Re: Faster source of random numbers for blanking harddrives

[Rob Mahurin <rob@utk.edu>]

On Thu, Oct 25, 2007 at 02:19:59PM +0800, Bob wrote:
> if you type
> dd if=/dev/urandom of=/dev/null count=100000
> 100000+0 records in
> 100000+0 records out
> 51200000 bytes (51 MB) copied, 14.0076 seconds, 3.7 MB/s
> 
> whereas
> dd if=/dev/zero of=/dev/null count=100000
> 100000+0 records in
> 100000+0 records out
> 51200000 bytes (51 MB) copied, 0.0932449 seconds, 549 MB/s
> 
> Is there a faster source or random or pseudo-random numbers because at 
> 4MB/s it'll take a loooong time to blank a 60GB drive let alone a 500GB job?

Most scientific libraries have fast, high-quality random number
generators.  If you have libgsl0-dev installed, compile the attached
program with

	$ make gsl_rng_info CFLAGS+="-lm -lgsl -lcblas -O6"

I get about 150 MB/s with the generator gfsr4:

	$ time GSL_RNG_TYPE=gfsr4 ./gsl_random_spew spew | head -c 500m > /dev/null 
	GSL_RNG_TYPE=gfsr4
	generator type: gfsr4
	seed = 0
	first value = 2901276280

	real  0m3.467s
	user  0m2.612s
	sys   0m0.672s

My computer has a different bus speed from yours, etc., so experiment.
I don't know whether this generator fills all 32 bits of each
generated number, or its period; those are in the GSL info manual
(from which I cribbed most of this program).  If you care about those
sorts of details, you might follow another poster's advice and
consider an existing solution.

Nota bene: don't call this program without redirecting stdout :)

Rob

-- 
Rob Mahurin
Dept. of Physics & Astronomy
University of Tennessee 	phone:  865 207 2594
Knoxville, TN  37996   		email:	rob@utk.edu

#include <stdio.h>
#include <gsl/gsl_rng.h>
     
gsl_rng * r;  /* global generator */
#define CHUNKSIZE	(1024*1024)

/*
 * Calling this program with any arguments will spew infinite amounts
 * of binary data to your terminal.  Caveat emptor.
 */
int main (int argc, char* argv[])
{
	const gsl_rng_type * T;
     
	gsl_rng_env_setup();
     
	T = gsl_rng_default;
	r = gsl_rng_alloc (T);
       
	fprintf (stderr,"generator type: %s\n", gsl_rng_name (r));
	fprintf (stderr,"seed = %lu\n", gsl_rng_default_seed);
	fprintf (stderr,"first value = %lu\n", gsl_rng_get (r));

	if (argc==1)	return 0;
	else {
		/* spew binary data to stdout */
		unsigned long random_chunk[CHUNKSIZE];
		int i =0; 
		while (1) {
			for (i = 0; i<CHUNKSIZE; ++i)
				random_chunk[i] = gsl_rng_get(r); 
			fwrite(random_chunk, sizeof(unsigned long), 
			       CHUNKSIZE, stdout); 
		}
	}
	/* never get here */
	return 1;
}