wehub-resource-sync
36 lines
1.4 KiB
ReStructuredText
36 lines
1.4 KiB
ReStructuredText
.. _nested-tasks:
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Pattern: Using nested tasks to achieve nested parallelism
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=========================================================
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In this pattern, a remote task can dynamically call other remote tasks (including itself) for nested parallelism.
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This is useful when sub-tasks can be parallelized.
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Keep in mind, though, that nested tasks come with their own cost: extra worker processes, scheduling overhead, bookkeeping overhead, etc.
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To achieve speedup with nested parallelism, make sure each of your nested tasks does significant work. See :doc:`too-fine-grained-tasks` for more details.
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Example use case
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----------------
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You want to quick-sort a large list of numbers.
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By using nested tasks, we can sort the list in a distributed and parallel fashion.
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.. figure:: ../images/tree-of-tasks.svg
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Tree of tasks
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Code example
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------------
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.. literalinclude:: ../doc_code/pattern_nested_tasks.py
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:language: python
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:start-after: __pattern_start__
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:end-before: __pattern_end__
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We call :func:`ray.get() <ray.get>` after both ``quick_sort_distributed`` function invocations take place.
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This allows you to maximize parallelism in the workload. See :doc:`ray-get-loop` for more details.
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Notice in the execution times above that with smaller tasks, the non-distributed version is faster. However, as the task execution
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time increases, i.e. because the lists to sort are larger, the distributed version is faster.
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