5.4 Merge window, part 1

Posted Sep 26, 2019 2:24 UTC (Thu) by naptastic (guest, #60139)
Parent article: 5.4 Merge window, part 1

> The Arm64 architecture can now use 52-bit addresses on hardware that supports them.

Who's making ARM systems with 4 TiB of RAM?

5.4 Merge window, part 1

Posted Sep 26, 2019 6:10 UTC (Thu) by jem (subscriber, #24231) [Link]

Well, obviously nobody, since there was no way of using it until now. Besides, 640k ought to be enough for anybody.

5.4 Merge window, part 1

Posted Sep 26, 2019 10:00 UTC (Thu) by excors (subscriber, #95769) [Link] (1 responses)

> Who's making ARM systems with 4 TiB of RAM?

I guess you meant 2^52 = 4 PiB, not 4 TiB? The latter is easy: https://www.gigabyte.com/ARM-Server/R281-T94-rev-100 is a dual-socket ARM server which supports "Up to 16 DIMMS per socket, up to 4 TB of memory per system in dual socket configuration".

Then there's things like Optane DIMMs, which seem to go up to 512 GiB now, so you could physically fit 8 TiB of addressable memory per socket into that server (although I assume it won't actually work on non-Intel platforms).

The kernel is being increased from 48-bit which is 256 TiB, and it sounds like we're not too many years away from reaching that limit.

5.4 Merge window, part 1

Posted Sep 30, 2019 16:27 UTC (Mon) by cesarb (subscriber, #6266) [Link]

To make things even more interesting, ARM can have three different base page sizes: 4KB, 16KB, and 64KB (this has led to some fun compatibility issues when packages autodetect and hardcode the page size during their build, for instance jemalloc: https://github.com/jemalloc/jemalloc/issues/467). In some distributions (for instance, current Fedora, if I read their kernel config correctly), the kernel is compiled to use 4KB pages; in other distributions (for instance, current RedHat, if I read their kernel config correctly), the kernel is compiled to use 64KB pages. The 52-bit addresses are only available when the kernel is compiled to use 64KB pages (which is probably why RedHat chose that option).