[LegionMammal978]

You do need to opt-in when compiling the kernel, but on Linux it doesn't take anything particularly special on the program's side to enable it. The rule is that non-fixed mmap() will only ever yield 48-bit addresses, until the program specifies a 57-bit address as a hint, after which it can yield 57-bit addresses at will. (This rule has lots of curious consequences for the location of the vDSO, when 5-level paging is enabled and an ELF program uses big addresses for its segments.)

[vlovich123]

Any idea what the use case is for such large addresses? Is it RDMA or something else? Even with RDMA I find it hard to believe that companies have > 256TiB of RAM installed behind a single switch.

[LegionMammal978]

I recall hearing one use case where you have a whole lot of threads, and most of them will only use a little memory, but a few will need a whole lot of memory. So you assign each thread a very large segment of the address space upfront, so that they never have to coordinate with other threads at runtime. At 1 GiB of space allocated to each thread, it takes only 262k threads to use up the whole 256 TiB address space.

[menaerus]

Good question. I also may not understand that since as of today, and to my knowledge, top of the line Intel Xeon's can support up to 4TB per socket. This means that for a 8-socket system, largest amount of physical RAM would equate to 32TB ... which is not even close to the addressable (virtual) memory on even 48-bit systems (256TB).