Comment by henning

They built a large codebase on a language that doesn't let you control memory, because that makes you "more productive". So just having Rails allocate a per-request arena that is asynchronously freed which would force the programmer not to have any objects that outlive the request, or just pre-allocating memory for a fixed amount of request handling per server instance, or whatever allocation behavior you want to do that is generally possible in C/C++/Zig/Rust/Odin/etc, requires hacking on the language itself. Which means your changes have to go through the Ruby team first. Any additional changes would also need to go through them, which increases the cost of change. Then there is a permanent layer of indirection between your GC callbacks and the semantics of what those callbacks do. Instead of just writing out the custom allocators you want, because that's impossible. How depressing.