topic Multiple calls to DGEEV in Intel® oneAPI Math Kernel Library

Multiple calls to DGEEV

Cansey50 — Thu, 23 Nov 2023 14:55:51 GMT

I have not used oneMKL before, and only recently begun experimenting with it. Ultimately I would like to create DLLs for some of the functions including the ones for the eigenvalue/eigenvector calculations.

I took the dgeev.c example from \examples_core_c\c\lapack\source\ zipped folder in my oneAPI installation folder, and compiled it in a Visual Studio 2019 console project. It runs ok and gives the exact results mentioned at the top of the dgeev.c file.

However, if I call dgeev again the results are different! I have attached the source code in which dgeev is called 3 times and the output of the program. I was expecting to get exactly the same results every single time the dgeev is called as the matrix a has not changed.

Can you please help me understand why the results are different, and what I am doing wrong?

Thanks beforehand.

In case it is needed:

My Processor: Intel(R) Core(TM) i5-2500 CPU

My OS: 64-bit operating system, x64-based processor

Re: Multiple calls to DGEEV

mecej4 — Thu, 23 Nov 2023 16:39:42 GMT

Please read the conventions of the Lapack/BLAS routines that are called in your code. For DGEEV, the documentation says:

[in,out]

          A is DOUBLE PRECISION array, dimension (LDA,N)
          On entry, the N-by-N matrix A.
          On exit, A has been overwritten.

If you wish to call DGEEV after the first call, you have to restore the original values in A before the second call (and subsequent ones, if any).

Most of the Lapack and BLAS routines were designed and written at a time when "main memory" was a precious and scarce resource. The computers of the time, such as the CDC6600, had memory measured in kilowords (132 kWords, 60 bits each).

Re: Multiple calls to DGEEV

Cansey50 — Thu, 23 Nov 2023 17:05:48 GMT

Thank you mercej4! I admit I have to read the documentation carefully.

Re:Multiple calls to DGEEV

ShanmukhS_Intel — Mon, 27 Nov 2023 05:32:37 GMT

Hi Khosro,

Thanks for posting in Intel Communities.

It’s great to know that the issue has been resolved, in case you run into any other issues please feel free to create a new thread.

Best Regards,

Shanmukh.SS