# Slow-down of a program built with gfortran in an Intel oneAPI command window

**URL:** <https://fortran-lang.discourse.group/t/slow-down-of-a-program-built-with-gfortran-in-an-intel-oneapi-command-window/10421>\
**Category:** Help\
**Created:** [September 29, 2025, 7:24pm UTC](https://fortran-lang.discourse.group/t/slow-down-of-a-program-built-with-gfortran-in-an-intel-oneapi-command-window/10421 "2025-09-29T19:24:51Z")\
**Posts on this page:** 4\
**Page:** 3

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**Author:** ![greenrongreen](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/greenrongreen/32/1249_2.png) [@greenrongreen](https://fortran-lang.discourse.group/u/greenrongreen)\
**Post date:** [October 6, 2025, 5:14pm UTC](https://fortran-lang.discourse.group/t/slow-down-of-a-program-built-with-gfortran-in-an-intel-oneapi-command-window/10421/41 "2025-10-06T17:14:01Z")

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One other perhaps silly question: do you have a newer processor that has both P-cores (Performance cores) and E-cores (efficiency cores)? If so, perhaps the oneAPI window is defaulting to a E-core or is by default launching processes on e-cores. Just a possibility.

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**Author:** ![Arjen](https://avatars.discourse-cdn.com/v4/letter/a/b9bd4f/32.png) [@Arjen](https://fortran-lang.discourse.group/u/Arjen)\
**Post date:** [October 6, 2025, 5:48pm UTC](https://fortran-lang.discourse.group/t/slow-down-of-a-program-built-with-gfortran-in-an-intel-oneapi-command-window/10421/42 "2025-10-06T17:48:28Z")

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Yes, it does have both types. I should perhaps try to work with the affinity option to get the processes to run on the right one. I have done experiments with that for the threaded version, but there did not seem to be much influence. And I would not expect an 8 to 10-fold slow-down if the process happens to be assigned to the wrong processor type. But it is the most likely reason that I have seen so far.

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**Author:** ![Arjen](https://avatars.discourse-cdn.com/v4/letter/a/b9bd4f/32.png) [@Arjen](https://fortran-lang.discourse.group/u/Arjen)\
**Post date:** [October 7, 2025, 1:05pm UTC](https://fortran-lang.discourse.group/t/slow-down-of-a-program-built-with-gfortran-in-an-intel-oneapi-command-window/10421/43 "2025-10-07T13:05:22Z")

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I just tried it by specifying the affinity to the program’s run (FYI: start /b /wait /affinity 0xAAAAAA program - the string of A’s is what I gathered from some website to be the selection of the performant cores). Result: 100 seconds for the 700x700 matrix.  
But the plain run, so no affinity, took 70 seconds in the same Intel oneAPI command window.  
The other runs, the ones I was complaining about, took 163 seconds for this size.  
I should probably try a wider set of cases, but my eariler observation that the effect is reproducible seems less solid.

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**Author:** ![Arjen](https://avatars.discourse-cdn.com/v4/letter/a/b9bd4f/32.png) [@Arjen](https://fortran-lang.discourse.group/u/Arjen)\
**Post date:** [October 7, 2025, 2:27pm UTC](https://fortran-lang.discourse.group/t/slow-down-of-a-program-built-with-gfortran-in-an-intel-oneapi-command-window/10421/44 "2025-10-07T14:27:47Z")

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Quick check: the time I recorded from a plain window is 15 seconds. So, all the other timings I reported today are far longer.

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