Strange gpu arrayfun behavior

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nealm on 20 Jul 2018
Commented: Joss Knight on 26 Jul 2018
I have existing code that works but I'd like to accelerate using the GPU version of arrayfun. However the behavior of GPU arrayfun is difficult to understand. According to the documentation the code within the helper function performs scalar operations on the input gpuArray values. However if I try to write my own scalar values the result is unpredictable. To demonstrate, if I have a helper function that look like:
function outArray = simpleTest(inArray)
outArray = inArray * 3.14159;
x = gpuArray(1:4);
y = arrayfun(@simpleTest, x);
y = gather(y)
y =
Columns 1 through 2
0.0000 + 0.0000i 0.0000 + 0.0000i
Columns 3 through 4
0.0000 + 0.0000i 0.0000 + 0.0000i
If I pass the scalar value to the helper function, as shown in the second helper function, it does seem to work:
function outArray = simpleTest2(inArray, scalarValue)
outArray = inArray * scalarValue;
y = arrayfun(@simpleTest2, x, 3.14159);
y = gather(y)
y =
3.1416 6.2832 9.4248 12.5664
Reading the documentation, I know that the scalar values in the calling function are expanded (similar to bsxfun) however I have been unable to get that function, or any other function, to work within the helper function. I know I'm missing something here, I would appreciate the help.
I have Matlab R2018a with Parallel Computing Toolbox and Cuda version 9.0 installed.
Joss Knight
Joss Knight on 24 Jul 2018
Okay, so the next thing to do is to check and upgrade your driver. What is your device? What is your driver version?

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Accepted Answer

nealm on 25 Jul 2018
I found the source of the problem. I use my own "startup.m" file (which only has "addpath" commands) that is executed at the beginning of a Matlab start. The directories that have been added during the startup process appear to interfere with the way gpu arrayfun executes.
If I don't use the startup.m file gpu arrayfun works fine. Curiously, if I manually run the contents of startup.m AFTER starting Matlab - gpu arrayfun also works fine.
Joss Knight
Joss Knight on 26 Jul 2018
Thanks for the investigation! I'll have a look at why str2double is involved and whether I can reproduce your issue. MTree is used to do static analysis of your arrayfun function for the GPU, and presumably str2double is being used to convert the text of your literal value. It may well be a bug that it's possible to break arrayfun by shadowing this function.

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