printf performance issue in openmp

问题

I have been told not to use printf in openmp programs as it degrades the performance of parallel simulation program.

I want to know what is the substitute for that. I mean how to display the output of a program without using printf.

I have the following AES-128 simulation problem using openmp which needs further comments Parallel simulation of AES in C using Openmp

I want to know how to output the cipher text without degrading the simulation performance?

Thanks in advance.

回答1:

You cannot both have your pie and eat it. Decide if you want to have great parallel performance or if it's important to see the output of the algorithm while running the parallel loop.

The obvious offline solution is to store the plaintexts, keys and ciphertexts in arrays. In your case that would require 119 MiB (= 650000*(3*4*16) bytes) in the original case and only 12 MiB in the case with 65000 trials. Nothing that a modern machine with GiBs of RAM cannot handle. The latter case even even fits in the last-level cache of some server-class CPUs.

#define TRIALS 65000

int (*key)[16];
int (*pt)[16];
int (*ct)[16];

double timer;

key = malloc(TRIALS * sizeof(*key));
pt = malloc(TRIALS * sizeof(*pt));
ct = malloc(TRIALS * sizeof(*ct));

timer = -omp_get_wtime();

#pragma omp parallel for private(rnd,j)
for(i = 0; i < TRIALS; i++)
{
   ...

   for(j = 0; j < 4; j++)
   {
      key[i][4*j]   = (rnd[j] & 0xff);
      pt[i][4*j]    = key[i][4*j];
      key[i][4*j+1] = ((rnd[j] >> 8)  & 0xff) ; 
      pt[4*j+1]     = key[i][4*j+1];
      key[i][4*j+2] = ((rnd[j] >> 16) & 0xff) ;
      pt[i][4*j+2]  = key[i][4*j+2];
      key[i][4*j+3] = ((rnd[j] >> 24) & 0xff) ;
      pt[i][4*j+3]  = key[i][4*j+3];
   }

   encrypt(key[i],pt[i],ct[i]);
}

timer += omp_get_wtime();
printf("Encryption took %.6f seconds\n", timer);

// Now display the results serially
for (i = 0; i < TRIALS; i++)
{
    display pt[i], key[i] -> ct[i]
}

free(key); free(pt); free(ct);

To see the speed-up, you have to measure only the time spent in the parallel region. If you also measure the time it takes to display the results, you will be back to where you started.

来源：https://stackoverflow.com/questions/20086728/printf-performance-issue-in-openmp

标签

performance

printf

openmp

simulation