This is quite an interesting question so let me set the scene. I work at The National Museum of Computing, and we have just managed to get a Cray Y-MP EL super computer from 1992 running, and we really want to see how fast it can go!

We decided the best way to do this was to write a simple C program that would calculate prime numbers and show how long it took to do so, then run the program on a fast modern desktop PC and compare the results.

We quickly came up with this code to count prime numbers:

#include <stdio.h>

#include <time.h>

void main() {

clock_t start, end;

double runTime;

start = clock();

int i, num = 1, primes = 0;

while (num <= 1000) {

i = 2;

while (i <= num) {

if(num % i == 0)

break;

i++;

}

if (i == num)

primes++;

system("clear");

printf("%d prime numbers calculated\n",primes);

num++;

}

end = clock();

runTime = (end - start) / (double) CLOCKS_PER_SEC;

printf("This machine calculated all %d prime numbers under 1000 in %g seconds\n", primes, runTime);

}

Which on our dual core laptop running Ubuntu (The Cray runs UNICOS), worked perfectly, getting 100% CPU usage and taking about 10 minutes or so. When I got home I decided to try it on my hex-core modern gaming PC, and this is where we get our first issues.

I first adapted the code to run on Windows since that is what the gaming PC was using, but was saddened to find that the process was only getting about 15% of the CPU's power. I figured that must be Windows being Windows, so I booted into a Live CD of Ubuntu thinking that Ubuntu would allow the process to run with its full potential as it had done earlier on my laptop.

However I only got 5% usage! So my question is, how can I adapt the program to run on my gaming machine in either Windows 7 or live Linux at 100% CPU utilisation? Another thing that would be great but not necessary is if the end product can be one .exe that could be easily distributed and ran on Windows machines.

Thanks a lot!

P.S. Of course this program didn't really work with the Crays 8 specialist processors, and that is a whole other issue... If you know anything about optimising code to work on 90's Cray super computers give us a shout too!

Source:

*Tips4all*
If you want 100% CPU, you need to use more than 1 core. To do that, you need multiple threads.

ReplyDeleteHere's a parallel version using OpenMP:

I had to increase the limit to 1000000 to make it take more than 1 second on my machine.

#include <stdio.h>

#include <time.h>

#include <omp.h>

int main() {

double start, end;

double runTime;

start = omp_get_wtime();

int num = 1,primes = 0;

int limit = 1000000;

#pragma omp parallel for schedule(dynamic) reduction(+ : primes)

for (num = 1; num <= limit; num++) {

int i = 2;

while(i <= num) {

if(num % i == 0)

break;

i++;

}

if(i == num)

primes++;

// printf("%d prime numbers calculated\n",primes);

}

end = omp_get_wtime();

runTime = end - start;

printf("This machine calculated all %d prime numbers under %d in %g seconds\n",primes,limit,runTime);

return 0;

}

Output:

This machine calculated all 78498 prime numbers under 1000000 in 29.753 seconds

Here's your 100% CPU:

You're running one process on a multi-core machine - so it only runs on one core.

ReplyDeleteThe solution is easy enough, since you're just trying to peg the processor - if you have N cores, run your program N times (in parallel, of course).

Example

Here is some code that runs your program NUM_OF_CORES times in parallel. It's POSIXy code - it uses fork - so you should run that under Linux. If what I'm reading about the Cray is correct, it might be easier to port this code than the OpenMP code in the other answer.

#include <stdio.h>

#include <time.h>

#include <stdlib.h>

#include <unistd.h>

#include <errno.h>

#define NUM_OF_CORES 8

#define MAX_PRIME 100000

void do_primes()

{

unsigned long i, num, primes = 0;

for (num = 1; num <= MAX_PRIME; ++num) {

for (i = 2; (i <= num) && (num % i != 0); ++i);

if (i == num)

++primes;

}

printf("Calculated %d primes.\n", primes);

}

int main(int argc, char ** argv)

{

time_t start, end;

time_t run_time;

unsigned long i;

pid_t pids[NUM_OF_CORES];

/* start of test */

start = time(NULL);

for (i = 0; i < NUM_OF_CORES; ++i) {

if (!(pids[i] = fork())) {

do_primes();

exit(0);

}

if (pids[i] < 0) {

perror("Fork");

exit(1);

}

}

for (i = 0; i < NUM_OF_CORES; ++i) {

waitpid(pids[i], NULL, 0);

}

end = time(NULL);

run_time = (end - start);

printf("This machine calculated all prime numbers under %d %d times "

"in %d seconds\n", MAX_PRIME, NUM_OF_CORES, run_time);

return 0;

}

Output

$ ./primes

Calculated 9592 primes.

Calculated 9592 primes.

Calculated 9592 primes.

Calculated 9592 primes.

Calculated 9592 primes.

Calculated 9592 primes.

Calculated 9592 primes.

Calculated 9592 primes.

This machine calculated all prime numbers under 100000 8 times in 8 seconds

we really want to see how fast it can go!

ReplyDeleteYour algorithm to generate prime numbers is very inefficient. Compare it to primegen that generates the 50847534 primes up to 1000000000 in just 8 seconds on a Pentium II-350.

To consume all CPUs easily you could solve an embarrassingly parallel problem e.g., compute Mandelbrot set or use genetic programming to paint Mona Lisa in multiple threads (processes).

Another approach is to take an existing benchmark program for the Cray supercomputer and port it to a modern PC.

The reason you're getting 15% on a hex core processor is because your code uses 1 core at 100%. 100/6 = 16.67%, which using a moving average with process scheduling (your process would be running under normal priority) could easily be reported as 15%.

ReplyDeleteTherefore, in order to use 100% cpu, you would need to use all the cores of your CPU - launch 6 parallel execution code paths for a hex core CPU and have this scale right up to however many processors your Cray machine has :)

Try to parallelize your program using, e.g., OpenMP. It is a very simple and effective framework for making up parallel programs.

ReplyDeleteFor a quick improvement on one core, remove system calls to reduce context-switching. Remove these lines:

ReplyDeletesystem("clear");

printf("%d prime numbers calculated\n",primes);

The first is particularly bad, as it will spawn a new process every iteration.