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openclgpuintel
user6345890
user6345890

Reputation: 13

OpenCL allcoation flag CL_MEM_USE_HOST_PTR usage not referencing my pointer

I was trying to use the flag CL_MEM_USE_HOST_PTR with the OpenCL function clCreateBuffer() in order to avoid multiple memory allocation. After a little research (reverse engineering), I found that the framework calls the operating system allocation function no matter what flag I use.

Maybe my concept is wrong? But from documentation it's supposed to use DMA to access the host memory instead of allocating new memory.

I am using opencl 1.2 on an Intel device (HD5500)

Upvotes: 1

Views: 860

Answers (1)

Tim
Tim

Reputation: 2796

On Intel GPUs ensure the allocated host pointer is page aligned and page length*. In fact I think the buffer size can actually be an even number of cache lines, but I always round up.

Use something like:

void *host_ptr = _aligned_malloc(align_to(size,4096),4096));

Here's a good article for this: In the "Key Takeaways".

If you already have the data and want to load the data into an OpenCL buffer object, then use CL_MEM_USE_HOST_PTR with a buffer allocated at a 4096 byte boundary (aligned to a page and cache line boundary) and a total size that is a multiple of 64 bytes (cache line size).

You can also use CL_MEM_ALLOC_HOST_PTR and let the driver handle the allocation. But to get at the pointer you'll have to map and unmap it (but at no copy cost).

Upvotes: 2

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