From https://www.gnu.org/software/hurd/faq/64-bit.html

> There are currently no plan for 64-bit userland, but there are plans for 64-bit kernelland with 32-bit userland, which will notably permit to efficiently make use of more than 2 GiB memory and provide 4 GiB userland addressing space. Work on this is currently in the master-x86_64 and port-amd64 branches for GNU Mach.

> That being said, you can always run a 32-bit version on a 64-bit machine, it just works, processes are just limited to a couple GiB available memory.

That's ironic, given that last I heard anything about Hurd, it was that partition size was constrained by bus width (i.e., it couldn't access disks larger than 2 GiB on most computers at the time), but since everyone was moving to 64-bit architectures soon enough, that wouldn't be an issue. (Not sure where I heard that particular excuse, but the explanation why is here: [1])

32-bit userland makes Hurd basically unusable for many server applications, where multi-gigabyte in-memory lookup tables are critical for performance.

[1] https://developers.slashdot.org/comments.pl?sid=44492&cid=46...