Is there a way to load a vector equal by size to global memory size of GPU in OpenCl?

Question

My GPU has 12 GB global memory (CL_DEVICE_GLOBAL_MEM_SIZE), but only 3 GB of memory which it can allocate (CL_DEVICE_MAX_MEM_ALLOC_SIZE). When I try to load a vector of size exceeding 3 GB, the program crashes. The question is, if it is possible to load a bigger vector into GPU memory to utilize it completely, how to do it?

Answer 1

By default, CL_DEVICE_MAX_MEM_ALLOC_SIZE reports 1/4 of CL_DEVICE_GLOBAL_MEM_SIZE , meaning it would only be allowed to allocate four 3GB buffers on a 12GB GPU.

However, Nvidia GPUs allow to allocate their full memory capacity in a single buffer, even though they also report to have the 1/4 limit.

Some AMD GPUs have the limit set higher, for example the Radeon VII lets you use 14/16GB for a single buffer.

The only devices I have ever seen that really inforce the 1/4 limit are Intel HD 4600 and 5500, so older Intel integrated GPUs. If you go above 1/4 in buffer size there, the cl::Buffer constructor throws error -61 .

In case you are stuck with the 1/4 memory limit on your device, split your large 12GB buffer in 4 smaller 3GB buffers (for example one vector for x, y, z, w components of the vector each). If you use Windows, note that you might only be able to use ~11.5GB in total as some VRAM is reserved for the operating system.

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I think your issue might not be CL_DEVICE_MAX_MEM_ALLOC_SIZE though, but 32-bit integer overflow for the array size above 4GB. Use the uint64_t data type to set the array size instead.

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You might also be interested in this lightweight OpenCL-Wrapper for C++. There, the length of vectors always is in 64-bit integer, and it automatically keeps track on howm much memory you use in total on each device, telling you if you allocate too much. It also catches that -61 error on Intel iGPUs and tells you the maximum allowed buffer size then.