IIRC glibc default malloc doesn't use per-thread arenas as they would waste too much memory on programs spawning tens of thousands of threads, and glibc can't really make too many workload assumptions. Instead I think it uses a fixed pool of arenas and tries to minimize contention.

These days on Linux, with restartable sequences, you can have true per-cpu arenas with zero contention. Not sure which allocator use them though.