An elastic rod beside a robot runs far slower than the chain on the CPU

What is wrong

A rod on the elastic-rod route (rod_vbd) that a robot can touch steps in its own solver. The robot steps in MuJoCo, and the two exchange forces many times per control step. On the CPU that makes a rod-and-robot scene much slower than the same rod on the chain route, which steps inside MuJoCo with the robot.

Number

CPU, one core, warm, a 0.4 m USB cable (r 3 mm, 20 segments), alloy.Scene + alloy.build. Measured with a development API-speed script (not shipped).

scene	chain	rod_vbd
the cable dropped on the ground	0.90 s per simulated second	2.49 s
the cable's end squeezed by two 0.2 kg pads on a MuJoCo wrist	1.00 s	94.9 s (95x)

The clamped gripper rig of tests/api/test_rods_grip.py runs 82.9 s per simulated second. A rod-only solver entry runs 4 VBD iterations; at 10, on the same machine, the three rows read 4.49, 161.2 and 137.1 s (before a cached attribute lookup in the coupled model view, the squeezed cable took 216 s).

On one L4 GPU (MuJoCo Warp), in simulated s per wall s summed over worlds, at 4 iterations. Both scenes were measured in one run on one tree (2026-11-09; development scripts, not shipped); every row passed sim.check().

The squeezed cable (the CPU table's second row):

worlds	1	64	256	1,024	4,096
chain	1.2	35	105	168	172
rod_vbd	0.062	2.6	6.2	17.6	not measured
chain / rod_vbd	20x	13x	17x	10x

The clamped gripper rig of tests/api/test_rods_grip.py, the chain by clamp="start", route="chain":

worlds	64	1,024
chain	55	225
rod_vbd	2.6	17.7
chain / rod_vbd	21x	13x

So on a GPU rod_vbd beside a robot runs 10-21x slower than the chain. An earlier run of the squeezed cable read 22-55x: its rod_vbd rows were at 10 iterations (half the speed), and its chain rows read 61 and 160 at 64 and 256 worlds, on an earlier tree.

Why

The exchange step is the largest dt / 2^j no longer than 0.5 sqrt(m / k), where m is the lightest robot body that could touch the rod and k the contact stiffness (2e6 N/m). Two 0.2 kg pads give 158 us, so j = 6: 64 exchanges per 10 ms control step, each one a MuJoCo step and two coupling passes of a 4-iteration VBD solve of the rod and the robot bodies that touch it. The chain needs none of this. - On the CPU the time is in the solver's launches, not the physics: an exchange at 10 iterations was about 380 small kernel launches, about 90 us each, and MuJoCo takes 3 % of the wall. - One coupling pass instead of two, or 2 VBD iterations instead of 4, is faster but loses the grip: the cable stops following the wrist (tests/api/test_rods_grip.py).

Workaround

• Short elastic rods next to a robot route to the chain by default; keep them there unless the scene needs the elastic rod (more than 40 segments, both ends clamped, a yielding wire).
• A rod clamped at one end can stay on the chain: add_rod(..., clamp="start", route="chain"). The gripper rig then runs 1.3 s per simulated second on the CPU instead of 83 s, and holds its twist as the rod does (on the CPU a held chain creeps back about 0.14 % per second; docs/guide/rods.md, Robots).
• Give pads and links near the rod realistic masses: the clock grows with sqrt(m).
• Run elastic-rod-and-robot scenes on a GPU, many worlds at once: on one L4 the squeezed cable ran 17.6 simulated s per wall s over 1,024 worlds (the table above; docs/guide/rods.md, Routes).

Planned fix

• The per-period exchange ladder (a world re-runs only the period that needs a finer step) does not change these rows: none of their periods refined.
• A cheaper exchange. Done: a cached attribute lookup in the coupled model view (216 -> 161 s, bit for bit the same result) and 4 VBD iterations for a rod-only entry (161 -> 95 s; the grip law holds); a clamped short rod on the chain as an opt-in (Workaround). Next: fewer launches per exchange, or a coupling that holds a stiff pad contact at a coarser step.
• rod_vbd beside a robot at 4,096 worlds was not run in this pass.
• The default stays the chain for short rods until this gap closes.
• GPU free rods were named the next candidate to move to rod_vbd. Through alloy.build on an L4 the chain is faster at 1,024 worlds (178 against 148 simulated s per wall s) and rod_vbd at 4,096 (303 against 180; docs/guide/rods.md, Routes).