qiskit/releasenotes/notes/0.20/bump-symengine-8ca362f5b9fe...

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---
upgrade:
- |
The minimum supported version of ``symengine`` is now 0.9.0. This was
necessary to improve compatibility with Python's ``pickle`` module which
is used internally as part of parallel dispatch with :func:`.parallel_map`.
- |
The default value of ``QISKIT_PARALLEL`` when running with Python 3.9 on
Linux is now set to ``TRUE``. This means when running :func:`.parallel_map`
or functions that call it internally, such as :func:`.transpile` and
:func:`.assemble`, the function will be executed in multiple processes and
should have better run time performance. This change was made because the
issues with reliability of parallel dispatch appear to have been resolved
(see `#6188 <https://github.com/Qiskit/qiskit-terra/issues/6188>`__ for
more details). If you still encounter issues because of this you can disable
multiprocessing and revert to the previous default behavior by setting the
``QISKIT_PARALLEL`` environment variable to ``FALSE``, or setting the
``parallel`` option to ``False`` in your user config file (also please file
an issue so we can track any issues related to multiprocessing).