Beginning sometime during the last week of July or so, some sort of change went out that altered how collisions between MeshParts and other objects were calculated. This change resulted in our MeshPart surface roads becoming heinously bumpy at high speeds, rendering players completely unable to control their vehicles in Ultimate Driving.
BEFORE ROBLOX CHANGE
AFTER ROBLOX CHANGE
Due to some scaling changes we made between the before & after videos, the speed in the AFTER video never once exceeded the actual speed achieved in the BEFORE video, which further proves a degradation in performance.
The bumpiness resembles the sort of instability you’d expect from a road made up of many parts. Roblox’s physics engine is not good at smoothly calculating wheels over a surface made up of many parts, which results in jumpiness over part seams.
This is not an issue of tuning the vehicle suspension tuning, as this issue was not present prior to late July, and old builds of my game that predate this engine bug are now exhibiting this behavior.
EDIT - It’s also not an issue of the switch to adaptive physics, as we’ve been using the Adaptive setting for most of the period its been available with no issue.
Expected behavior
As shown in the BEFORE video above, prior to this change being made vehicles would glide over these MeshPart-based roads as though they were a smooth surface, with wheels reliably following the surface of the mesh without bouncing over seams between triangles within the collision mesh. Players vehicles should not be randomly bouncing around over these smooth roadways.
I and my QA team are pretty confident this improved things, but we’ll keep an eye out just in case. The issue was exacerbated in certain vehicles due to differences in stability tuning / top speeds, we had a much easier time handling one of the worst offenders once this flag was fixed.
Is there a chance this could be contributing to server crashes?
Since this flag was reverted our servers have been struggling to survive past the 4 hour mark. I understand the platform experienced outages both yesterday and today, but this issue has been plaguing the game outside of those outages. The only indication I can find in my own analytics that something’s wrong is elevated Heartbeat numbers, aside that memory usage is nominal and players are reporting the crashes happen without warning.
Given there’s been outages, internal flag flips, and sporadic data issues on my end, I’m having a hard time gathering from my community when this issue picked up given there’s so many factors all coalescing at once.
That specific flag is unrelated to any crashes, but we do see elevated server crashes in UD having to do with network replication. I’m raising this with the networking team.
Hello! We’ve identified a separate bug that is causing these other crashes you mentioned. We are working on it but unfortunately do not have a resolution yet. I will keep you updated on the status.
Any update on this issue? This bug has been nearly halving my session lengths ever since it cropped up which is knocking me out of the discovery algorithm.
Sorry, I don’t have a substantive update yet. This one is very difficult to root-cause. We have noticed that there is a specific access pattern that causes the bug to reproduce, but we are not sure yet what the exact pattern is. (We know that there is a pattern because the distribution of experiences affected by this bug does not match the “normal” distribution.)
The crash somehow involves Lua Events, but seeing as every experience uses Events, that’s not really useful debug information.
