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## Multiprocessing with gRPC Python
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Multiprocessing allows application developers to sidestep the Python global
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interpreter lock and achieve true parallelism on multicore systems.
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Unfortunately, using multiprocessing and gRPC Python is not yet as simple as
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instantiating your server with a `futures.ProcessPoolExecutor`.
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The library is implemented as a C extension, maintaining much of the state that
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drives the system in native code. As such, upon calling
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[`fork`](http://man7.org/linux/man-pages/man2/fork.2.html), any threads in a
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critical section may leave the state of the gRPC library invalid in the child
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process. See this [excellent research
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paper](https://www.microsoft.com/en-us/research/uploads/prod/2019/04/fork-hotos19.pdf)
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for a thorough discussion of the topic.
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Calling `fork` without `exec` in your process *is* supported
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before any gRPC servers have been instantiated. Application developers can
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take advantage of this to parallelize their CPU-intensive operations.
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## Calculating Prime Numbers with Multiple Processes
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This example calculates the first 10,000 prime numbers as an RPC. We instantiate
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one server per subprocess, balancing requests between the servers using the
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[`SO_REUSEPORT`](https://lwn.net/Articles/542629/) socket option.
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```python
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_PROCESS_COUNT = multiprocessing.cpu_count()
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```
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On the server side, we detect the number of CPUs available on the system and
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spawn exactly that many child processes. If we spin up fewer, we won't be taking
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full advantage of the hardware resources available.
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## Running the Example
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To run the server,
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[ensure `bazel` is installed](https://docs.bazel.build/versions/master/install.html)
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and run:
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```
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bazel run //examples/python/multiprocessing:server &
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```
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Note the address at which the server is running. For example,
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```
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...
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[PID 107153] Binding to '[::]:33915'
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[PID 107507] Starting new server.
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[PID 107508] Starting new server.
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...
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```
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Note that several servers have been started, each with its own PID.
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Now, start the client by running
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```
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bazel run //examples/python/multiprocessing:client -- [SERVER_ADDRESS]
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```
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For example,
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```
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bazel run //examples/python/multiprocessing:client -- [::]:33915
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```
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Alternatively, generate code using the following and then run the client and server
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directly:
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```python
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cd examples/python/helloworld
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python -m grpc_tools.protoc -I . prime.proto --python_out=. --grpc_python_out=.
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```
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