3 ms·

Noob question: What kind of optimizations are even responsible for those kinds of performance improvements? 32 bits is enough to utilize all 4 GB of the Raspbe

Noob question: What kind of optimizations are even responsible for those kinds of performance improvements? 32 bits is enough to utilize all 4 GB of the Raspberry Pi4, so I figured the only benefit of using a 64 bit OS would be to support 64 bit software. Why would a 64 bit build perform better than a 32 bit build on the same hardware?

yalok 7y ago
64-bit ARM NEON (SIMD instruction set) is much more efficient vs 32-bit. And not just NEON, there are more registers as well, deeper pipeline, etc.
- xeeeeeeeeeeenu 7y ago
  Also, 32-bit ARMs often (but not always) don't have hardware integer division.
  - pm215 7y ago
    Integer division is mandatory from v7VE on upwards (so roughly Cortex-A9, A15 and later), and it's definitely in the 32-bit support of any 64-bit capable core. If Raspbian is compiling its packages for the lowest common denominator arm v6 cpu some Pis have, then you'd get some slowdown from division being an out-of-line call (to library code that can use the hw insn), but you'd see that part of the speedup just from building for 32-bit but optimised for newer cores than v6, I think.
    - xeeeeeeeeeeenu 7y ago
      >If Raspbian is compiling its packages for the lowest common denominator arm v6 cpu some Pis have It is definitely the case, the official Raspbian images support Raspberry Pi 1.
    - my123 7y ago
      Cortex-A9 does not have hardware division. Cortex-A7 and Cortex-A15 onwards do.
      - pm215 7y ago
        Yes, you're right, I misremembered. (It came in with CPUs with the virtualization extension, so not A9.)
cesarb 7y ago
> 32 bits is enough to utilize all 4 GB of the Raspberry Pi4 For 4 GB of memory, 32 bits of address space are not good enough, as discussed a couple of days ago (https://lwn.net/Articles/813201/ https://lwn.net/Articles/813201/ discussed here at https://news.ycombinator.com/item?id=22440733 https://news.ycombinator.com/item?id=22440733).

yalok 7y ago

64-bit ARM NEON (SIMD instruction set) is much more efficient vs 32-bit. And not just NEON, there are more registers as well, deeper pipeline, etc.

xeeeeeeeeeeenu 7y ago
Also, 32-bit ARMs often (but not always) don't have hardware integer division.
- pm215 7y ago
  Integer division is mandatory from v7VE on upwards (so roughly Cortex-A9, A15 and later), and it's definitely in the 32-bit support of any 64-bit capable core. If Raspbian is compiling its packages for the lowest common denominator arm v6 cpu some Pis have, then you'd get some slowdown from division being an out-of-line call (to library code that can use the hw insn), but you'd see that part of the speedup just from building for 32-bit but optimised for newer cores than v6, I think.
  - xeeeeeeeeeeenu 7y ago
    >If Raspbian is compiling its packages for the lowest common denominator arm v6 cpu some Pis have It is definitely the case, the official Raspbian images support Raspberry Pi 1.
  - my123 7y ago
    Cortex-A9 does not have hardware division. Cortex-A7 and Cortex-A15 onwards do.
    - pm215 7y ago
      Yes, you're right, I misremembered. (It came in with CPUs with the virtualization extension, so not A9.)

cesarb 7y ago

> 32 bits is enough to utilize all 4 GB of the Raspberry Pi4 For 4 GB of memory, 32 bits of address space are not good enough, as discussed a couple of days ago (https://lwn.net/Articles/813201/ https://lwn.net/Articles/813201/ discussed here at https://news.ycombinator.com/item?id=22440733 https://news.ycombinator.com/item?id=22440733).