When creating a new heap because an allocation cannot fit into all existing heaps associated with an arena, the glibc allocator does not extend the mprotect area of the existing heap to the maximum in all cases. As a result, the kernel will not be able to merge VMAs of adjacent heaps.

It looks like that this might restrict the heap size artificially to 2 TiB in a default configuration.

The heapdumper should produce statistics to check if this is indeed something that should be fixed in the allocator (which would likely be a fairly localized change).