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clarified OMP_NUM_THREADS #835

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clarified OMP_NUM_THREADS #835

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9 changes: 8 additions & 1 deletion docs/Multithreading.md
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Expand Up @@ -147,7 +147,14 @@ $ GOMP_CPU_AFFINITY="0-15" BLIS_NUM_THREADS=16 ./my_blis_program
```
Either of these approaches causes BLIS to automatically determine a reasonable threading strategy based on what is known about the operation and problem size. If `BLIS_NUM_THREADS` is not set, BLIS will attempt to query the value of `BLIS_NT` (a shorthand alternative to `BLIS_NUM_THREADS`). If neither variable is defined, then BLIS will attempt to read `OMP_NUM_THREADS`. If none of these variables is set, the default number of threads is 1.

**Note**: We *highly* discourage use of the `OMP_NUM_THREADS` environment variable to specify multithreading within BLIS and may remove support for it in the future. If you wish to set parallelism globally via environment variables, please use `BLIS_NUM_THREADS`.
**Note**: If none of `BLIS_NT`/`BLIS_NUM_THREADS` are defined, BLIS will fall back to use
the standardized `OMP_NUM_THREADS` environment variable.
By having an application specific environment variable one can fine-tune the thread
utilization, e.g. to run OpenMP constructs using 4 threads, and BLIS with 2 threads:
```
$ OMP_NUM_THREADS=4 BLIS_NUM_THREADS=2 ./my_omp_blis_program
```


### Environment variables: the manual way

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