[GitHub] leleamol commented on issue #13798: C++ generate wrong prediction results on GPU

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leleamol commented on issue #13798: C++ generate wrong prediction results on GPU
URL: https://github.com/apache/incubator-mxnet/issues/13798#issuecomment-459920883
 
 
   Hello @jacksonxliu 
   I could not reproduce this problem. I tried outputting the results from "inception_inference" example in CPU and GPU context but the output was same.
   Can you please confirm that the above code when ran as it is on CPU and GPU instances outputs a different result?
   
   I noticed that the even though we can get the copy of output in GPU context as follows:
   
   `auto ids = exec->outputs[0].Copy(Context(kGPU, 0));`
   
   We wouldn't be able to access the individual members of "ids" NDArray directly. This is because the underlying "GetData()" function requires the NDArray to be in CPU context. The overloaded operator "<<" of NDArray converts the array in GPU context to CPU context before accessing the individual members. 
   
   In addition, I noticed that when printing or accessing the NDArray elements, Python uses "MXNDArraySyncCopyToCPU" API where as C++ implementation uses "_copy" operator.
   
   Will it possible for you to try following code to see if it produces different results on CPU and GPU?
   
   ```
   std::vector<mx_float> ids;
   exec->outputs[0].SyncCopyToCPU(&ids, 0L);
   
   for (size_t i = 0; i < ids.size(); i++) {
      std::cout << ids[i];
   }
   ```
   Thanks.
   
   

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