I have written a sample program to understand the effects of GPU/CPU pinned memory and Heap memory. The following code illustrates this. I have allocated three buffers of dimensions say 1280x720. I have filled buffers 1 & 2 with some data and in turn used these buffers to fill buffer 3. The mathematical operatio involved in filling buffer 3 is insignificant. In case 1, the memory allocated from these buffers are from heap (malloc call). In case 2, the memory for these buffers are allocated from OpenCL API calls (clCreateBuffer()). There is a performance difference between these 2 cases. I tested it on Intel integrated GPU's. I am unable to explain this difference in performance. Does it have some thing to do with cacheable properties of CPU/GPU pinned memory vs Heap memory.
Have you encountered such behavior before or am i doing something wrong?
#include <stdio.h>
#include <malloc.h>
#include <string.h>
#include <stdlib.h>
#include <inttypes.h>
#define OPENCL
#if defined(_WIN32)
/*
* Win32 specific includes
*/
#ifndef WIN32_LEAN_AND_MEAN
#define WIN32_LEAN_AND_MEAN
#endif
#include <windows.h>
#else
#include <sys/time.h>
/* timersub is not provided by msys at this time. */
#ifndef timersub
#define timersub(a, b, result) \
do { \
(result)->tv_sec = (a)->tv_sec - (b)->tv_sec; \
(result)->tv_usec = (a)->tv_usec - (b)->tv_usec; \
if ((result)->tv_usec < 0) { \
--(result)->tv_sec; \
(result)->tv_usec += 1000000; \
} \
} while (0)
#endif
#endif
struct usec_timer {
#if defined(_WIN32)
LARGE_INTEGER begin, end;
#else
struct timeval begin, end;
#endif
};
static void usec_timer_start(struct usec_timer *t) {
#if defined(_WIN32)
QueryPerformanceCounter(&t->begin);
#else
gettimeofday(&t->begin, NULL);
#endif
}
static void usec_timer_mark(struct usec_timer *t) {
#if defined(_WIN32)
QueryPerformanceCounter(&t->end);
#else
gettimeofday(&t->end, NULL);
#endif
}
static int64_t usec_timer_elapsed(struct usec_timer *t) {
#if defined(_WIN32)
LARGE_INTEGER freq, diff;
diff.QuadPart = t->end.QuadPart - t->begin.QuadPart;
QueryPerformanceFrequency(&freq);
return diff.QuadPart * 1000000 / freq.QuadPart;
#else
struct timeval diff;
timersub(&t->end, &t->begin, &diff);
return diff.tv_sec * 1000000 + diff.tv_usec;
#endif
}
#ifdef OPENCL
#include ".\CL\cl.h"
int opencl_init(cl_context *context, cl_command_queue *cmd_queue) {
cl_int status;
cl_uint num_platforms = 0;
cl_platform_id platform;
cl_uint num_devices = 0;
cl_device_id device;
cl_command_queue_properties command_queue_properties = 0;
// Get the number of platforms in the system.
status = clGetPlatformIDs(0, NULL, &num_platforms);
if (status != CL_SUCCESS || num_platforms == 0)
goto fail;
// Get the platform ID for one platform
status = clGetPlatformIDs(1, &platform, NULL);
if (status != CL_SUCCESS)
goto fail;
// Get the number of devices available on the platform
status = clGetDeviceIDs(platform, CL_DEVICE_TYPE_GPU, 0, NULL, &num_devices);
if (status != CL_SUCCESS || num_devices == 0)
goto fail;
// Get the device ID for one device
status = clGetDeviceIDs(platform, CL_DEVICE_TYPE_GPU, 1, &device, NULL);
if (status != CL_SUCCESS)
goto fail;
// Create OpenCL context for one device
*context = clCreateContext(NULL, 1, &device, NULL, NULL, &status);
if (status != CL_SUCCESS || *context == NULL)
goto fail;
// Create command queues for the device
*cmd_queue = clCreateCommandQueue(*context, device, command_queue_properties, &status);
if (status != CL_SUCCESS || *cmd_queue == NULL)
goto fail;
return 0;
fail:
return 1;
}
#endif
int main(int argc, char **argv) {
int x, y, z;
int width = 1280, height = 720;
unsigned char *buffer[3];
int use_gpu;
cl_mem opencl_mem[3];
cl_context context;
cl_command_queue cmd_queue;
cl_int status;
if (argc != 2)
return 0;
use_gpu = atoi(argv[1]);
if (use_gpu) {
if (opencl_init(&context, &cmd_queue))
printf("OpenCL init failure");
}
if (use_gpu) {
for (x = 0; x < 3; x++) {
opencl_mem[x] = clCreateBuffer(context,
CL_MEM_READ_ONLY | CL_MEM_ALLOC_HOST_PTR,
width * height * sizeof(*buffer[x]), NULL,
&status);
if (status != CL_SUCCESS)
return 0;
buffer[x] = clEnqueueMapBuffer(cmd_queue, opencl_mem[x], CL_TRUE,
CL_MAP_READ | CL_MAP_WRITE, 0,
width * height * sizeof(*buffer[x]), 0,
NULL, NULL, &status);
if (status != CL_SUCCESS) {
clReleaseMemObject(opencl_mem[x]);
opencl_mem[x] = NULL;
return 0;
}
}
} else {
for (x = 0; x < 3; x++) {
buffer[x] = malloc(width * height * sizeof(*buffer[x]));
if (buffer[x] == NULL) {
printf("Unable to alloc memory");
}
}
}
memset(buffer[0], 1, width * height * sizeof(*buffer[0]));
memset(buffer[1], 2, width * height * sizeof(*buffer[1]));
memset(buffer[2], 0, width * height * sizeof(*buffer[2]));
{
struct usec_timer emr_timer;
usec_timer_start(&emr_timer);
for (z = 0; z < 600; z++) {
for (y = 0; y < height; y++) {
for (x = 0; x < width; x++) {
// don't worry about overflows
buffer[2][y * width + x] += buffer[0][y * width + x]
+ buffer[1][y * width + x];
}
}
}
usec_timer_mark(&emr_timer);
printf("Elapsed time %"PRIu64"\n", usec_timer_elapsed(&emr_timer));
}
if (use_gpu) {
for (x = 0; x < 3; x++) {
if (buffer[x] != NULL) {
status = clEnqueueUnmapMemObject(cmd_queue, opencl_mem[0], buffer[0], 0,
NULL, NULL);
status |= clFinish(cmd_queue);
if (status != CL_SUCCESS)
return 0;
buffer[0] = NULL;
}
if (opencl_mem[0] != NULL) {
status = clReleaseMemObject(opencl_mem[0]);
if (status != CL_SUCCESS)
return 0;
opencl_mem[0] = NULL;
}
}
clReleaseCommandQueue(cmd_queue);
clReleaseContext(context);
} else {
for (x = 0; x < 3; x++) {
free(buffer[x]);
buffer[x] = NULL;
}
}
return 0;
}
