Question

My code is a parallel implmentation that calculates the nth digit of pi. When I finish the kernel and try to copy the memory back to the host I get a “the launch timed out and was terminated” error.
I used this code for error checking for each cudamalloc, cudamemcpy, and kernal launch.

std::string error = cudaGetErrorString(cudaGetLastError());
printf("%s\n", error);

These calls were saying everything was fine until the first cudamemcpy call after returning from the kernel. the error happens in the line “cudaMemcpy(avhost, avdev, size, cudaMemcpyDeviceToHost);” in main. Any help is appreciated.

#include <stdlib.h>
#include <stdio.h>
#include <math.h>

#define mul_mod(a,b,m) fmod( (double) a * (double) b, m)
///////////////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////////////
/* return the inverse of x mod y */
__device__ int inv_mod(int x,int y) {
  int q,u,v,a,c,t;

  u=x;
  v=y;
  c=1;
  a=0;
  do {
    q=v/u;

    t=c;
    c=a-q*c;
    a=t;

    t=u;
    u=v-q*u;
    v=t;
  } while (u!=0);
  a=a%y;
  if (a<0) a=y+a;
  return a;
}
///////////////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////////////
/* return the inverse of u mod v, if v is odd */
__device__ int inv_mod2(int u,int v) {
  int u1,u3,v1,v3,t1,t3;

  u1=1;
  u3=u;

  v1=v;
  v3=v;

  if ((u&1)!=0) {
    t1=0;
    t3=-v;
    goto Y4;
  } else {
    t1=1;
    t3=u;
  }

  do {

    do {
      if ((t1&1)==0) {
    t1=t1>>1;
    t3=t3>>1;
      } else {
    t1=(t1+v)>>1;
    t3=t3>>1;
      }
      Y4:;
    } while ((t3&1)==0);

    if (t3>=0) {
      u1=t1;
      u3=t3;
    } else {
      v1=v-t1;
      v3=-t3;
    }
    t1=u1-v1;
    t3=u3-v3;
    if (t1<0) {
      t1=t1+v;
    }
  } while (t3 != 0);
  return u1;
}


/* return (a^b) mod m */
__device__ int pow_mod(int a,int b,int m)
{
  int r,aa;

  r=1;
  aa=a;
  while (1) {
    if (b&1) r=mul_mod(r,aa,m);
    b=b>>1;
    if (b == 0) break;
    aa=mul_mod(aa,aa,m);
  }
  return r;
}
///////////////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////////////
/* return true if n is prime */
int is_prime(int n)
{
   int r,i;
   if ((n % 2) == 0) return 0;

   r=(int)(sqrtf(n));
   for(i=3;i<=r;i+=2) if ((n % i) == 0) return 0;
   return 1;
}
///////////////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////////////
/* return the prime number immediatly after n */
int next_prime(int n)
{
   do {
      n++;
   } while (!is_prime(n));
   return n;
}
///////////////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////////////
#define DIVN(t,a,v,vinc,kq,kqinc)       \
{                       \
  kq+=kqinc;                    \
  if (kq >= a) {                \
    do { kq-=a; } while (kq>=a);        \
    if (kq == 0) {              \
      do {                  \
    t=t/a;                  \
    v+=vinc;                \
      } while ((t % a) == 0);           \
    }                       \
  }                     \
}

///////////////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////////////

__global__ void digi_calc(int *s, int *av, int *primes, int N, int n, int nthreads){
    int a,vmax,num,den,k,kq1,kq2,kq3,kq4,t,v,i,t1, h;
    unsigned int tid = blockIdx.x*blockDim.x + threadIdx.x;
// GIANT LOOP
    for (h = 0; h<1; h++){
    if(tid > nthreads) continue;
    a = primes[tid];
    vmax=(int)(logf(3*N)/logf(a));
    if (a==2) {
      vmax=vmax+(N-n);
      if (vmax<=0) continue;
    }
    av[tid]=1;
    for(i=0;i<vmax;i++) av[tid]*= a;

    s[tid]=0;
    den=1;
    kq1=0;
    kq2=-1;
    kq3=-3;
    kq4=-2;
    if (a==2) {
      num=1;
      v=-n; 
    } else {
      num=pow_mod(2,n,av[tid]);
      v=0;
    }

    for(k=1;k<=N;k++) {

      t=2*k;
      DIVN(t,a,v,-1,kq1,2);
      num=mul_mod(num,t,av[tid]);

      t=2*k-1;
      DIVN(t,a,v,-1,kq2,2);
      num=mul_mod(num,t,av[tid]);

      t=3*(3*k-1);
      DIVN(t,a,v,1,kq3,9);
      den=mul_mod(den,t,av[tid]);

      t=(3*k-2);
      DIVN(t,a,v,1,kq4,3);
      if (a!=2) t=t*2; else v++;
      den=mul_mod(den,t,av[tid]);

      if (v > 0) {
    if (a!=2) t=inv_mod2(den,av[tid]);
    else t=inv_mod(den,av[tid]);
    t=mul_mod(t,num,av[tid]);
    for(i=v;i<vmax;i++) t=mul_mod(t,a,av[tid]);
    t1=(25*k-3);                                                                                                                                                                                                                                                                                                                                                                       
    t=mul_mod(t,t1,av[tid]);
    s[tid]+=t;
    if (s[tid]>=av[tid]) s-=av[tid];
      }
    }

    t=pow_mod(5,n-1,av[tid]);
    s[tid]=mul_mod(s[tid],t,av[tid]);
    }
    __syncthreads();
}
///////////////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////////////
int main(int argc,char *argv[])
{
  int N,n,i,totalp, h;
  double sum;
  const char *error;
  int *sdev, *avdev, *shost, *avhost, *adev, *ahost;
    argc = 2;
    argv[1] = "2";
  if (argc<2 || (n=atoi(argv[1])) <= 0) {
    printf("This program computes the n'th decimal digit of pi\n"
       "usage: pi n , where n is the digit you want\n"
       );
    exit(1);
  }
    sum = 0;
    N=(int)((n+20)*logf(10)/logf(13.5));
    totalp=(N/logf(N))+10;
    ahost = (int *)calloc(totalp, sizeof(int));
    i = 0;
    ahost[0]=2;
    for(i=1; ahost[i-1]<=(3*N); ahost[i+1]=next_prime(ahost[i])){
        i++;
    }
    // allocate host memory
    size_t size = i*sizeof(int);
    shost = (int *)malloc(size);
    avhost = (int *)malloc(size);

  //allocate memory on device
    cudaMalloc((void **) &sdev, size);
    cudaMalloc((void **) &avdev, size);
    cudaMalloc((void **) &adev, size);
    cudaMemcpy(adev, ahost, size, cudaMemcpyHostToDevice);

    if (i >= 512){
        h = 512;
    }
    else h = i;
    dim3 dimGrid(((i+512)/512),1,1);                   
    dim3 dimBlock(h,1,1);

    // launch kernel
    digi_calc <<<dimGrid, dimBlock >>> (sdev, avdev, adev, N, n, i);

    //copy memory back to host
    cudaMemcpy(avhost, avdev, size, cudaMemcpyDeviceToHost);
    cudaMemcpy(shost, sdev, size, cudaMemcpyDeviceToHost);

  // end malloc's, memcpy's, kernel calls
    for(h = 0; h <=i; h++){
    sum=fmod(sum+(double) shost[h]/ (double) avhost[h],1.0);
    }
  printf("Decimal digits of pi at position %d: %09d\n",n,(int)(sum*1e9));
    //free memory
    cudaFree(sdev);
    cudaFree(avdev);
    cudaFree(adev);
    free(shost);
    free(avhost);
    free(ahost);
  return 0;
}

Answer 1

This is exactly the same problem you asked about in this question. The kernel is getting terminated early by the driver because it is taking too long to finish. If you read the documentation for any of these runtime API functions you will see the following note:

Note:
Note that this function may also return error codes from previous,
asynchronous launches.

All that is happening is that the first API call after the kernel launch is returning the error incurred while the kernel was running – in this case the cudaMemcpy call. The way you can confirm this for yourself is to do something like this directly after the kernel launch:

// launch kernel
digi_calc <<<dimGrid, dimBlock >>> (sdev, avdev, adev, N, n, i);
std::string error = cudaGetErrorString(cudaPeekAtLastError());
printf("%s\n", error);
error = cudaGetErrorString(cudaThreadSynchronize());
printf("%s\n", error);

The cudaPeekAtLastError() call will show you if there are any errors in the kernel launch, and the error code returned by the cudaThreadSynchronize() call will show whether any errors were generated while the kernel was executing.

The solution is exactly as outlined in the previous question: probably the simplest way is redesign the code so it is “re-entrant” so you can split the work over several kernel launches, with each kernel launch safely under the display driver watchdog timer limit.