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62 lines
2.8 KiB
Markdown
62 lines
2.8 KiB
Markdown
# Thread Pools
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A thread pool ([Wikipedia][thread_pools_wikipedia]) accepts and executes
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lightweight work items called "tasks", using a carefully managed group
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of dedicated long-running worker threads. The worker threads perform
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the work items in parallel without forcing each work item to assume the
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burden of starting and destroying a dedicated thead.
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## Classes
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### `ThreadPoolInterface`
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The [`ThreadPoolInterface`][thread_pool_interface.h] abstract interface is
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an extension of the `OutOfLineExecutor` (see [the executors architecture
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guide][executors]) abstract interface, adding `startup`, `shutdown`, and
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`join` virtual member functions. It is the base class for our thread
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pool classes.
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### `ThreadPool`
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[`ThreadPool`][thread_pool.h] is the most basic concrete thread pool. The
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number of worker threads is adaptive, but configurable with a min/max
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range. Idle worker threads are reaped (down to the configured min), while
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new worker threads can be created when needed (up to the configured max).
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### `ThreadPoolTaskExecutor`
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[`ThreadPoolTaskExecutor`][thread_pool_task_executor.h] is not a thread
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pool, but rather a `TaskExecutor` that uses a `ThreadPoolInterface` and
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a `NetworkInterface` to execute scheduled tasks. It's configured with a
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`ThreadPoolInterface` over which it _takes_ ownership, and a
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`NetworkInterface`, of which it _shares_ ownership. With these resources
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it implements the elaborate `TaskExecutor` interface (see [executors]).
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### `NetworkInterfaceThreadPool`
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[`NetworkInterfaceThreadPool`][network_interface_thread_pool.h] is a
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thread pool implementation that doesn't actually own any worker threads.
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It runs its tasks on the background thread of a
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[`NetworkInterface`][network_interface.h].
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Incoming tasks that are scheduled from the `NetworkInterface`'s thread
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are run immediately. Otherwise they are queued to be run by the
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`NetworkInterface` thread when it is available.
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### `ThreadPoolMock`
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[`ThreadPoolMock`][thread_pool_mock.h] is a `ThreadPoolInterface`. It is not
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a mock of a `ThreadPool`. It has no configurable stored responses. It has
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one worker thread and a pointer to a `NetworkInterfaceMock`, and with these
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resources it simulates a thread pool well enough to be used by a
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`ThreadPoolTaskExecutor` in unit tests.
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[thread_pools_wikipedia]: https://en.wikipedia.org/wiki/Thread_pool
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[executors]: ../src/mongo/executor/README.md
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[thread_pool_interface.h]: ../src/mongo/util/concurrency/thread_pool_interface.h
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[thread_pool.h]: ../src/mongo/util/concurrency/thread_pool.h
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[thread_pool_task_executor.h]: ../src/mongo/executor/thread_pool_task_executor.h
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[network_interface_thread_pool.h]: ../src/mongo/executor/network_interface_thread_pool.h
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[network_interface.h]: ../src/mongo/executor/network_interface.h
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[thread_pool_mock.h]: ../src/mongo/executor/thread_pool_mock.h
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