Update the mlx.launch and mlx.distributed_config docs

docs/src/usage/launching_distributed.rst

@@ -7,13 +7,106 @@ Launching Distributed Programs
 
 .. currentmodule:: mlx.core.distributed
 
-Installing the MLX python package provides a helper script ``mlx.launch`` that
-can be used to run python scripts distributed on several nodes. It allows
-launching using either the MPI backend or the ring backend. See the
-:doc:`distributed docs <distributed>` for the different backends.
+Installing the MLX python package provides two utilities to help you configure
+your Macs for distributed computation and also launch distributed programs on
+multiple nodes or with many processes in a single node. These utilities are aptly named
 
-Usage
------
+- ``mlx.launch``
+- ``mlx.distributed_config``
+
+See the :doc:`distributed docs <distributed>` for an introduction and
+getting-started guides to the various backends.
+
+``mlx.distributed_config`` 
+---------------------------
+
+Unless you are launching distributed jobs locally for development or multi-gpu
+CUDA environments, then you have several Macs that you need to configure for
+distributed communication with MLX.
+
+``mlx.distributed_config`` aims to automate the process of configuring the
+network interfaces (especially for communication over thunderbolt) and also
+creating the hostfile to be used with ``mlx.launch``.
+
+We will analyse 3 cases of using ``mlx.distributed_config``
+
+1. RDMA over thunderbolt using JACCL
+2. TCP/IP over thunderbolt using the ring backend
+3. TCP/IP over ethernet using the ring backend
+
+JACCL
+^^^^^^^
+
+After following :ref:`the steps to enable RDMA <jaccl_section>` you can run the
+following command to configure the nodes and create the hostfile.
+
+.. code-block::
+
+   mlx.distributed_config --verbose --backend jaccl \
+        --hosts m3-ultra-1,m3-ultra-2,m3-ultra-3,m3-ultra-4 --over thunderbolt \
+        --auto-setup --output m3-ultra-jaccl.json
+
+Let's walk through the steps that the script takes to configure the nodes.
+
+1. Ssh to all nodes to verify that they are reachable
+2. Extract the thunderbolt connectivity. Namely run commands on each node to
+   calculate which node is connected to which other node.
+3. Verify that we have a valid fully connected mesh
+4. Check that RDMA is enabled
+5. Extract the ethernet IP from interface en0
+6. Disable the thunderbolt bridge and set up peer to peer networks for each
+   thunderbolt cable
+7. Write the hostfile
+
+Knowing the above steps allows you to manually configure the nodes but also
+debug any configuration issue. For instance changing the Ethernet IP to a
+different interface directly in the config is possible (as long as it is
+reachable from all nodes).
+
+The ``--auto-setup`` argument requires password-less sudo on each node. If it
+isn't available then the configuration script will print commands to be run on
+each node.
+
+Ring over thunderbolt
+^^^^^^^^^^^^^^^^^^^^^
+
+Setting up a ring backend over thunderbolt only requires changing the
+``--backend`` from ``jaccl`` to ``ring``.
+
+The steps are very similar with the main difference being that instead of
+verifying that the nodes are fully connected, the script attempts to identify a
+ring topology (or multiple rings).
+
+Ring over Ethernet
+^^^^^^^^^^^^^^^^^^
+
+Configuring the ring backend over ethernet doesn't require setting up network
+interface and as such it simply extracts the ``en0`` IP from each node and
+writes the hostfile.
+
+Debugging cable connections
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+``mlx.distributed_config`` can help you debug the connectivity of your nodes
+over thunderbolt by exporting a graph of the connections.
+
+Running
+
+.. code-block::
+
+   mlx.distributed_config --verbose \
+        --hosts host1,host2,host3,host4 \
+        --over thunderbolt --dot
+
+will export a `GraphViz <https://graphviz.org>`_ representation of the
+connections between the nodes which makes it very easy to figure out which
+cable is not connected correctly.
+
+See :ref:`the JACCL section <jaccl_section>` for an example.
+
+
+``mlx.launch``
+--------------
 
 The minimal usage example of ``mlx.launch`` is simply
 
@@ -33,6 +126,10 @@ the rest if one of them fails unexpectedly or if ``mlx.launch`` is terminated.
 It also takes care of forwarding the output of each remote process to stdout
 and stderr respectively.
 
+Importantly, it also broadcasts stdin to each process which enables interactive
+programs to work in distributed mode as well as debugging using the interactive
+debugger.
+
 Providing Hosts
 ^^^^^^^^^^^^^^^^
 
@@ -63,10 +160,62 @@ host and on the same path. A good checklist to debug errors is the following:
   ``mlx.launch --print-python`` to see what that path is.
 * the script you want to run is available on all hosts at the same path
 
+If you are launching from a node with a completely different setup than the
+nodes that the program will run on, you can specify ``--no-verify-script`` so
+that ``mlx.launch`` does not attempt to verify that the executable and script
+exist locally before launching the distributed job.
+
+.. _ring_specifics:
+
+Ring Specifics
+^^^^^^^^^^^^^^
+
+The :ref:`ring <ring_section>` backend, which is also the default
+backend, can be explicitly selected with the argument ``--backend ring``. The
+ring backend has some specific requirements and arguments that are different to
+other backends:
+
+* The argument ``--hosts`` only accepts IPs and not hostnames. If we need to
+  ssh to a hostname that does not correspond to the IP we want to bind to we
+  have to provide a hostfile.
+* ``--starting-port`` defines the port to bind to on the remote hosts.
+  Specifically rank 0 for the first IP will use this port and each subsequent
+  IP or rank will add 1 to this port.
+* ``--connections-per-ip`` allows us to increase the number of connections
+  between neighboring nodes. This corresponds to ``--mca btl_tcp_links 2`` for
+  ``mpirun``.
+
+.. _jaccl_specifics:
+
+JACCL Specifics
+^^^^^^^^^^^^^^^^
+
+The :ref:`JACCL <jaccl_section>` backend can be selected with the argument
+``--backend jaccl``. A hostfile is necessary to launch with this backend
+because it needs to contain the RDMA devices connecting each node to each other
+node.
+
+NCCL Specifics
+^^^^^^^^^^^^^^
+
+The :ref:`NCCL <nccl_section>` backend is the default backend for CUDA
+environments. When launching from a Mac to a Linux machine with CUDA then the
+backend should be selected using ``--backend nccl``.
+
+The ``--repeat-hosts, -n`` argument should be used to launch multi-node and
+multi-gpu jobs. For instance
+
+.. code-block::
+
+   mlx.launch --backend nccl --hosts linux-1,linux-2 -n 8 --no-verify-script -- ./my-job.sh
+
+will attempt to launch 16 processes, 8 on each node that will all run
+``my-job.sh``.
+
 .. _mpi_specifics:
 
 MPI Specifics
--------------
+^^^^^^^^^^^^^
 
 One can use MPI by passing ``--backend mpi`` to ``mlx.launch``. In that case,
 ``mlx.launch`` is a thin wrapper over ``mpirun``. Moreover,
@@ -83,23 +232,3 @@ to choose a specific interface for the byte-transfer-layer of MPI we can call
 .. code:: shell
 
     mlx.launch --backend mpi --mpi-arg '--mca btl_tcp_if_include en0' --hostfile hosts.json my_script.py
-
-
-.. _ring_specifics:
-
-Ring Specifics
---------------
-
-The ring backend, which is also the default backend, can be explicitly selected
-with the argument ``--backend ring``. The ring backend has some specific
-requirements and arguments that are different to MPI:
-
-* The argument ``--hosts`` only accepts IPs and not hostnames. If we need to
-  ssh to a hostname that does not correspond to the IP we want to bind to we
-  have to provide a hostfile.
-* ``--starting-port`` defines the port to bind to on the remote hosts.
-  Specifically rank 0 for the first IP will use this port and each subsequent
-  IP or rank will add 1 to this port.
-* ``--connections-per-ip`` allows us to increase the number of connections
-  between neighboring nodes. This corresponds to ``--mca btl_tcp_links 2`` for
-  ``mpirun``.