Determinism support 1/N

[mar-yan24]

Add opt.deterministic flag with post-narrowphase contact sort (#562)

I was previously working on differentiation support for MJWarp but I am taking a break from that because the contacts are giving me a hard time. I can't seem to figure out how to optimize the gradient landscape while keeping good dynamics from rigid contact and coulombic friction. Thus, I have decided spending some time on this would be of more use for now lol.

Summary

This is one of several phased additions. This is a basic PR that just adds an opt-in opt.deterministic flag that sorts contacts after narrowphase by (worldid, geom0, geom1, geomcollisionid) using wp.utils.radix_sort_pairs. This fixes the most upstream source of run-to-run non-determinism on GPU: contact index permutation from atomic_add counters in narrowphase and CCD. After sorting, d.contact.* is rewritten in canonical order before any downstream kernel reads it.

Downstream state (qacc, qvel, qpos, constraint force, solver reductions) is not yet bitwise reproducible. Follow-ups needed, see Roadmap below.

Changes

• types.py: Option.deterministic: bool (default False). Docstring notes phase 1 scope and ~5-10% overhead.
• io.py: Wires the default in put_model, adds the field to override_model so opt.deterministic=True works from the CLI.
• collision_driver.py: _sort_contacts() runs after _narrowphase() when the flag is set. Composite 32-bit key: ((world * ngeom + geom0) * ngeom + geom1) * gcid_max + gcid. Falls back to gcid_max = 1 on int32 overflow. Three gather-permute kernels rewrite d.contact.* from temp buffers.
• determinism_test.py: 8 parameterized tests -> contact ordering, field bitwise equality across repeat runs, sort key monotonicity, default-false smoke check.

Test results

8/8 pass on RTX 4060 Laptop (sm_89, Ada Lovelace), Warp 1.13.0.dev20260302:

test_contact_ordering_deterministic[collision.xml, nworld=1]   PASSED
test_contact_ordering_deterministic[collision.xml, nworld=4]   PASSED
test_contact_ordering_deterministic[humanoid.xml, nworld=1]    PASSED
test_contact_ordering_deterministic[humanoid.xml, nworld=4]    PASSED
test_contact_fields_deterministic[collision.xml, nworld=1]     PASSED
test_contact_fields_deterministic[humanoid.xml, nworld=1]      PASSED
test_contacts_sorted_by_geom                                   PASSED
test_deterministic_flag_default_false                          PASSED

Coverage: contact geom arrays bitwise identical across 3 runs x 10 steps at two nworld sizes. All contact fields (dist, pos, frame, dim, worldid, geomcollisionid) bitwise identical. Sort key monotonicity verified. Default False confirmed (no cost unless opted in).

Benchmarks

I had claude help me formulate some benchmarks to see the potential overhead with this implementation. 3 trials x 500 steps, 50-step warmup, wp.synchronize() fences around the timing window.

Newton + Dense, RTX 4060 Laptop (sm_89)

model	nworld	nacon	off (us/step)	on (us/step)	overhead
humanoid.xml	1	7	3,459	3,903	+12.8%
humanoid.xml	64	448	3,570	4,006	+12.2%
humanoid.xml	512	3,584	3,866	4,550	+17.7%
collision.xml	1	6	4,185	4,449	+6.3%
collision.xml	64	384	4,219	4,610	+9.3%
collision.xml	512	3,072	5,258	5,829	+10.9%

CG + Sparse, RTX 4060 Laptop (sm_89)

model	nworld	nacon	off (us/step)	on (us/step)	overhead
humanoid.xml	1	7	6,445	6,858	+6.4%
humanoid.xml	64	413	7,205	7,612	+5.6%
humanoid.xml	512	3,385	7,245	7,600	+4.9%
collision.xml	1	6	4,505	4,930	+9.4%
collision.xml	64	384	5,037	5,399	+7.2%
collision.xml	512	3,072	6,342	6,833	+7.7%

All configs under 25% overhead. Worst case is +17.7% (humanoid nworld=512, Newton+Dense); actually one trial in that config hit +28.9% but had 208 ms stdev vs ~65 ms for adjacent configs. Im pretty sure that is likely thermal throttling on my crappy laptop lol.

Overhead % is roughly flat across nworld within each solver path. The bottleneck is the 17 wp.empty_like calls in _sort_contacts , not the GPU sort itself. I am planning on implementing pre-allocated scratch buffers and will fix this in a follow-up, let me know thoughts.

Roadmap

Full reproducibility obviously needs more phases:

My rough plan at the moment is to work on constraint row allocation next, this is probably what will help open up downstream effects. After that I will work on actuator moment allocation. Both of these will be done using prefix-sum.

The biggest fix later will be implementing solver reductions, i.e. cost, grad_dot, search_dot. This should make d.qacc bitwise stable and thus follows qpos and qvel as well.

This current PR does not make simulation bitwise reproducible end to end. It guarantees only that d.contact.* is stable across runs of the same input. End to end full state reproducibility will probably come after some more phases are released.

[thowell]

@mar-yan24 thank you for contributing this feature to mujoco warp!

[thowell]

lets update this to remove '(opt-in, ...' and please add a todo to update this comment when more parts of the simulation pipeline have optional deterministic results

[thowell]

@mar-yan24 fyi there is a warp draft pr for introducing determinism in warp NVIDIA/warp#1355

[erikfrey]

We just discussed this - it could be worth pursuing this approach in parallel to Warp's low-level support for determinism as they two different approaches may have different performance tradeoffs.

[mar-yan24]

@thowell, thanks for the input! Actually I haven't kept up with Warp as closely recently so I'll take a look at the PR brought up there and see if there are similar ideas compared to what I have in my current plan.

Regarding @erikfrey's comment, I don't mind working on the rest of the determinism implementation for this PR and comparing the performance once finished. I'll probably continue working on this for the week and I'll try to finish by around a week from now for the full end-to-end implementation.

Thank you both for the info/updates!

[mar-yan24]

Thanks for the review @thowell! The changes should be good to go. I am planning the next determinism steps after this PR like constraint row allocation and actuator moment allocation. Before I continue building, would you prefer that I keep extending this branch/PR so the work is all on this PR or split it up into separate requests for review. Either works for me.

[thowell]

@mar-yan24 lets create separate prs for the next deterministic features. thanks!