.. _Partitioner_Summary:
Partitioner Summary
===================
The parallel loop templates ``parallel_for`` and ``parallel_reduce``
take an optional *partitioner* argument, which specifies a strategy for
executing the loop. The following table summarizes partitioners and
their effect when used in conjunction with ``blocked_range``.
.. container:: tablenoborder
.. list-table::
:header-rows: 1
* - Partitioner
- Description
- When Used with ``blocked_range(i,j,g)``
* - ``simple_partitioner``
- Chunksize bounded by grain size.
- ``g/2 ≤ chunksize ≤ g``
* - ``auto_partitioner`` (default)
- Automatic chunk size.
- ``g/2 ≤ chunksize``
* - ``affinity_partitioner``
- Automatic chunk size, cache affinity and uniform distribution of iterations.
- ``g/2 ≤ chunksize``
* - ``static_partitioner``
- Deterministic chunk size, cache affinity and uniform distribution of iterations without load balancing.
- ``max(g/3, problem_size/num_of_resources) ≤ chunksize``
An ``auto_partitioner`` is used when no partitioner is specified. In
general, the ``auto_partitioner`` or ``affinity_partitioner`` should be
used, because these tailor the number of chunks based on available
execution resources. ``affinity_partitioner`` and ``static_partitioner``
may take advantage of ``Range`` ability to split in a given ratio (see
"Advanced Topic: Other Kinds of Iteration Spaces") for distributing
iterations in nearly equal chunks between computing resources.
``simple_partitioner`` can be useful in the following situations:
- The subrange size for ``operator()`` must not exceed a limit. That
might be advantageous, for example, if your ``operator()`` needs a
temporary array proportional to the size of the range. With a limited
subrange size, you can use an automatic variable for the array
instead of having to use dynamic memory allocation.
- A large subrange might use cache inefficiently. For example, suppose
the processing of a subrange involves repeated sweeps over the same
memory locations. Keeping the subrange below a limit might enable the
repeatedly referenced memory locations to fit in cache.
- You want to tune to a specific machine.
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