Update the mlx.launch and mlx.distributed_config docs

docs/src/usage/distributed.rst

@@ -117,6 +117,8 @@ The following examples aim to clarify the backend initialization logic in MLX:
     world_ring = mx.distributed.init(backend="ring")
     world_any = mx.distributed.init()  # same as MPI because it was initialized first!
 
+.. _training_example:
+
 Training Example
 ----------------
 
@@ -289,7 +291,7 @@ Enabling RDMA
 ^^^^^^^^^^^^^
 
 Until the feature matures, enabling RDMA over thunderbolt is slightly more
-involved and **cannot** be done remotely even with sudo. In fact it has to be
+involved and **cannot** be done remotely even with sudo. In fact, it has to be
 done in macOS recovery:
 
 1. `Start your computer in recovery <https://support.apple.com/en-us/102518>`_.
@@ -316,8 +318,8 @@ Defining a Mesh
 ^^^^^^^^^^^^^^^
 
 The JACCL backend supports only fully connected topologies. Namely, there needs
-to be a thunderbolt cable connecting all pairs of Macs directly. For example in
+to be a thunderbolt cable connecting all pairs of Macs directly. For example, in
-the following topology visualizations the left one is valid because there is a
+the following topology visualizations, the left one is valid because there is a
 connection from any node to any other node, while for the one on the right M3
 Ultra 1 is not connected to M3 Ultra 2.
 
@@ -372,7 +374,7 @@ Even though TCP/IP is not used when communicating with Thunderbolt RDMA,
 disabling the thunderbolt bridge is still required as well as setting up
 isolated local networks for each thunderbolt connection.
 
-All of the above can be done instead via ``mlx.distributed_config``. The helper
+All of the above can be done instead via ``mlx.distributed_config``. This helper
 script will
 
 - ssh into each node
@@ -382,15 +384,15 @@ script will
 - generate the hostfile to be used with ``mlx.launch``
 
 Putting it All Together
-^^^^^^^^^^^^^^^^^^^^^^
+^^^^^^^^^^^^^^^^^^^^^^^^
 
-For example to launch a distributed MLX script that uses JACCL is fairly simple
+For example launching a distributed MLX script that uses JACCL is fairly simple
 if the nodes are reachable via ssh and have password-less sudo.
 
 First, connect all the thunderbolt cables. Then we can verify the connections
 by using the ``mlx.distributed_config`` script to visualize them.
 
-.. code-block:: bash
+.. code-block::
 
    mlx.distributed_config --verbose \
         --hosts m3-ultra-1,m3-ultra-2,m3-ultra-3,m3-ultra-4 \
@@ -399,7 +401,7 @@ by using the ``mlx.distributed_config`` script to visualize them.
 After making sure that everything looks right we can auto-configure the nodes
 and save the hostfile to ``m3-ultra-jaccl.json`` by running:
 
-.. code-block:: bash
+.. code-block::
 
    mlx.distributed_config --verbose \
         --hosts m3-ultra-1,m3-ultra-2,m3-ultra-3,m3-ultra-4 \
@@ -409,7 +411,7 @@ and save the hostfile to ``m3-ultra-jaccl.json`` by running:
 And now we are ready to run a distributed MLX script such as distributed inference
 of a gigantic model using MLX-LM.
 
-.. code-block:: bash
+.. code-block::
 
    mlx.launch --verbose --backend jaccl --hostfile m3-ultra-jaccl.json \
         --env MLX_METAL_FAST_SYNCH=1 -- \  # <--- important
@@ -428,6 +430,32 @@ of a gigantic model using MLX-LM.
 Getting Started with NCCL
 -------------------------
 
+MLX on CUDA environments ships with the ability to talk to `NCCL
+<https://developer.nvidia.com/nccl>`_ which is a high-performance collective
+communication library that supports both multi-gpu and multi-node setups.
+
+For CUDA environments, NCCL is the default backend for ``mlx.launch`` and all
+it takes to run a distributed job is
+
+.. code-block::
+
+   mlx.launch -n 8 test.py
+
+   # perfect for interactive scripts
+   mlx.launch -n 8 python -m mlx_lm chat --model my-model --shard
+
+You can also use ``mlx.launch`` to ssh to a remote node and launch a script
+with the same ease
+
+.. code-block::
+
+   mlx.launch --hosts my-cuda-node -n 8 test.py
+
+In many cases you may not want to use ``mlx.launch`` with the NCCL backend
+because the cluster scheduler will be the one launching the processes. You can
+:ref:`see which environment variables need to be defined <no_mlx_launch>` in
+order for the MLX NCCL backend to be initialized correctly.
+
 .. _mpi_section:
 
 Getting Started with MPI
@@ -507,9 +535,116 @@ Force MPI to use the most performant network interface by setting ``--mca
 btl_tcp_if_include <iface>`` where ``<iface>`` should be the interface you want
 to use.
 
+.. _no_mlx_launch:
+
 Distributed Without ``mlx.launch``
 ----------------------------------
 
+None of the implementations of the distributed backends require launching with
+``mlx.launch``. The script simply connects to each host. Starts a process per
+rank and sets up the necessary environment variables before delegating to your
+MLX script. See the :doc:`dedicated documentation page <launching_distributed>`
+for more details.
 
-Using the helper scripts
+For many use-cases this will be the easiest way to perform distributed
--------------------------
+computations in MLX. However, there may be reasons that you cannot or should
+not use ``mlx.launch``. A common such case is the use of a scheduler that
+starts all the processes for you on machines undetermined at the time of
+scheduling the job.
+
+Below we list the environment variables required to use each backend.
+
+Ring
+^^^^^^
+
+**MLX_RANK** should contain a single 0-based integer that defines the rank of
+the process.
+
+**MLX_HOSTFILE** should contain the path to a json file that contains IPs and
+ports for each rank to listen to, something like the following:
+
+.. code-block:: json
+
+   [
+     ["123.123.1.1:5000", "123.123.1.2:5000"],
+     ["123.123.2.1:5000", "123.123.2.2:5000"],
+     ["123.123.3.1:5000", "123.123.3.2:5000"],
+     ["123.123.4.1:5000", "123.123.4.2:5000"]
+   ]
+
+**MLX_RING_VERBOSE** is optional and if set to 1 it enables some more logging
+from the distributed backend.
+
+JACCL
+^^^^^
+
+**MLX_RANK** should contain a single 0-based integer that defines the rank of
+the process.
+
+**MLX_JACCL_COORDINATOR** should contain the IP and port that rank 0 can listen
+to all the other ranks connect to in order to establish the RDMA connections.
+
+**MLX_IBV_DEVICES** should contain the path to a json file that contains the
+ibverbs device names that connect each node to each other node, something like
+the following:
+
+.. code-block:: json
+
+   [
+      [null, "rdma_en5", "rdma_en4", "rdma_en3"],
+      ["rdma_en5", null, "rdma_en3", "rdma_en4"],
+      ["rdma_en4", "rdma_en3", null, "rdma_en5"],
+      ["rdma_en3", "rdma_en4", "rdma_en5", null]
+   ]
+
+
+NCCL
+^^^^^
+
+**MLX_RANK** should contain a single 0-based integer that defines the rank of
+the process.
+
+**MLX_WORLD_SIZE** should contain the total number of processes that will be
+launched.
+
+**NCCL_HOST_IP** and **NCCL_PORT** should contain the IP and port that all
+hosts can connect to to establish the NCCL communication.
+
+**CUDA_VISIBLE_DEVICES** should contain the local index of the gpu that
+corresponds to this process.
+
+Of course any `other environment variable
+<https://docs.nvidia.com/deeplearning/nccl/user-guide/docs/env.html>`_ that is
+used by NCCL can be set.
+
+.. _tips_and_tricks:
+
+Tips and Tricks
+----------------
+
+This is a small collection of tips to help you utilize better the distributed
+communication capabilities of MLX.
+
+- *Test locally first.*
+
+  You can use the pattern ``mlx.launch -n2 -- my_script.py`` to run a small
+  scale test on a single node first.
+
+- *Batch your communication.*
+
+  As described in the :ref:`training example <training_example>`, performing a
+  lot of small communication can hurt performance. Copy the approach of
+  :func:`mlx.nn.average_gradients` to gather many small communications in a
+  single large one.
+
+- *Visualize the connectivity.*
+
+  Use ``mlx.distributed_config --hosts h1,h2,h3 --over thunderbolt --dot`` to
+  visualize the connnections and make sure that the cables are connected
+  correctly. See the :ref:`JACCL section <jaccl_section>` for examples.
+
+- *Use the debugger.*
+
+  ``mlx.launch`` is meant for interactive use. It broadcasts stdin to all
+  processes and gathers stdout from all processes. This makes using ``pdb`` a
+  breeze.

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
+Installing the MLX python package provides two utilities to help you configure
-can be used to run python scripts distributed on several nodes. It allows
+your Macs for distributed computation and also launch distributed programs on
-launching using either the MPI backend or the ring backend. See the
+multiple nodes or with many processes in a single node. These utilities are aptly named
-:doc:`distributed docs <distributed>` for the different backends.
 
-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``.