Re:Playing with a Raspberry Pi 4 64-bit
by moosingin3space on Friday July 12, 2019 @07:46PM
(#20425524)
I'm no Linus Torvalds fan, but his rant (and follow-up) on PAE explains why this is actually a terrible idea:
1) https://www.realworldtech.com/forum/?threadid=76912&curposti... 2) https://www.realworldtech.com/forum/?threadid=76912&curposti...
So cute. The times before Meltdown.
Anyways, Linus is being... Linus. He is of course right in some ways, from kernel point of view it does suck, when you can't map all of the caches and buffers simultaneously. But PAE is pretty good for a bunch of large processes.
1 TB RAM supported by 32-bit ARM is not enough? I don't think increasing process virtual address size is the most pressing need at this point. Although it would help with things like WebAssembly, and anything else that benefits from mapping a lot of stuff in the address space.
The myth that you need to have 64-bit to access more than 4 GB RAM seems to still be very strong, despite not being true for a long time on modern x86 (since Pentium Pro in 1995) and ARM platforms (since LPAE designs, like 2013 Cortex A7/A15, etc.).
Of course I'd also like to get AArch64 on RPi4, but for entirely different reasons. To support double precision NEON SIMD and to get more registers to help Cortex A72 core out-of-order engine to keep the execution units busy. More registers helps to reduce dependencies, giving OoO more freedom to rearrange instructions for optimal execution.