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Tweaked/added notes to docs/Multithreading.md.

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Details:
- Added language to docs/Multithreading.md cautioning the reader about
  the nuances of setting multithreading parameters via the manual and
  automatic ways simultaneously, and also about how these parameters
  behave when multithreading is disabled at configure-time. These
  changes are an attempt to address the issues that arose in issue #362.
  Thanks to Jérémie du Boisberranger for his feedback on this topic.
- CREDITS file update.
This commit is contained in:
Field G. Van Zee
2019-11-12 15:32:57 -06:00
parent bdc7ee3394
commit 8f399c8940
2 changed files with 80 additions and 75 deletions
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docs/Multithreading.md
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@@ -107,7 +107,11 @@ This pattern--automatic or manual--holds regardless of which of the three method
Regardless of which method is employed, and which specific way within each method, after setting the number of threads, the application may call the desired level-3 operation (via either the [typed API](docs/BLISTypedAPI.md) or the [object API](docs/BLISObjectAPI.md)) and the operation will execute in a multithreaded manner. (When calling BLIS via the BLAS API, only the first two (global) methods are available.)
**Note**: Please be aware of what happens if you try to specify both the automatic and manual ways, as it could otherwise confuse new users. Regardless of which broad method is used, **if multithreading is specified via both the automatic and manual ways, the manual way will always take precedence.** Also, specifying parallelism for even *one* loop counts as specifying the manual way (in which case the ways of parallelism for the remaining loops will be assumed to be 1).
**Note**: Please be aware of what happens if you try to specify both the automatic and manual ways, as it could otherwise confuse new users. Here are the important points:
* Regardless of which broad method is used, **if multithreading is specified via both the automatic and manual ways, the values set via the manual way will always take precedence.**
* Specifying parallelism for even *one* loop counts as specifying the manual way (in which case the ways of parallelism for the remaining loops will be assumed to be 1).
* If you have specified multithreading via *both* the automatic and manual ways, BLIS will **not** complain if the values are inconsistent with one another. (For example, you may request 8 total threads be used while also specifing 4 ways of parallelism within each of two matrix multiplication loops, for a total of 16 ways.) Furthermore, you will be able to query these inconsistent values via the runtime API both before and after multithreading executes.
* If multithreading is disabled, you **may still** specify multithreading values via either the manual or automatic ways. However, BLIS will silently ignore **all** of these values. A BLIS library that is built with multithreading disabled at configure-time will always run sequentially (from the prespective of a single application thread).
## Globally via environment variables