Strange sparse matrix error on GPU

I have a sparse matrix with a relatively small (16 MB) memory footprint

>> Whos A

  Name   Size              Kilobytes     Class    Attributes
                                                            
  A      100000x100000         16406     double   sparse    

When I send it to the GPU and add the scalar 0, I get a puzzling error

>> gpuArray(A)+sparse(0);

Error using  + 
Maximum variable size allowed on the device is exceeded.

I really cannot see why this operation would cause any excessively large memory allocations. This is in Matlab version R2018a and the following GPU details. Does anyone have any insights as to why this occurs?

>> gpuDevice

ans = 

  CUDADevice with properties:
  
                      Name: 'GeForce GTX TITAN X'
                     Index: 1
         ComputeCapability: '5.2'
            SupportsDouble: 1
             DriverVersion: 9.2000
            ToolkitVersion: 9
        MaxThreadsPerBlock: 1024
          MaxShmemPerBlock: 49152
        MaxThreadBlockSize: [1024 1024 64]
               MaxGridSize: [2.1475e+09 65535 65535]
                 SIMDWidth: 32
               TotalMemory: 1.2885e+10
           AvailableMemory: 1.2259e+10
       MultiprocessorCount: 24
              ClockRateKHz: 1076000
               ComputeMode: 'Default'
      GPUOverlapsTransfers: 1
    KernelExecutionTimeout: 0
          CanMapHostMemory: 1
           DeviceSupported: 1
            DeviceSelected: 1

[Matt J]

Here's a simple command to reproduce the problem,

>>  gpuArray(speye(1e5))+sparse(0);

Error using  + 
Maximum variable size allowed on the device is exceeded.

Answer 1

[Edric Ellis]

The maximum number of elements in a gpuArray is intmax('int32'). In this case, the scalar addition is forcing your sparse matrix to become a full array of size 1e5×1e5. (In this particular case, because the scalar you are adding is precisely zero, then it could theoretically become a no-op.)

>> (1e5*1e5) <= intmax('int32')
ans =
  logical
   0

[Matt J]

Hi Edric,

Sorry, I mistyped the test. It should be,

>> gpuArray(speye(1e5))+sparse(0);

Error using  + 
Maximum variable size allowed on the device is exceeded.

Because the added 0 is in sparse form, it should not be triggering the creation of a full matrix.

[Edric Ellis]

Ok, but even so, number of non-zeros required for the result exceeds intmax('int32') because unfortunately the gpuArray implementation of plus doesn't make a special case for adding zero - sparse gpuArray plus scalar always results in a "big sparse" array which is essentially a full array but with sparse storage.

Not terribly useful perhaps, but the following does work correctly:

>> gpuArray(speye(1e5)) + spalloc(1e5,1e5,0);

[Matt J]

OK. Well, I hope it's on the todo list of things to fix. :-)