Background thread GC?

[adinosaur]

Is it possible to use another os thread(not in main thread) run gcstep? Maybe need a GIL like Python?

[zeux]

This is currently impossible because a lot of data structures GC uses aren't thread safe. What we recommend right now (and deploy in Roblox more or less) for interactive applications is to have a section of your frame where scripts can not run, and run GC during that section via lua_gc (there's some subtlety with how to set it up which we should document...). This ensures that scripts rarely trigger what we call "assist GC" since most of the GC work is done separately, and the application has an opportunity to do extra computation while GC is running.

[adinosaur]

We use lua in a game app and already do some modify in lua GC's code:

1. Disable macro luaC_checkGC for avoid GC's assist;
2. Use lua_gc to run GC per frame;
3. Limit GC time less then 3ms;

My next idea is that we use a GIL(Global Interpreter Lock), when run scripts first acquire the GIL, and release it when scripts finish. And have a background thread, it loop to run GC via lua_gc while GIL is free. GCState propagate/sweep is safe, callfin is not, Maybe this state still run in main thread?

[zeux]

A couple notes:

• What you're suggesting is possible. Luau doesn't have a callfin stage, so you can simply use the mutex with try_lock in the GC thread - however, note that you're still essentially waiting on the GC in the main thread if it already started the step. You can probably reduce this problem by calling lua_gc with short intervals. Of course either way you're relying on a portion of the frame to not run GC. All of this doesn't require VM changes, you can do this from the embedding application.
• In Luau commenting out luaC_checkGC is not very important because lua_gc already properly accounts for this. Moreover, in any Lua implementation this strategy is dangerous because if scripts allocate faster than GC can collect memory you're risking a memory leak without assists.