Erik Frey
3c0a56c1e5
PiperOrigin-RevId: 578738033 Change-Id: Id0e68c7b961380cb1ed88b40d99f202c3fa8e228
322 lines
13 KiB
ReStructuredText
322 lines
13 KiB
ReStructuredText
.. _Mjx:
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================
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MuJoCo XLA (MJX)
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================
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Starting with version 3.0.0, MuJoCo includes MuJoCo XLA (MJX) under the
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`mjx <https://github.com/google-deepmind/mujoco/tree/main/mjx>`__ directory. MJX allows MuJoCo to run on compute
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hardware supported by the `XLA <https://www.tensorflow.org/xla>`__ compiler via the
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`JAX <https://github.com/google/jax#readme>`__ framework. MJX runs on a
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`all platforms supported by JAX <https://jax.readthedocs.io/en/latest/installation.html#supported-platforms>`__: Nvidia
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and AMD GPUs, Apple Silicon, and `Google Cloud TPUs <https://cloud.google.com/tpu>`__.
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The MJX API is consistent with the main simulation functions in the MuJoCo API, although it is currently missing some
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features. While the :ref:`API documentation <Mainsimulation>` is applicable to both libraries, we indicate features
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unsupported by MJX in the :ref:`notes <MjxFeatureParity>` below.
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MJX is distributed as a separate package called ``mujoco-mjx`` on `PyPI <https://pypi.org/project/mujoco-mjx>`__.
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Although it depends on the main ``mujoco`` package for model compilation and visualization, it is a re-implementation of
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MuJoCo that uses the same algorithms as the MuJoCo implementation. However, in order to properly leverage JAX, MJX
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deliberately diverges from the MuJoCo API in a few places, see below.
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MJX is a successor to the `generalized physics pipeline <https://github.com/google/brax/tree/main/brax/generalized>`__
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in Google's `Brax <https://github.com/google/brax>`__ physics and reinforcement learning library. MJX was built
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by core contributors to both MuJoCo and Brax, who will together continue to support both Brax (for its reinforcement
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learning algorithms and included environments) and MJX (for its physics algorithms). A future version of Brax will
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depend on the ``mujoco-mjx`` package, and Brax's existing
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`generalized pipeline <https://github.com/google/brax/tree/main/brax/generalized>`__ will be deprecated. This change
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will be largely transparent to users of Brax.
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.. _MjxNotebook:
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Tutorial notebook
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=================
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The following IPython notebook demonstrates the use of MJX along with reinforcement learning to train humanoid and
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quadruped robots to locomote: |colab|.
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.. |colab| image:: https://colab.research.google.com/assets/colab-badge.svg
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:target: https://colab.research.google.com/github/google-deepmind/mujoco/blob/main/mjx/tutorial.ipynb
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.. _MjxInstallation:
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Installation
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============
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The recommended way to install this package is via `PyPI <https://pypi.org/project/mujoco-mjx/>`__:
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.. code-block:: shell
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pip install mujoco-mjx
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A copy of the MuJoCo library is provided as part of this package's depdendencies and does **not** need to be downloaded
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or installed separately.
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.. _MjxUsage:
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Basic usage
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===========
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Once installed, the package can be imported via ``from mujoco import mjx``. Structs, functions, and enums are available
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directly from the top-level ``mjx`` module.
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.. _MjxStructs:
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Structs
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-------
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Before running MJX functions on an accelerator device, structs must be copied onto the device via the ``mjx.device_put``
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function. Placing an :ref:`mjModel` on device yields an ``mjx.Model``. Placing an :ref:`mjData` on device yields
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an ``mjx.Data``:
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.. code-block:: python
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model = mujoco.MjModel.from_xml_string("...")
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data = mujoco.MjData(model)
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mjx_model = mjx.device_put(model)
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mjx_data = mjx.device_put(data)
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These MJX variants mirror their MuJoCo counterparts but have three key differences:
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#. Fields in ``mjx.Model`` and ``mjx.Data`` are JAX arrays copied onto device, instead of numpy arrays.
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#. Some fields are missing from ``mjx.Model`` and ``mjx.Data`` for features that are
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:ref:`unsupported <mjxFeatureParity>` in MJX.
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#. Arrays in ``mjx.Model`` and ``mjx.Data`` support adding batch dimensions. Batch dimensions are a natural way to
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express domain randomization (in the case of ``mjx.Model``) or high-throughput simulation for reinforcement learning
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(in the case of ``mjx.Data``).
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Neither ``mjx.Model`` nor ``mjx.Data`` are meant to be constructed manually. An ``mjx.Data`` may be created by calling
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``mjx.make_data``, which mirrors the :ref:`mj_makeData` function in MuJoCo:
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.. code-block:: python
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model = mujoco.MjModel.from_xml_string("...")
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mjx_model = mjx.device_put(model)
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mjx_data = mjx.make_data(model)
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Using ``mjx.make_data`` may be preferable when constructing batched ``mjx.Data`` structures inside of a ``vmap``.
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.. _MjxFunctions:
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Functions
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---------
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MuJoCo functions are exposed as MJX functions of the same name, but following
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`PEP 8 <https://peps.python.org/pep-0008/>`__-compliant names. Most of the :ref:`main simulation <Mainsimulation>` and
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some of the :ref:`sub-components <Subcomponents>` for forward simulation are available from the top-level ``mjx`` module.
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MJX functions are not `JIT compiled <https://jax.readthedocs.io/en/latest/jax-101/02-jitting.html>`__ by default -- we
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leave it to the user to JIT MJX functions, or JIT their own functions that reference MJX functions. See the
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:ref:`minimal example <MjxExample>` below.
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.. _MjxEnums:
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Enums and constants
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-------------------
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MJX enums are available as ``mjx.EnumType.ENUM_VALUE``, for example ``mjx.JointType.FREE``. Enums for unsupported MJX
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features are omitted from the MJX enum declaration. MJX declares no constants but references MuJoCo constants directly.
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.. _MjxExample:
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Minimal example
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---------------
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.. code-block:: python
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# Throw a ball at 100 different velocities.
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import jax
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import mujoco
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from mujoco import mjx
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XML=r"""
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<mujoco>
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<worldbody>
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<body>
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<freejoint/>
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<geom size=".15" mass="1" type="sphere"/>
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</body>
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</worldbody>
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</mujoco>
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"""
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model = mujoco.MjModel.from_xml_string(XML)
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mjx_model = mjx.device_put(model)
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@jax.vmap
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def batched_step(vel):
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mjx_data = mjx.make_data(mjx_model)
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qvel = mjx_data.qvel.at[0].set(vel)
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mjx_data = mjx_data.replace(qvel=qvel)
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pos = mjx.step(mjx_model, mjx_data).qpos[0]
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return pos
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vel = jax.numpy.arange(0.0, 1.0, 0.01)
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pos = jax.jit(batched_step)(vel)
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print(pos)
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.. _MjxFeatureParity:
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Feature Parity
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==============
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MJX supports most of the main simulation features of MuJoCo, with a few exceptions. MJX will raise an exception if
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asked to copy to device an :ref:`mjModel` with field values referencing unsupported features.
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The following features are **fully supported** in MJX:
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.. list-table::
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:width: 90%
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:align: left
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:widths: 1 5
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:header-rows: 1
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* - Category
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- Feature
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* - Dynamics
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- :ref:`Forward <mj_forward>`
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* - :ref:`Joint <mjtJoint>`
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- ``FREE``, ``BALL``, ``SLIDE``, ``HINGE``
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* - :ref:`Transmission <mjtTrn>`
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- ``TRN_JOINT``
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* - :ref:`Actuation <geactuation>`
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- ``DYN_NONE``, ``DYN_INTEGRATOR``, ``DYN_FILTER``, ``GAIN_FIXED``, ``GAIN_AFFINE``, ``BIAS_NONE``,
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``BIAS_AFFINE``
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* - :ref:`Geom <mjtGeom>`
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- ``PLANE``, ``SPHERE``, ``CAPSULE``, ``BOX``, ``MESH``
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* - :ref:`Constraint <mjtConstraint>`
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- ``EQUALITY``, ``LIMIT_JOINT``, ``CONTACT_PYRAMIDAL``
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* - :ref:`Equality <mjtEq>`
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- ``CONNECT``, ``WELD``, ``JOINT``
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* - :ref:`Integrator <mjtIntegrator>`
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- ``EULER``, ``RK4``
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* - :ref:`Cone <mjtCone>`
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- ``PYRAMIDAL``
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* - :ref:`Condim <coContact>`
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- 3
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* - :ref:`Solver <mjtSolver>`
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- ``CG``, ``NEWTON``
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* - Fluid Model
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- :ref:`flInertia`
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The following features are **in development** and coming soon:
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.. list-table::
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:width: 90%
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:align: left
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:widths: 1 5
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:header-rows: 1
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* - Category
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- Feature
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* - Dynamics
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- :ref:`Inverse <mj_inverse>`
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* - :ref:`Transmission <mjtTrn>`
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- ``TRN_TENDON``
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* - :ref:`Geom <mjtGeom>`
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- ``HFIELD``, ``ELLIPSOID``, ``CYLINDER``, ``SDF``
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* - :ref:`Constraint <mjtConstraint>`
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- ``CONTACT_FRICTIONLESS``, ``CONTACT_ELLIPTIC``, ``FRICTION_DOF``
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* - :ref:`Integrator <mjtIntegrator>`
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- ``IMPLICIT``, ``IMPLICITFAST``
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* - :ref:`Cone <mjtCone>`
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- ``ELLIPTIC``
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* - :ref:`Condim <coContact>`
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- 1, 4, 6
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* - Fluid Model
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- :ref:`flEllipsoid`
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* - :ref:`Tendons <tendon>`
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- :ref:`Spatial <tendon-spatial>`, :ref:`Fixed <tendon-fixed>`
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* - :ref:`Equality <mjtEq>`
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- ``TENDON``
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* - :ref:`Sensors <mjtSensor>`
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- All except ``mjSENS_PLUGIN``, ``mjSENS_USER``
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The following features are **unsupported**:
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.. list-table::
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:width: 90%
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:align: left
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:widths: 1 5
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:header-rows: 1
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* - Category
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- Feature
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* - :ref:`Transmission <mjtTrn>`
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- ``TRN_JOINTINPARENT``, ``TRN_SLIDERCRANK``, ``TRN_SITE``, ``TRN_BODY``, ``MUSCLE``
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* - :ref:`Solver <mjtSolver>`
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- ``PGS``
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* - :ref:`Callbacks <glphysics>`
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- ``mjDYN_USER``, ``mjGAIN_USER``, ``mjBIAS_USER``, ``mjSENS_USER``
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.. _MjxSharpBits:
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🔪 MJX - The Sharp Bits 🔪
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==========================
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GPUs and TPUs have unique performance tradeoffs that MJX is subject to. MJX specializes in simulating big batches of
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parallel identical physics scenes using algorithms that can be efficiently vectorized on
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`SIMD hardware <https://en.wikipedia.org/wiki/Single_instruction,_multiple_data>`__. This specialization is useful
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for machine learning workloads such as `reinforcement learning <https://en.wikipedia.org/wiki/Reinforcement_learning>`__
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that require massive data throughput.
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There are certain workflows that MJX is ill-suited for:
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Single scene simulation
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Simulating a single scene (1 instance of :ref:`mjData`), MJX can be **10x** slower than MuJoCo, which has been
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carefully optimized for CPU. MJX works best when simulating thousands or tens of thousands of scenes in parallel.
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Large, complex scenes with many contacts
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Accelerators exhibit poor performance for
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`branching code <https://aschrein.github.io/jekyll/update/2019/06/13/whatsup-with-my-branches-on-gpu.html#tldr>`__.
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Branching is used in broad-phase collision detection, when identifying potential collisions between large numbers of
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bodies in a scene. MJX ships with a simple branchless broad-phase algorithm (see performance tuning) but it is not as
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powerful as the one in MuJoCo.
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To see how this affects simulation, let us consider a physics scene with increasing numbers of physics bodies. We
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simulate a scene with a variable number of humanoids (from 1 to 10) and then compare MJX's performance on an Nvidia
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A100 GPU to MuJoCo on a 12-core workstation:
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.. figure:: images/mjx/mujoco_vs_mjx_large_scene.png
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:width: 658px
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:align: center
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Notice that as we increase the number of humanoids (which increases the number of potential contacts in a scene), MJX
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performance degrades more rapidly than MuJoCo. At the limit, for such a large scene, MuJoCo performance nearly
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matches MJX.
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Scenes with collisions between meshes with many vertices
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MJX supports mesh geometries and can determine if two meshes are colliding using branchless versions of
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`mesh collision algorithms <https://ubm-twvideo01.s3.amazonaws.com/o1/vault/gdc2013/slides/822403Gregorius_Dirk_TheSeparatingAxisTest.pdf>`__.
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These algorithms work well for smaller meshes (with hundreds of vertices) but suffer with large meshes. With careful
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tuning, MJX can simulate scenes with mesh collisions well -- see the MJX
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`shadow hand <https://github.com/google-deepmind/mujoco/tree/main/mjx/mujoco/mjx/benchmark/model/shadow_hand>`__
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config for an example.
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.. _MjxPerformance:
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Performance tuning
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==================
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For MJX to perform well, some configuration parameters should be adjusted from their default MuJoCo values:
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:ref:`option` element
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The ``iterations`` and ``ls_iterations`` attributes---which control solver and linesearch iterations, respectively---
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should be brought down to just low enough that the simulation remains stable. Accurate solver forces are not so
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important in reinforcement learning in which domain randomization is often used to add noise to physics for sim-to-real.
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The ``NEWTON`` :ref:`Solver <mjtSolver>` often delivers reasonable convergence with one solver iteration, and performs
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well on GPU. ``CG`` is currently a better choice for TPU.
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:ref:`contact-pair` element
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Consider explicitly marking geoms for collision detection to reduce the number of contacts that MJX must consider
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during each step. Enabling only an explicit list of valid contacts can have a dramatic effect on simulation
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performance in MJX. Doing this well often requires an understanding of the task -- for example, the
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`OpenAI Gym Humanoid <https://github.com/openai/gym/blob/master/gym/envs/mujoco/humanoid_v4.py>`__ task resets when
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the humanoid starts to fall, so full contact with the floor is not needed.
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:ref:`option-flag` element
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Disabling ``eulerdamp`` can help performance and is often not needed for stability.
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