Why does complex Matlab gpuArray take twice as much memory than it should? - matlab

Question

I noticed that a large complex array takes twice as much memory on GPU than on CPU.
Here is a minimal example:
%-- First Try: Complex Single
gpu = gpuDevice(1);
m1 = gpu.FreeMemory;
test = complex(single(zeros(600000/8,1000))); % 600 MByte complex single
whos('test')
test = gpuArray(test);
fprintf(' Used memory on GPU: %e\n', m1-gpu.FreeMemory);
Now I do the same with a twice as big array which is not complex:
%-- Second Try:, Single
gpu = gpuDevice(1);
m1 = gpu.FreeMemory;
test = single(zeros(600000/4,1000)); % 600MB MByte real single
whos('test')
test = gpuArray(test);
fprintf(' Used memory on GPU: %e\n', m1-gpu.FreeMemory);
The output is:
Name Size Bytes Class Attributes
test 75000x1000 600000000 single complex
Used memory on GPU: 1.200095e+09
Name Size Bytes Class Attributes
test 150000x1000 600000000 single
Used memory on GPU: 6.000476e+08
On the CPU both arrays are 600MB - on the GPU the complex array uses 1.2 GByte.
I tested this on two graphics cards: GeForce GTX 680 and Tesla K20 using Matlab 2013a.
How can I avoid this? Is this a bug in Matlab?

Answer 1

This was answered on MATLAB central. To summarize MathWorks developer Edric Ellis's answer:
gpu.FreeMemory may not be an accurate measure of the available GPU memory because MATLAB does not immediately free up memory when it's done using it. gpu.AvailableMemory is a more accurate measure of available memory.
Transferring complex data to/from the GPU still requires 2x the memory because there is a format conversion that is done on the GPU. Specifically, complex arrays in CPU host memory are stored with the real/imaginary parts split into 2 separate vectors, whereas complex arrays on the GPU device are stored in interleaved format.
Testing on R2017a, I confirmed that:
Switching from gpu.FreeMemory to gpu.AvailableMemory indeed addresses this discrepancy in reported memory usage that prompted the original question.
With 8 GB of GPU memory, copying...
6 GB real array: success
6 GB complex array: "Out of memory on device" error
6 separate 1 GB complex arrays: success