[docs] Fix num_threads/n_jobs semantics and default #6863
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@@ -240,14 +240,18 @@ Core Parameters | |||
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| - refer to `Distributed Learning Guide <./Parallel-Learning-Guide.rst>`__ to get more details | ||||
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| - ``num_threads`` :raw-html:`<a id="num_threads" title="Permalink to this parameter" href="#num_threads">🔗︎</a>`, default = ``None``, type = int, aliases: ``num_thread``, ``nthread``, ``nthreads``, ``n_jobs`` | ||||
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| - used only in ``train``, ``prediction`` and ``refit`` tasks or in correspondent functions of language-specific packages | ||||
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| - number of threads for LightGBM | ||||
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| - ``None`` means number of physical cores | ||||
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Let's remove this, in favor of a single new note for the |
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| - ``0`` means default number of threads in OpenMP | ||||
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| - Negative integers are interpreted as following joblib's formula (``n_cpus + 1 + n_jobs``), just like scikit-learn (so e.g. -1 means using all cores, physical or logical). | ||||
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There was a problem hiding this comment. Please move this all the way to the bottom of the docs for this parameter, and change it to something like the following. These I think this is worth calling out in a note pointing to those docs though, to avoid confusion like what you faced. |
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| - for the best speed, set this to the number of **real CPU cores**, not the number of threads (most CPUs use `hyper-threading <https://en.wikipedia.org/wiki/Hyper-threading>`__ to generate 2 threads per CPU core) | ||||
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| - do not set it too large if your dataset is small (for instance, do not use 64 threads for a dataset with 10,000 rows) | ||||
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This is not correct.
Noneis a Python-specific concept, and LightGBM is not just a Python package. It's a C++ library with a C API, a command line interface (CLI), an R package, and many more extensions.This parameters page documents the impact of these parameters for the core LightGBM library. It's fine to add a note here that the
scikit-learnestimators in the Python package specially have a different behavior, but this default should not be changed. It is really 0:LightGBM/include/LightGBM/config.h
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