cuda - Zero-copy memory, memory-mapped file

Question

I am trying to create a mapped memory file, containing uint32_ts, and then use that as zero-copy pinned memory as shown below for CUDA. I am getting the c

Answer 1

One problem is that the logical sequence of your program is incorrect. This line assigns a value to data provided by the CUDA API:

err = cudaHostAlloc((void**)&data, data_file_size, cudaHostAllocMapped);

This line then overwrites that value, with a new one:

data = (uint32_t*) mmap(0, data_file_size, PROT_READ, MAP_PRIVATE, data_file, 0);

At that point, the value of data is not recognized by the CUDA API as being a pinned memory space anymore, so when you call this:

err = cudaHostGetDevicePointer((void**)&dev_data, (void*)data, 0);

you get an error, because the value contained in data is not recognized.

EDIT: (based on this question) Apart from that issue, it seems that if you change the file handling from read-only, to read-write, then this process can be made to work (throws no runtime errors). Here's a complete code (which doesn't contain the above logical flaw) that demonstrates this (I have previously created a test.dat file of size 566316 bytes):

$ cat t706.cu
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdint.h>

int main(void)
{
  struct stat buf;

  char *dev_data;

  cudaDeviceProp cuda_prop;
  cudaGetDeviceProperties(&cuda_prop, 0);
  if (!cuda_prop.canMapHostMemory)
    exit(EXIT_FAILURE);

  cudaSetDeviceFlags(cudaDeviceMapHost);


  int data_file = open("test.dat", O_RDWR);
  int stat = fstat(data_file, &buf);
  int data_file_size = buf.st_size;
  printf("data_file_size = %d\n", data_file_size);
  char *data = (char *) mmap(0, data_file_size, PROT_READ|PROT_WRITE, MAP_PRIVATE, data_file, 0);
  if (data == NULL) {
    printf("mmap failure\n");
    exit(EXIT_FAILURE);}
  cudaError_t err = cudaHostRegister(data, data_file_size, cudaHostRegisterDefault);
  if (err != cudaSuccess) { //ERROR HERE.
    printf("cudaHostRegister fail\n");
    exit(EXIT_FAILURE);}

  err = cudaHostGetDevicePointer((void**)&dev_data, (void*)data, 0);
  if (err == cudaErrorMemoryAllocation)
  {
    printf("cudaHostGetDevicePointer - Mem Alloc Err\n");
    exit(EXIT_FAILURE);
  }
  else if (err == cudaErrorInvalidValue)
  {
    printf("cudaHostGetDevicePointer - Invalid Val Err\n");
    exit(EXIT_FAILURE);
  }

}
$ nvcc -arch=sm_30 -o t706 t706.cu
$ ./t706
data_file_size = 566316
$