I’m not so sure the usefulness of custom allocators really relates to the complexity of the problem. Take the scratch arena example: ifaik it’s just free performance (& simplicity) whenever you need dynamic allocations with lifetimes that begin and end on a critical path. Presumably this is why Jai/Odin focus on this situation so much since it applies to per-frame stuff in videogames, but generally I’d expect this to come up more often in nontrivial problems.