Making a memory visualizer/profiler with luau

[Polanas]

Greetings!
I'm looking for any kind of hints/help with getting memory allocation data (ideally per table/function/thread/etc) from a luau instance. Roblox has introduced this feature last year (announcement link).

I've managed to get some very general info (total memory used/freed, total allocations amount) via a custom allocator function. There also seems to be C api for retrieving total bytes per type (lua_setmemcat, lua_totalbytes). I'm totally stuck on how to proceed from here. How would one approach this (and is it even possible to make something similar without modifying source)?

If there's an open source implementation of something like this, please share it :) (the ones I've look at were quite bare-bones). Any help at all would be greatly appreciated!

[vegorov-rbx]

Hello!

The main function used in those tools is luaC_enumheap, you can use that to build a whole graph of objects in memory.

For an open-source implementation, it is also similar to the way luaC_dump generates a JSON file you can send into https://github.com/luau-lang/luau/blob/master/tools/perfstat.py and https://github.com/luau-lang/luau/blob/master/tools/perfgraph.py

Finally, you can use lua_callbacks and onallocate callback to collect information on call stack of every allocation (expensive for performance, but useful for other kinds of analysis).

We have plans to make these luaC_* functions a part of the public C API in the future, but our roadmap is filled with other things for now.

[Polanas]

Hi! Wow, thanks for such a quick answer!! luaC_enumheap looks very promising. I hope it'll become a part of public API :)
perfstat and perfgraph are also gonna be helpful.

If you have some time available, could you briefly describe how luaC_enumheap is supposed to be used?

Upd: just found it used in a test in Conformance.test.cpp. Yay!

[Polanas]

Okay, got it to work finally. That's sooo powerful!!!! Luau rocks ❤️

[Polanas]

Also worth noting that luaC_enumheap seems to require 0 garbage to be present upon calling it. I was getting random Illegal Instruction segfaults, and they stopped only after I started calling lua_gc with LUA_GCCOLLECT right before luaC_enumheap

[vegorov-rbx]

We have always called lua_gc before it, but we can check what can be done to make it not required.