Improved MuJoCo Simulation Scaling about mujoco HOT 1 OPEN

Hi @bd-mdelasa,

First a meta-comment: this sort of detailed request from valued users is exactly what we need in order to correctly prioritize our roadmap.

Addressing your high-level question

Are there any efforts in the area of scaling MuJoCo to larger scenes?

The answer is a resounding "yes", as follows.

1. At the compiler level, we are in late stages of designing a programmatic API to manipulate models. You can inspect the (unfinished, currently private) API here. The gist of the API is the introduction of a new high-level struct called mjSpec which is essentially a programmatic equivalent of an XML model description. Instead of the current workflow XML → mjModel, the new workflow will look like XML → mjSpec → (edit, add, remove elements) → mjModel. So we still have all the benefits of model compilation, but we keep the mjSpec object as a live, editable thing that can always be recompiled. By not touching things that have not changed (incremental compilation), making a new mjModel will be very fast, after the first compile. This should allow you to manually "freeze" subsets of dofs that are no longer relevant for the scene in progress. We are currently finalizing this API, which will of course also be accessible via the Python bindings.
2. At the engine level, the main items are:
   • Fix some memory allocation issues with the Newton solver. This will both give us some speed-up from better cache friendliness but most importantly allow the Newton solver to handle tens of thousands of contacts/dofs.
   • Enable contact islands for the Newton solver, which will allow multithreading within a single scene. After this is done contacts islands will likely be on by default.
3. Once both 1 and 2 are finished, we will add dof freezing functionality at the island level. We are not yet sure if this will be enabled as a runtime option (without changing the mjModel) or at the mjSpec API level, effectively "recompile the model, removing all dofs of island X".

The workstream in item 1 is already actively being worked on. Item 2 was recently up-prioritized following your feature request. We expect 1 and 2 to be complete some time during Q3 24'.

Regarding your specific question regarding welding boxes, I'm a bit surprised that this is not working as expected. First of all by "welding" I hope you mean removing the DoFs, not adding a "weld" constraint, yes? Assuming the answer is "yes", please try replacing the enclosing bodies with frames. These are similar to bodies but cheaper. Basically all static (dofless) bodies should be frames, rather than bodies. If you have followup question regarding this specific workflow, please open a new issue, since I'd like to keep this issue, along with #7, to track the high-level large-scene feature set.

Cheers!

from mujoco.