### Navigation - [index](# "General Index") - [modules](# "Python Module Index") | - [next](# "Integration with other services") | - [previous](# "tornado.queues – Queues for coroutines") | - [Tornado 4.4.dev1 documentation](#) » - [Coroutines and concurrency](#) » # `tornado.process` — Utilities for multiple processes Utilities for working with multiple processes, including both forkingthe server into multiple processes and managing subprocesses. *exception *`tornado.process.``CalledProcessError`[[source]](#) An alias for [`subprocess.CalledProcessError`](https://docs.python.org/3.4/library/subprocess.html#subprocess.CalledProcessError "(in Python v3.4)") [https://docs.python.org/3.4/library/subprocess.html#subprocess.CalledProcessError]. `tornado.process.``cpu_count`()[[source]](#) Returns the number of processors on this machine. `tornado.process.``fork_processes`(*num_processes*, *max_restarts=100*)[[source]](#) Starts multiple worker processes. If `num_processes` is None or <= 0, we detect the number of coresavailable on this machine and fork that number of childprocesses. If `num_processes` is given and > 0, we fork thatspecific number of sub-processes. Since we use processes and not threads, there is no shared memorybetween any server code. Note that multiple processes are not compatible with the autoreloadmodule (or the `autoreload=True` option to [`tornado.web.Application`](# "tornado.web.Application")which defaults to True when `debug=True`).When using multiple processes, no IOLoops can be created orreferenced until after the call to `fork_processes`. In each child process, `fork_processes` returns its *task id*, anumber between 0 and `num_processes`. Processes that exitabnormally (due to a signal or non-zero exit status) are restartedwith the same id (up to `max_restarts` times). In the parentprocess, `fork_processes` returns None if all child processeshave exited normally, but will otherwise only exit by throwing anexception. `tornado.process.``task_id`()[[source]](#) Returns the current task id, if any. Returns None if this process was not created by [`fork_processes`](# "tornado.process.fork_processes"). *class *`tornado.process.``Subprocess`(**args*, ***kwargs*)[[source]](#) Wraps `subprocess.Popen` with IOStream support. The constructor is the same as `subprocess.Popen` with the followingadditions: - `stdin`, `stdout`, and `stderr` may have the value`tornado.process.Subprocess.STREAM`, which will make the correspondingattribute of the resulting Subprocess a [`PipeIOStream`](# "tornado.iostream.PipeIOStream"). - A new keyword argument `io_loop` may be used to pass in an IOLoop. Changed in version 4.1: The `io_loop` argument is deprecated. `set_exit_callback`(*callback*)[[source]](#) Runs `callback` when this process exits. The callback takes one argument, the return code of the process. This method uses a `SIGCHLD` handler, which is a global settingand may conflict if you have other libraries trying to handle thesame signal. If you are using more than one `IOLoop` it maybe necessary to call [`Subprocess.initialize`](# "tornado.process.Subprocess.initialize") first to designateone `IOLoop` to run the signal handlers. In many cases a close callback on the stdout or stderr streamscan be used as an alternative to an exit callback if thesignal handler is causing a problem. `wait_for_exit`(*raise_error=True*)[[source]](#) Returns a [`Future`](# "tornado.concurrent.Future") which resolves when the process exits. Usage: ~~~ ret = yield proc.wait_for_exit() ~~~ This is a coroutine-friendly alternative to [`set_exit_callback`](# "tornado.process.Subprocess.set_exit_callback")(and a replacement for the blocking [`subprocess.Popen.wait`](https://docs.python.org/3.4/library/subprocess.html#subprocess.Popen.wait "(in Python v3.4)") [https://docs.python.org/3.4/library/subprocess.html#subprocess.Popen.wait]). By default, raises [`subprocess.CalledProcessError`](https://docs.python.org/3.4/library/subprocess.html#subprocess.CalledProcessError "(in Python v3.4)") [https://docs.python.org/3.4/library/subprocess.html#subprocess.CalledProcessError] if the processhas a non-zero exit status. Use `wait_for_exit(raise_error=False)`to suppress this behavior and return the exit status without raising. New in version 4.2. *classmethod *`initialize`(*io_loop=None*)[[source]](#) Initializes the `SIGCHLD` handler. The signal handler is run on an [`IOLoop`](# "tornado.ioloop.IOLoop") to avoid locking issues.Note that the [`IOLoop`](# "tornado.ioloop.IOLoop") used for signal handling need not be thesame one used by individual Subprocess objects (as long as the`IOLoops` are each running in separate threads). Changed in version 4.1: The `io_loop` argument is deprecated. *classmethod *`uninitialize`()[[source]](#) Removes the `SIGCHLD` handler. © Copyright 2009-2016, The Tornado Authors. Created using [Sphinx](http://sphinx-doc.org/) 1.3.5.