topic How to allocate memory in MIC correctly? in Software Archive

How to allocate memory in MIC correctly?

Chang_X_ — Sun, 16 Feb 2014 01:53:48 GMT

I'm trying MIC and encountered a strange problem about how to allocate memory on MIC.

I write a example program like THIS:

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

__attribute__ ((target(mic:0))) float *a;
__attribute__ ((target(mic:0))) float *b;
__attribute__ ((target(mic:0))) float *c;

int main() {

#pragma offload target(mic:0) \
	nocopy(a:length(1000) alloc_if(1) free_if(0)) \
	nocopy(b:length(1000) alloc_if(1) free_if(0)) \
	nocopy(c:length(1000) alloc_if(1) free_if(0))
	{
		printf("a: 0x%x b:0x%x, c:0x%x\n", a,b,c);
		for(int i=0; i<1000; i++) {
			a = 1;
			b = 2;
			c = 3;
		}
	}

#pragma offload target(mic:0) \
	nocopy(a: alloc_if(0) free_if(0)) \
	nocopy(b: alloc_if(0) free_if(0)) \
	nocopy(c: alloc_if(0) free_if(0))
	{
		printf("a: 0x%x b:0x%x, c:0x%x\n", a,b,c);
		for(int i=0; i<1000; i++) printf("i: %d a: %f b: %f c: %f\n",i, a,b,c);
	}

	return 0;
}

The output shows that all of pointers of a,b and c pointed to the same place, so the assignment of ‘c' overwrite the assignments of a and b. Why the program runs like this? How to allocate memory on MIC correctly?

THX.

Chang,

TaylorIoTKidd — Wed, 19 Feb 2014 01:53:41 GMT

Chang,

I suggest reading http://software.intel.com/en-us/blogs/2013/03/27/behind-the-scenes-offload-memory-management-on-the-intel-xeon-phi-coprocessor and looking at the coding examples in your composer installation ( /opt/intel/composer_xe_2013_sp1.2.144/Samples/en_US/C++/mic_samples ).

If you still have issues, please continue this thread.

Regards
--
Taylor

I modified your program below

Kevin_D_Intel — Wed, 19 Feb 2014 09:47:20 GMT

I modified your program below adding the corresponding host-side allocations and using IN() vs. NOCOPY() for the first transfer where the allocation on the MIC-side occurs.

I'm not certain whether it satisfies your interest so in addition to the blog Taylor cited, you can review the details and examples on the Effective Use of Compiler Features for Offloading article which may offer additional insights.

#include <stdio.h>
#include <stdlib.h>
__attribute__ ((target(mic:0))) float *a;
__attribute__ ((target(mic:0))) float *b;
__attribute__ ((target(mic:0))) float *c;

int main()
{

a = (float *)_mm_malloc(sizeof(float) * 1000,64);
b = (float *)_mm_malloc(sizeof(float) * 1000,64);
c = (float *)_mm_malloc(sizeof(float) * 1000,64);

#pragma offload target(mic:0) \
        in(a:length(1000) alloc_if(1) free_if(0)) \
        in(b:length(1000) alloc_if(1) free_if(0)) \
        in(c:length(1000) alloc_if(1) free_if(0))
{
    printf("a: 0x%x b:0x%x, c:0x%x\n", a,b,c);

    for(int i=0; i<1000; i++) {
      a = 1;
      b = 2;
      c = 3;
    }
}

#pragma offload target(mic:0) \
        nocopy(a: alloc_if(0) free_if(0)) \
        nocopy(b: alloc_if(0) free_if(0)) \
        nocopy(c: alloc_if(0) free_if(0))
{
     printf("a: 0x%x b:0x%x, c:0x%x\n", a,b,c);

for(int i=0; i<1000; i++)
printf("i: %d a: %f b: %f c: %f\n",i, a,b,c);
}

return 0;
}