That isn't relevant to what we're discussing here. The problem of fragmentation across arenas that that paper describes is a problem with any system that uses t

That isn't relevant to what we're discussing here. The problem of fragmentation across arenas that that paper describes is a problem with any system that uses thread-local caching, including Go's tcmalloc. The type of fragmentation we're discussing is fragmentation within a memory arena. That is the kind of fragmentation that only a copying/compacting collector can eliminate.