> As pressure from thread collisions increases, additional arenas are created via mmap to relieve the pressure. The number of arenas is capped at eight times the number of CPUs in the system (unless the user specifies otherwise, see mallopt), which means a heavily threaded application will still see some contention, but the trade-off is that there will be less fragmentation.

https://sourceware.org/glibc/wiki/MallocInternals

So glibc's malloc will use up to 8x #CPUs arenas. If you have 10_000 threads, there is likely to be contention.