.. _parallel_process: =========================================== Starting the IPython controller and engines =========================================== To use IPython for parallel computing, you need to start one instance of the controller and one or more instances of the engine. The controller and each engine can run on different machines or on the same machine. Because of this, there are many different possibilities. Broadly speaking, there are two ways of going about starting a controller and engines: * In an automated manner using the :command:`ipcluster` command. * In a more manual way using the :command:`ipcontroller` and :command:`ipengine` commands. This document describes both of these methods. We recommend that new users start with the :command:`ipcluster` command as it simplifies many common usage cases. General considerations ====================== Before delving into the details about how you can start a controller and engines using the various methods, we outline some of the general issues that come up when starting the controller and engines. These things come up no matter which method you use to start your IPython cluster. Let's say that you want to start the controller on ``host0`` and engines on hosts ``host1``-``hostn``. The following steps are then required: 1. Start the controller on ``host0`` by running :command:`ipcontroller` on ``host0``. 2. Move the JSON file (:file:`ipcontroller-engine.json`) created by the controller from ``host0`` to hosts ``host1``-``hostn``. 3. Start the engines on hosts ``host1``-``hostn`` by running :command:`ipengine`. This command has to be told where the JSON file (:file:`ipcontroller-engine.json`) is located. At this point, the controller and engines will be connected. By default, the JSON files created by the controller are put into the :file:`~/.ipython/profile_default/security` directory. If the engines share a filesystem with the controller, step 2 can be skipped as the engines will automatically look at that location. The final step required to actually use the running controller from a client is to move the JSON file :file:`ipcontroller-client.json` from ``host0`` to any host where clients will be run. If these file are put into the :file:`~/.ipython/profile_default/security` directory of the client's host, they will be found automatically. Otherwise, the full path to them has to be passed to the client's constructor. Using :command:`ipcluster` =========================== The :command:`ipcluster` command provides a simple way of starting a controller and engines in the following situations: 1. When the controller and engines are all run on localhost. This is useful for testing or running on a multicore computer. 2. When engines are started using the :command:`mpiexec` command that comes with most MPI [MPI]_ implementations 3. When engines are started using the PBS [PBS]_ batch system (or other `qsub` systems, such as SGE). 4. When the controller is started on localhost and the engines are started on remote nodes using :command:`ssh`. 5. When engines are started using the Windows HPC Server batch system. .. note:: Currently :command:`ipcluster` requires that the :file:`~/.ipython/profile_/security` directory live on a shared filesystem that is seen by both the controller and engines. If you don't have a shared file system you will need to use :command:`ipcontroller` and :command:`ipengine` directly. Under the hood, :command:`ipcluster` just uses :command:`ipcontroller` and :command:`ipengine` to perform the steps described above. The simplest way to use ipcluster requires no configuration, and will launch a controller and a number of engines on the local machine. For instance, to start one controller and 4 engines on localhost, just do:: $ ipcluster start n=4 To see other command line options, do:: $ ipcluster -h Configuring an IPython cluster ============================== Cluster configurations are stored as `profiles`. You can create a new profile with:: $ ipython profile create --parallel profile=myprofile This will create the directory :file:`IPYTHONDIR/profile_myprofile`, and populate it with the default configuration files for the three IPython cluster commands. Once you edit those files, you can continue to call ipcluster/ipcontroller/ipengine with no arguments beyond ``profile=myprofile``, and any configuration will be maintained. There is no limit to the number of profiles you can have, so you can maintain a profile for each of your common use cases. The default profile will be used whenever the profile argument is not specified, so edit :file:`IPYTHONDIR/profile_default/*_config.py` to represent your most common use case. The configuration files are loaded with commented-out settings and explanations, which should cover most of the available possibilities. Using various batch systems with :command:`ipcluster` ------------------------------------------------------ :command:`ipcluster` has a notion of Launchers that can start controllers and engines with various remote execution schemes. Currently supported models include :command:`ssh`, :command:`mpiexec`, PBS-style (Torque, SGE), and Windows HPC Server. .. note:: The Launchers and configuration are designed in such a way that advanced users can subclass and configure them to fit their own system that we have not yet supported (such as Condor) Using :command:`ipcluster` in mpiexec/mpirun mode -------------------------------------------------- The mpiexec/mpirun mode is useful if you: 1. Have MPI installed. 2. Your systems are configured to use the :command:`mpiexec` or :command:`mpirun` commands to start MPI processes. If these are satisfied, you can create a new profile:: $ ipython profile create --parallel profile=mpi and edit the file :file:`IPYTHONDIR/profile_mpi/ipcluster_config.py`. There, instruct ipcluster to use the MPIExec launchers by adding the lines: .. sourcecode:: python c.IPClusterEngines.engine_launcher = 'IPython.parallel.apps.launcher.MPIExecEngineSetLauncher' If the default MPI configuration is correct, then you can now start your cluster, with:: $ ipcluster start n=4 profile=mpi This does the following: 1. Starts the IPython controller on current host. 2. Uses :command:`mpiexec` to start 4 engines. If you have a reason to also start the Controller with mpi, you can specify: .. sourcecode:: python c.IPClusterStart.controller_launcher = 'IPython.parallel.apps.launcher.MPIExecControllerLauncher' .. note:: The Controller *will not* be in the same MPI universe as the engines, so there is not much reason to do this unless sysadmins demand it. On newer MPI implementations (such as OpenMPI), this will work even if you don't make any calls to MPI or call :func:`MPI_Init`. However, older MPI implementations actually require each process to call :func:`MPI_Init` upon starting. The easiest way of having this done is to install the mpi4py [mpi4py]_ package and then specify the ``c.MPI.use`` option in :file:`ipengine_config.py`: .. sourcecode:: python c.MPI.use = 'mpi4py' Unfortunately, even this won't work for some MPI implementations. If you are having problems with this, you will likely have to use a custom Python executable that itself calls :func:`MPI_Init` at the appropriate time. Fortunately, mpi4py comes with such a custom Python executable that is easy to install and use. However, this custom Python executable approach will not work with :command:`ipcluster` currently. More details on using MPI with IPython can be found :ref:`here `. Using :command:`ipcluster` in PBS mode --------------------------------------- The PBS mode uses the Portable Batch System [PBS]_ to start the engines. As usual, we will start by creating a fresh profile:: $ ipython profile create --parallel profile=pbs And in :file:`ipcluster_config.py`, we will select the PBS launchers for the controller and engines: .. sourcecode:: python c.IPClusterStart.controller_launcher = \ 'IPython.parallel.apps.launcher.PBSControllerLauncher' c.IPClusterEngines.engine_launcher = \ 'IPython.parallel.apps.launcher.PBSEngineSetLauncher' IPython does provide simple default batch templates for PBS and SGE, but you may need to specify your own. Here is a sample PBS script template: .. sourcecode:: bash #PBS -N ipython #PBS -j oe #PBS -l walltime=00:10:00 #PBS -l nodes={n/4}:ppn=4 #PBS -q {queue} cd $PBS_O_WORKDIR export PATH=$HOME/usr/local/bin export PYTHONPATH=$HOME/usr/local/lib/python2.7/site-packages /usr/local/bin/mpiexec -n {n} ipengine profile_dir={profile_dir} There are a few important points about this template: 1. This template will be rendered at runtime using IPython's :class:`EvalFormatter`. This is simply a subclass of :class:`string.Formatter` that allows simple expressions on keys. 2. Instead of putting in the actual number of engines, use the notation ``{n}`` to indicate the number of engines to be started. You can also use expressions like ``{n/4}`` in the template to indicate the number of nodes. There will always be ``{n}`` and ``{profile_dir}`` variables passed to the formatter. These allow the batch system to know how many engines, and where the configuration files reside. The same is true for the batch queue, with the template variable ``{queue}``. 3. Any options to :command:`ipengine` can be given in the batch script template, or in :file:`ipengine_config.py`. 4. Depending on the configuration of you system, you may have to set environment variables in the script template. The controller template should be similar, but simpler: .. sourcecode:: bash #PBS -N ipython #PBS -j oe #PBS -l walltime=00:10:00 #PBS -l nodes=1:ppn=4 #PBS -q {queue} cd $PBS_O_WORKDIR export PATH=$HOME/usr/local/bin export PYTHONPATH=$HOME/usr/local/lib/python2.7/site-packages ipcontroller profile_dir={profile_dir} Once you have created these scripts, save them with names like :file:`pbs.engine.template`. Now you can load them into the :file:`ipcluster_config` with: .. sourcecode:: python c.PBSEngineSetLauncher.batch_template_file = "pbs.engine.template" c.PBSControllerLauncher.batch_template_file = "pbs.controller.template" Alternately, you can just define the templates as strings inside :file:`ipcluster_config`. Whether you are using your own templates or our defaults, the extra configurables available are the number of engines to launch (``{n}``, and the batch system queue to which the jobs are to be submitted (``{queue}``)). These are configurables, and can be specified in :file:`ipcluster_config`: .. sourcecode:: python c.PBSLauncher.queue = 'veryshort.q' c.IPClusterEngines.n = 64 Note that assuming you are running PBS on a multi-node cluster, the Controller's default behavior of listening only on localhost is likely too restrictive. In this case, also assuming the nodes are safely behind a firewall, you can simply instruct the Controller to listen for connections on all its interfaces, by adding in :file:`ipcontroller_config`: .. sourcecode:: python c.RegistrationFactory.ip = '*' You can now run the cluster with:: $ ipcluster start profile=pbs n=128 Additional configuration options can be found in the PBS section of :file:`ipcluster_config`. .. note:: Due to the flexibility of configuration, the PBS launchers work with simple changes to the template for other :command:`qsub`-using systems, such as Sun Grid Engine, and with further configuration in similar batch systems like Condor. Using :command:`ipcluster` in SSH mode --------------------------------------- The SSH mode uses :command:`ssh` to execute :command:`ipengine` on remote nodes and :command:`ipcontroller` can be run remotely as well, or on localhost. .. note:: When using this mode it highly recommended that you have set up SSH keys and are using ssh-agent [SSH]_ for password-less logins. As usual, we start by creating a clean profile:: $ ipython profile create --parallel profile=ssh To use this mode, select the SSH launchers in :file:`ipcluster_config.py`: .. sourcecode:: python c.IPClusterEngines.engine_launcher = \ 'IPython.parallel.apps.launcher.SSHEngineSetLauncher' # and if the Controller is also to be remote: c.IPClusterStart.controller_launcher = \ 'IPython.parallel.apps.launcher.SSHControllerLauncher' The controller's remote location and configuration can be specified: .. sourcecode:: python # Set the user and hostname for the controller # c.SSHControllerLauncher.hostname = 'controller.example.com' # c.SSHControllerLauncher.user = os.environ.get('USER','username') # Set the arguments to be passed to ipcontroller # note that remotely launched ipcontroller will not get the contents of # the local ipcontroller_config.py unless it resides on the *remote host* # in the location specified by the `profile_dir` argument. # c.SSHControllerLauncher.program_args = ['--reuse', 'ip=0.0.0.0', 'profile_dir=/path/to/cd'] .. note:: SSH mode does not do any file movement, so you will need to distribute configuration files manually. To aid in this, the `reuse_files` flag defaults to True for ssh-launched Controllers, so you will only need to do this once, unless you override this flag back to False. Engines are specified in a dictionary, by hostname and the number of engines to be run on that host. .. sourcecode:: python c.SSHEngineSetLauncher.engines = { 'host1.example.com' : 2, 'host2.example.com' : 5, 'host3.example.com' : (1, ['profile_dir=/home/different/location']), 'host4.example.com' : 8 } * The `engines` dict, where the keys are the host we want to run engines on and the value is the number of engines to run on that host. * on host3, the value is a tuple, where the number of engines is first, and the arguments to be passed to :command:`ipengine` are the second element. For engines without explicitly specified arguments, the default arguments are set in a single location: .. sourcecode:: python c.SSHEngineSetLauncher.engine_args = ['profile_dir=/path/to/profile_ssh'] Current limitations of the SSH mode of :command:`ipcluster` are: * Untested on Windows. Would require a working :command:`ssh` on Windows. Also, we are using shell scripts to setup and execute commands on remote hosts. * No file movement - Using the :command:`ipcontroller` and :command:`ipengine` commands ==================================================================== It is also possible to use the :command:`ipcontroller` and :command:`ipengine` commands to start your controller and engines. This approach gives you full control over all aspects of the startup process. Starting the controller and engine on your local machine -------------------------------------------------------- To use :command:`ipcontroller` and :command:`ipengine` to start things on your local machine, do the following. First start the controller:: $ ipcontroller Next, start however many instances of the engine you want using (repeatedly) the command:: $ ipengine The engines should start and automatically connect to the controller using the JSON files in :file:`~/.ipython/profile_default/security`. You are now ready to use the controller and engines from IPython. .. warning:: The order of the above operations may be important. You *must* start the controller before the engines, unless you are reusing connection information (via `--reuse`), in which case ordering is not important. .. note:: On some platforms (OS X), to put the controller and engine into the background you may need to give these commands in the form ``(ipcontroller &)`` and ``(ipengine &)`` (with the parentheses) for them to work properly. Starting the controller and engines on different hosts ------------------------------------------------------ When the controller and engines are running on different hosts, things are slightly more complicated, but the underlying ideas are the same: 1. Start the controller on a host using :command:`ipcontroller`. 2. Copy :file:`ipcontroller-engine.json` from :file:`~/.ipython/profile_/security` on the controller's host to the host where the engines will run. 3. Use :command:`ipengine` on the engine's hosts to start the engines. The only thing you have to be careful of is to tell :command:`ipengine` where the :file:`ipcontroller-engine.json` file is located. There are two ways you can do this: * Put :file:`ipcontroller-engine.json` in the :file:`~/.ipython/profile_/security` directory on the engine's host, where it will be found automatically. * Call :command:`ipengine` with the ``file=full_path_to_the_file`` flag. The ``file`` flag works like this:: $ ipengine file=/path/to/my/ipcontroller-engine.json .. note:: If the controller's and engine's hosts all have a shared file system (:file:`~/.ipython/profile_/security` is the same on all of them), then things will just work! Make JSON files persistent -------------------------- At fist glance it may seem that that managing the JSON files is a bit annoying. Going back to the house and key analogy, copying the JSON around each time you start the controller is like having to make a new key every time you want to unlock the door and enter your house. As with your house, you want to be able to create the key (or JSON file) once, and then simply use it at any point in the future. To do this, the only thing you have to do is specify the `--reuse` flag, so that the connection information in the JSON files remains accurate:: $ ipcontroller --reuse Then, just copy the JSON files over the first time and you are set. You can start and stop the controller and engines any many times as you want in the future, just make sure to tell the controller to reuse the file. .. note:: You may ask the question: what ports does the controller listen on if you don't tell is to use specific ones? The default is to use high random port numbers. We do this for two reasons: i) to increase security through obscurity and ii) to multiple controllers on a given host to start and automatically use different ports. Log files --------- All of the components of IPython have log files associated with them. These log files can be extremely useful in debugging problems with IPython and can be found in the directory :file:`~/.ipython/profile_/log`. Sending the log files to us will often help us to debug any problems. Configuring `ipcontroller` --------------------------- Ports and addresses ******************* Database Backend **************** .. seealso:: Configuring `ipengine` ----------------------- .. note:: TODO .. [PBS] Portable Batch System. http://www.openpbs.org/ .. [SSH] SSH-Agent http://en.wikipedia.org/wiki/ssh-agent