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DOC: site.cfg: warn against OpenBLAS and multiprocessing

Fixes #654 by not fixing it; I don't think NumPy *can* actually fix
the problem as it's a design flaw in Python's multiprocessing. Listed
various alternatives (Python 3.4 forkserver, single-threaded OpenBLAS,
Python threading).
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commit 7488ebeee75f98a01dc6c9bab6e98e254ff54c95 1 parent 4b4796a
@larsmans larsmans authored
Showing with 11 additions and 0 deletions.
  1. +11 −0 site.cfg.example
11 site.cfg.example
@@ -83,6 +83,17 @@
# for your configuration (in the following example we installed OpenBLAS with
# ``make install PREFIX=/opt/OpenBLAS``.
+# **Warning**: OpenBLAS, by default, is built in multithreaded mode. Due to the
+# way Python's multiprocessing is implemented, a multithreaded OpenBLAS can
+# cause programs using both to hang as soon as a worker process is forked on
+# POSIX systems (Linux, Mac). Python 3.4 will introduce a new feature in
+# multiprocessing, called the "forkserver", which solves this problem. For
+# older versions, either compile OpenBLAS with multithreading turned off or
+# use Python threads instead of multiprocessing.
+# (This problem does not exist with multithreaded ATLAS.)
# [openblas]
# libraries = openblas
# library_dirs = /opt/OpenBLAS/lib
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