How to Fix Server CPU Spikes From Runaway Physics Substeps

A server that hums along and then suddenly pegs a core under a pile-up is often in a physics death spiral: a slow frame causes more substeps, which causes slower frames. Bound it. Here is how.

How to fix it

1. Cap substeps and max delta

Limit the maximum number of physics substeps per frame and clamp the accumulated delta. This stops a single slow frame from demanding an unbounded catch-up that compounds into a spiral.

2. Throttle distant and idle bodies

Lower the simulation rate or disable physics for bodies far from any player, and allow resting bodies to sleep. Most server CPU on physics goes to objects nobody is interacting with.

3. Profile the hot frame

Use the server profiler to find which contacts or queries spike. Often a single overlapping spawn or a stack of rigidbodies generates thousands of contacts; fixing that one case removes the spike.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.