Negligible difference in performance between RDSEED and RDRAND

user_1467632 0
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Recent Intel chips (Ivy Bridge and up) have instructions for generating (pseudo) random bits. RDSEED outputs "true" random bits generated from entropy gathered from a sensor on the chip. RDRAND outputs bits generated from a pseudorandom number generator seeded by the true random number generator. According to Intel's documentation, RDSEED is slower, since gathering entropy is costly. Thus, RDRAND is offered as a cheaper alternative, and its output is sufficiently secure for most cryptographic applications. (This is analogous to the /dev/random versus /dev/urandom on Unix systems.)

I was curious about the performance difference between the two instructions, so I wrote some code to compare them. To my surprise, I find there is virtually no difference in performance. Could anyone provide an explanation? Code and system details follow.

Benchmark

/* Compare the performance of RDSEED and RDRAND.
 *
 * Compute the CPU time used to fill a buffer with (pseudo) random bits 
 * using each instruction.
 *
 * Compile with: gcc -mdrnd -mdseed
 */
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <x86intrin.h>

#define BUFSIZE (1<<24)

int main() {

  unsigned int ok, i;
  unsigned long long *rand = malloc(BUFSIZE*sizeof(unsigned long long)), 
                     *seed = malloc(BUFSIZE*sizeof(unsigned long long)); 

  clock_t start, end, bm;

  // RDRAND (the benchmark)
  start = clock();
  for (i = 0; i < BUFSIZE; i++) {
    ok  = _rdrand64_step(&rand[i]);
  }
  bm = clock() - start;
  printf("RDRAND: %li\n", bm);

  // RDSEED
  start = clock();
  for (i = 0; i < BUFSIZE; i++) {
    ok = _rdseed64_step(&seed[i]);
  }
  end = clock();
  printf("RDSEED: %li, %.2lf\n", end - start, (double)(end-start)/bm);

  free(rand);
  free(seed);
  return 0;
}

System details

  • Intel Core i7-6700 CPU @ 3.40GHz
  • Ubuntu 16.04
  • gcc 5.4.0
x86-64 cryptography gcc random
3 Answers

Interesting - in my case with 3.6 GHz 10-Core Intel Core i9 (on iMac), with the above program (corrected to repeat RDRAND/RDSEED call in case of failure) I observe:

$ ./rdseed-test 
RDRAND: 1751837
RDSEED: 1752472, 1.00

Update

I must admit that I'm puzzled - trying this same executable a few days later gives me 3x difference, like the one reported by others above:

$ ./rdseed-test 
RDRAND: 1761312
RDSEED: 5309609, 3.01

No idea why sometimes RDSEED runs as fast as RDRAND, and sometimes - three times slower.

user_30030740
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