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Add opt-in MJX solver scan loop #3408

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Add opt-in MJX solver scan loop #3408

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1 change: 1 addition & 0 deletions mjx/mujoco/mjx/_src/io.py
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Original file line number Diff line number Diff line change
Expand Up @@ -270,6 +270,7 @@ def _put_option(
implicitfast = o.integrator == mujoco.mjtIntegrator.mjINT_IMPLICITFAST
if implicitfast and has_fluid_params:
raise NotImplementedError('implicitfast not implemented for fluid drag.')
impl_fields['solver_scan'] = bool(impl_fields.get('solver_scan', False))
impl_fields['has_fluid_params'] = has_fluid_params
return types.Option(**fields, _impl=types.OptionJAX(**impl_fields))

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4 changes: 4 additions & 0 deletions mjx/mujoco/mjx/_src/solver.py
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Expand Up @@ -598,6 +598,10 @@ def body(ctx: Context) -> Context:
ctx = Context.create(m, d)
if m.opt.iterations == 1:
ctx = body(ctx)
elif m.opt._impl.solver_scan:
# Opt-in scan-based loop for reverse-mode autodiff. The default remains
# `while_loop` to avoid changing solver performance characteristics.
ctx = _while_loop_scan(cond, body, ctx, m.opt.iterations)
else:
ctx = jax.lax.while_loop(cond, body, ctx)

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28 changes: 28 additions & 0 deletions mjx/mujoco/mjx/_src/solver_test.py
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Expand Up @@ -164,6 +164,34 @@ def test_condim(self, condim, cone):
m.opt.cone = cone
solver.solve(mjx.put_model(m), mjx.put_data(m, mujoco.MjData(m)))

def test_solver_reverse_mode_grad(self):
"""Reverse-mode autodiff works with the scan solver loop enabled."""
m = mujoco.MjModel.from_xml_string("""
<mujoco>
<worldbody>
<geom size="0 0 1e-5" type="plane" condim="1"/>
<body pos="0 0 0.09">
<freejoint/>
<geom size="0.1"/>
</body>
</worldbody>
</mujoco>
""")
# iterations > 1 exercises the outer loop that blocks reverse-mode autodiff.
m.opt.iterations = 4
m.opt.ls_iterations = 4
mx = mjx.put_model(m).tree_replace({'opt._impl.solver_scan': True})
dx = mjx.put_data(m, mujoco.MjData(m))

def loss(qvel):
d = dx.replace(qvel=qvel)
d = mjx.solve(mx, d)
return (d.qacc ** 2).sum()

grad = jax.jit(jax.grad(loss))(dx.qvel)
self.assertEqual(grad.shape, dx.qvel.shape)
self.assertTrue(np.all(np.isfinite(grad)))


if __name__ == '__main__':
absltest.main()
2 changes: 1 addition & 1 deletion mjx/mujoco/mjx/_src/types.py
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Expand Up @@ -485,6 +485,7 @@ class OptionJAX(PyTreeNode):
o_friction: jax.Array
disableactuator: int
sdf_initpoints: int
solver_scan: bool
has_fluid_params: bool


Expand Down Expand Up @@ -1213,4 +1214,3 @@ def tree_path_to_attr_str(path: jax.tree_util.KeyPath) -> str:
assert all(isinstance(p, jax.tree_util.GetAttrKey) for p in path)
path = [p for p in path if p.name != '_impl']
return '__'.join(p.name for p in path)

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