTAL %ld \n",start);
bool ad=true;
long nr;
for(i=0;i<NUMAR_SECUNDE_SIMULATE*dimensiuni_forme[9]*3;i++)
{
    if(i%3==0)
        nr=i;
    if(i%3==0 && buff[de_scris][i]!=i)
        ad=false;
    else
        if(i%3==1 &&buff[de_scris][i]!=(nr+0.5))
            ad=false;
        else
            if(i%3==2 && buff[de_scris][i]!=(nr+0.75))
                ad=false;
}
if(ad==false)
    printf("not correct \n");
start=clock();

    numarator=0;
//parallel version
while(numarator<60)
{


        if(numarator!=0)
            delete [] buff[de_scris];

    long index, limita,id;
    omp_set_num_threads(2);

    if(numarator!=0)
        de_scris=(de_scris+1)%2;
    limita=NUMAR_SECUNDE_SIMULATE*dimensiuni_forme[9]*3; //3-
    buff[de_scris]= new double [limita];
 #pragma omp parallel shared(worker_buffer,limita,buff) private(index,id)
    {
        printf("intram cu %d threaduri \n", omp_get_num_threads());
        id=omp_get_thread_num();
        //index=id;
        for(index=id;(index*3)<limita;index+=omp_get_num_threads())
        {
            buff[de_scris][index*3]=worker_buffer[0][index/dimensiuni_forme[9]][index%dimensiuni_forme[9]];  //aici va veni send_buff[index].x
            buff[de_scris][index*3+1]=buff[de_scris][index*3]+0.5;
            buff[de_scris][index*3+2]=buff[de_scris][index*3]+0.75;
        }

    //  index+=omp_get_num_threads();


    }//end parallel zone
    numarator++;
}

start=clock()-start;
printf("TICKS TOTAL %ld \n",start);
 ad=true;
    //testing for correctness
for(i=0;i<NUMAR_SECUNDE_SIMULATE*dimensiuni_forme[9]*3;i++)
{
    if(i%3==0)
        nr=i;
    if(i%3==0 && buff[de_scris][i]!=i)
        ad=false;
    else
        if(i%3==1 &&buff[de_scris][i]!=(nr+0.5))
            ad=false;
        else
            if(i%3==2 && buff[de_scris][i]!=(nr+0.75))
                ad=false;
}
if(ad==false)
    printf("not correct \n");
 return 0;
 }

Do you measure the time only with the clock function in your code? If so, you should know that clock returns the cpu time used, meaning if you use 2 prossesors the time returned by clock() will increase twice as fast as real time. I am not 100% sure if visucal c++ implements the clock() function the same way, could be the reason you measure different times. — Haatschii
– Haatschii, Commented Mar 24, 2012 at 11:36
I've tried using time() and clock_gettime() the result was the same — Ray
– Ray, Commented Mar 25, 2012 at 7:43

Tudor · Accepted Answer · 2012-03-24 22:12:04Z

1

Judging by how you organized this for loop:

    for(index=id;(index*3)<limita;index+=omp_get_num_threads())
    {
        buff[de_scris][index*3]=worker_buffer[0][index/dimensiuni_forme[9]][index%dimensiuni_forme[9]];  //aici va veni send_buff[index].x
        buff[de_scris][index*3+1]=buff[de_scris][index*3]+0.5;
        buff[de_scris][index*3+2]=buff[de_scris][index*3]+0.75;
    }

and assuming that you have 4 threads, your threads will get interleaved index values:

thread 0: 0, 4,  8, 12,...
thread 1: 1, 5,  9, 13,...
thread 2: 2, 6, 10, 14,...
thread 3: 3, 7, 11, 15,...

which may be causing cache ping-pong effects, since values written by different threads may land on the same cache line, thus slowing down your execution.

Try to use a simple for loop with static partitioning instead, in order to get continuous partitions:

    #pragma omp parallel for
    for(index = 0; index < limita / 3;index++)
    {
        buff[de_scris][index*3]=worker_buffer[0][index/dimensiuni_forme[9]][index%dimensiuni_forme[9]];  //aici va veni send_buff[index].x
        buff[de_scris][index*3+1]=buff[de_scris][index*3]+0.5;
        buff[de_scris][index*3+2]=buff[de_scris][index*3]+0.75;
    }

answered Mar 24, 2012 at 22:12

Tudor

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1 Comment

Ray Over a year ago

You were right thank-you very much , I used the loop that you have written and it showed a speed up. Strange how in visual it showed a speed without using the for directive.

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