New Article: ARM Goes 64-bit

Article: ARM Goes 64-bit
By: name99 (name99.delete@this.redheron.com), November 17, 2012 3:31 pm
Room: Moderated Discussions
anonymou5 (no.delete@this.spam.com) on August 14, 2012 11:03 am wrote:
> > Who uses x86 4MB pages?
>
> The use of PAE=0 is limited to scenarios in which PAE is not supported but PSE is (think
> historic x86 processors), scenarios in which PA space is less critical than the footprint
> of the page tables (the 32-bit paging entries limit 4 KiB pages to 32-bit PAs, and 4 MiB
> pages to 41-bit PAs, and they don't leave room for NX bits -- however, the tables consume
> less space), and/or scenarios in which page table walk performance matters (PDPTRs + two
> levels for PSE versus four levels for today's PAE).

I always assumed x86 large pages were used in essentially the same way BATs are used on POWER. They're not meant to be a general solution; they're a way to prevent the TLB being polluted in one particular case where this is easily avoided, namely where one has a
- large chunk of
- contiguous address space which is
- all used in the same way and which
- doesn't need to be swapped (or, unlikely, can be swapped in these large chunks)

SO two obvious use case: one might use large pages/BATs for
- covering video address space (including not just the frame buffer)
- the instruction and data footprints of the wired part of the OS.

An additional use case that sometimes arises, for dedicated machines is covering (some?) code and (some?) data that one essentially wants wired down and in constant use.
This could be the web server on some machines; the database server on some machines, perhaps even in special cases a shmem block of memory used for communication.

The point is, in ALL these cases, one knows ahead of the time what one is trying to do and why. For these uses cases, large pages work, and don't pollute the TLB. But it is fighting their design principle to assume that they are just large 4kB pages and can be handled the same way, in particular that they can usefully be AUTOMATICALLY managed by the OS --- automatically allocated, automatically swapped, automatically aggregated and de-aggregated from smaller pages.

As for 64kB pages --- bring it on. High time, and well done, ARM. Really the only thing I'd like to see in the new ISA that I don't see is
- support for multiple condition codes, like POWER
- a dedicated count register for speeding up inner loops, again like POWER.

I'm not a compiler guy, but for low level assembly hacking, multiple condition registers could be used with value. It always struck me that the compiler guys were not even trying to use them, with at least two obvious possibilities not explored:
- storing bools in conditions registers rather than standard "integer" registers
- testing conditions (when it makes sense) across basic block boundaries, rather than the paradigm of only looking within a single basic block for scheduling opportunities.

As for the dedicated count register, yes, you can get the same effect mostly with branch prediction, but this allows you to free up some of those limited branch prediction slots for more useful purposes, and gives you a register that lives close to the instruction side of the chip, not the data side, and thus a little faster when used for instruction purposes --- witness POWER's use of that register to call proc ptrs. I don't know --- maybe that sort of lack of orthogonality is just too much hassle? But count register never struck me as a bad idea, unlike some of what's in the POWER ISA.
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TopicPosted ByDate
New Article: ARM Goes 64-bitDavid Kanter2012/08/14 12:04 AM
  New Article: ARM Goes 64-bitnone2012/08/14 12:44 AM
    New Article: ARM Goes 64-bitDavid Kanter2012/08/14 01:04 AM
    MIPS MT-ASEPaul A. Clayton2012/08/14 09:01 AM
      MONITOR/MWAITEduardoS2012/08/14 10:08 AM
        MWAIT not specifically MTPaul A. Clayton2012/08/14 10:36 AM
          MWAIT not specifically MTEduardoS2012/08/15 03:16 PM
        MONITOR/MWAITanonymou52012/08/14 11:07 AM
          MONITOR/MWAITEduardoS2012/08/15 03:20 PM
      MIPS MT-ASErwessel2012/08/14 10:14 AM
  New Article: ARM Goes 64-bitSHK2012/08/14 02:01 AM
  New Article: ARM Goes 64-bitanon2012/08/14 02:37 AM
    New Article: ARM Goes 64-bitRichard Cownie2012/08/14 03:57 AM
      New Article: ARM Goes 64-bitanon2012/08/14 04:29 AM
      New Article: ARM Goes 64-bitnone2012/08/14 04:44 AM
        New Article: ARM Goes 64-bitanon2012/08/14 05:28 AM
          New Article: ARM Goes 64-bitanon2012/08/14 05:32 AM
            New Article: ARM Goes 64-bitEduardoS2012/08/14 06:06 AM
          New Article: ARM Goes 64-bitnone2012/08/14 05:40 AM
            AArch64 select better than cmovPaul A. Clayton2012/08/14 06:08 AM
            New Article: ARM Goes 64-bitanon2012/08/14 06:12 AM
              New Article: ARM Goes 64-bitnone2012/08/14 06:25 AM
                Predicated ld/store are usefulPaul A. Clayton2012/08/14 06:48 AM
                  Predicated ld/store are usefulnone2012/08/14 06:56 AM
                    Predicated ld/store are usefulanon2012/08/14 07:07 AM
                    Predicated stores might not be that badPaul A. Clayton2012/08/14 07:27 AM
                      Predicated stores might not be that badDavid Kanter2012/08/15 01:14 AM
                        Predicated stores might not be that badMichael S2012/08/15 11:41 AM
                        Predicated stores might not be that badR Byron2012/08/17 04:09 AM
                New Article: ARM Goes 64-bitanon2012/08/14 06:54 AM
                  New Article: ARM Goes 64-bitnone2012/08/14 07:04 AM
                    New Article: ARM Goes 64-bitanon2012/08/14 07:43 AM
          New Article: ARM Goes 64-bitEduardoS2012/08/14 06:07 AM
            New Article: ARM Goes 64-bitanon2012/08/14 06:20 AM
              New Article: ARM Goes 64-bitnone2012/08/14 06:29 AM
                New Article: ARM Goes 64-bitanon2012/08/14 07:00 AM
            New Article: ARM Goes 64-bitMichael S2012/08/14 03:43 PM
        New Article: ARM Goes 64-bitRichard Cownie2012/08/14 06:53 AM
          OT: Conrad's "Youth"Richard Cownie2012/08/14 07:20 AM
      New Article: ARM Goes 64-bitEduardoS2012/08/14 06:04 AM
        New Article: ARM Goes 64-bitmpx2012/08/14 08:59 AM
          New Article: ARM Goes 64-bitAntti-Ville Tuunainen2012/08/14 09:16 AM
        New Article: ARM Goes 64-bitanonymou52012/08/14 11:03 AM
          New Article: ARM Goes 64-bitname992012/11/17 03:31 PM
            Microarchitecting a counter registerPaul A. Clayton2012/11/17 07:37 PM
    New Article: ARM Goes 64-bitbakaneko2012/08/14 04:21 AM
      New Article: ARM Goes 64-bitname992012/11/17 03:40 PM
        New Article: ARM Goes 64-bitEduardoS2012/11/17 04:52 PM
        New Article: ARM Goes 64-bitDoug S2012/11/17 05:48 PM
        New Article: ARM Goes 64-bitbakaneko2012/11/18 05:40 PM
          New Article: ARM Goes 64-bitWilco2012/11/19 07:59 AM
            New Article: ARM Goes 64-bitEduardoS2012/11/19 08:23 AM
              New Article: ARM Goes 64-bitWilco2012/11/19 09:31 AM
                Downloading µarch-specific binaries?Paul A. Clayton2012/11/19 11:21 AM
                New Article: ARM Goes 64-bitEduardoS2012/11/19 11:41 AM
                  New Article: ARM Goes 64-bitWilco2012/11/21 07:44 AM
                    JIT vs. static compilation (Was: New Article: ARM Goes 64-bit)VMguy2012/11/22 03:21 AM
                      JIT vs. static compilation (Was: New Article: ARM Goes 64-bit)David Kanter2012/11/22 12:12 PM
                        JIT vs. static compilation (Was: New Article: ARM Goes 64-bit)Gabriele Svelto2012/11/23 03:50 AM
                    New Article: ARM Goes 64-bitEduardoS2012/11/23 10:09 AM
                      New Article: ARM Goes 64-bitEBFE2012/11/26 01:24 AM
                        New Article: ARM Goes 64-bitGabriele Svelto2012/11/26 03:33 AM
                          New Article: ARM Goes 64-bitEBFE2012/11/27 11:17 PM
                            New Article: ARM Goes 64-bitGabriele Svelto2012/11/28 02:32 AM
                        New Article: ARM Goes 64-bitEduardoS2012/11/26 12:16 PM
                          New Article: ARM Goes 64-bitEBFE2012/11/28 12:33 AM
                            New Article: ARM Goes 64-bitEduardoS2012/11/28 05:53 AM
                              New Article: ARM Goes 64-bitMichael S2012/11/28 06:15 AM
                                New Article: ARM Goes 64-bitEduardoS2012/11/28 07:33 AM
                                  New Article: ARM Goes 64-bitMichael S2012/11/28 09:16 AM
                                    New Article: ARM Goes 64-bitEduardoS2012/11/28 09:53 AM
                                    New Article: ARM Goes 64-bitEugene Nalimov2012/11/28 05:58 PM
                                      Amazing!EduardoS2012/11/28 07:25 PM
                                        Amazing! (non-italic response)EduardoS2012/11/28 07:25 PM
                                        Amazing!EBFE2012/11/28 08:20 PM
                                          Undefined behaviour doubles downEduardoS2012/11/28 09:10 PM
                              New Article: ARM Goes 64-bitEBFE2012/11/28 07:54 PM
                                New Article: ARM Goes 64-bitEduardoS2012/11/28 09:21 PM
                Have you heard of Transmeta?David Kanter2012/11/19 03:47 PM
            New Article: ARM Goes 64-bitbakaneko2012/11/19 09:08 AM
            New Article: ARM Goes 64-bitDavid Kanter2012/11/19 03:40 PM
              Semantic Dictionary EncodingRay2012/11/19 10:37 PM
              New Article: ARM Goes 64-bitRohit2012/11/20 04:48 PM
                New Article: ARM Goes 64-bitDavid Kanter2012/11/20 11:07 PM
                  New Article: ARM Goes 64-bitWilco2012/11/21 06:41 AM
                    New Article: ARM Goes 64-bitDavid Kanter2012/11/21 10:12 AM
                    A JIT exampleMark Roulo2012/11/21 10:30 AM
                      A JIT exampleWilco2012/11/21 07:04 PM
                        A JIT examplerwessel2012/11/21 09:05 PM
                        A JIT exampleGabriele Svelto2012/11/23 03:53 AM
                        A JIT exampleEduardoS2012/11/23 10:13 AM
                          A JIT exampleWilco2012/11/23 01:41 PM
                            A JIT exampleEduardoS2012/11/23 02:06 PM
                            A JIT exampleGabriele Svelto2012/11/23 04:09 PM
                              A JIT exampleSymmetry2012/11/26 05:58 AM
            New Article: ARM Goes 64-bitRay2012/11/19 10:27 PM
    New Article: ARM Goes 64-bitDavid Kanter2012/08/14 09:11 AM
  v7-M is Thumb-onlyPaul A. Clayton2012/08/14 06:58 AM
  Minor suggested correctionPaul A. Clayton2012/08/14 08:33 AM
    Minor suggested correctionanon2012/08/14 08:57 AM
  New Article: ARM Goes 64-bitExophase2012/08/14 08:33 AM
    New Article: ARM Goes 64-bitDavid Kanter2012/08/14 09:16 AM
      New Article: ARM Goes 64-bitjigal2012/08/15 01:49 PM
  Correction re ARM and BBC MicroPaul2012/08/14 08:59 PM
    Correction re ARM and BBC MicroPer Hesselgren2012/08/15 03:27 AM
  Memory BW so lowPer Hesselgren2012/08/15 03:14 AM
    Memory BW so lownone2012/08/15 11:16 AM
  New Article: ARM Goes 64-bitdado2012/08/15 10:25 AM
  Number of GPRsKenneth Jonsson2012/08/16 02:35 PM
    Number of GPRsExophase2012/08/16 02:52 PM
      Number of GPRsKenneth Jonsson2012/08/17 02:41 AM
        Ooops, missing link...Kenneth Jonsson2012/08/17 02:44 AM
        64-bit pointers eat some performancePaul A. Clayton2012/08/17 06:19 AM
          64-bit pointers eat some performancebakaneko2012/08/17 08:37 AM
            Brute force seems to workPaul A. Clayton2012/08/17 10:08 AM
              Brute force seems to workbakaneko2012/08/17 11:15 AM
          64-bit pointers eat some performanceRichard Cownie2012/08/17 08:46 AM
            Pointer compression is atypicalPaul A. Clayton2012/08/17 10:43 AM
              Pointer compression is atypicalRichard Cownie2012/08/17 12:57 PM
                Pointer compression is atypicalHoward Chu2012/08/22 10:17 PM
                  Pointer compression is atypicalRichard Cownie2012/08/23 04:48 AM
                    Pointer compression is atypicalHoward Chu2012/08/23 06:51 AM
              Pointer compression is atypicalWilco2012/08/17 02:41 PM
                Pointer compression is atypicalRichard Cownie2012/08/17 04:13 PM
                  Pointer compression is atypicalRicardo B2012/08/19 10:44 AM
                  Pointer compression is atypicalHoward Chu2012/08/22 10:08 PM
                    Unified libraries?Paul A. Clayton2012/08/23 07:49 AM
                    Pointer compression is atypicalRichard Cownie2012/08/23 08:44 AM
                      Pointer compression is atypicalHoward Chu2012/08/23 05:17 PM
                        Pointer compression is atypicalanon2012/08/23 08:15 PM
                          Pointer compression is atypicalHoward Chu2012/08/23 09:33 PM
            64-bit pointers eat some performanceFoo_2012/08/18 12:09 PM
              64-bit pointers eat some performanceRichard Cownie2012/08/18 05:25 PM
                64-bit pointers eat some performanceRichard Cownie2012/08/18 05:32 PM
            Page-related benefit of small pointersPaul A. Clayton2012/08/23 08:36 AM
        Number of GPRsWilco2012/08/17 06:31 AM
          Number of GPRsKenneth Jonsson2012/08/17 11:54 AM
            Number of GPRsExophase2012/08/17 12:44 PM
              Number of GPRsKenneth Jonsson2012/08/17 01:22 PM
                Number of GPRsWilco2012/08/17 02:53 PM
        What about dynamic utilization?Exophase2012/08/17 09:30 AM
          Compiler vs. assembly aliasing knowledge?Paul A. Clayton2012/08/17 10:20 AM
            Compiler vs. assembly aliasing knowledge?Exophase2012/08/17 11:09 AM
            Compiler vs. assembly aliasing knowledge?anon2012/08/18 02:23 AM
              Compiler vs. assembly aliasing knowledge?Ricardo B2012/08/19 11:02 AM
                Compiler vs. assembly aliasing knowledge?anon2012/08/19 06:07 PM
                  Compiler vs. assembly aliasing knowledge?Ricardo B2012/08/19 07:26 PM
                    Compiler vs. assembly aliasing knowledge?anon2012/08/19 10:03 PM
                      Compiler vs. assembly aliasing knowledge?anon2012/08/20 01:59 AM
        Number of GPRsDavid Kanter2012/08/17 12:46 PM
          RAT issues as part of reason 1Paul A. Clayton2012/08/17 02:18 PM
        Number of GPRsname992012/11/17 06:37 PM
          Large ARFs increase renaming costPaul A. Clayton2012/11/17 09:23 PM
    Number of GPRsDavid Kanter2012/08/16 03:31 PM
    Number of GPRsRichard Cownie2012/08/16 05:17 PM
    32 GPRs ~2-3%Paul A. Clayton2012/08/16 06:27 PM
      Oops, Message-ID: aaed6e38-c7bd-467e-ba41-f40cf1020e5e@googlegroups.com (NT)Paul A. Clayton2012/08/16 06:29 PM
      32 GPRs ~2-3%Exophase2012/08/16 10:06 PM
        R31 as SP/zero is kind of neat (NT)Paul A. Clayton2012/08/17 06:23 AM
        32 GPRs ~2-3%rwessel2012/08/17 08:24 AM
          32 GPRs ~2-3%Exophase2012/08/17 09:16 AM
            32 GPRs ~2-3%Max2012/08/17 04:19 PM
      32 GPRs ~2-3%name992012/11/17 07:43 PM
    Number of GPRsmpx2012/08/17 01:11 AM
      Latency and powerPaul A. Clayton2012/08/17 06:54 AM
    Number of GPRsbakaneko2012/08/17 03:09 AM
  New Article: ARM Goes 64-bitSteve2012/08/17 02:12 PM
    New Article: ARM Goes 64-bitDavid Kanter2012/08/19 12:42 PM
      New Article: ARM Goes 64-bitDoug S2012/08/19 02:02 PM
      New Article: ARM Goes 64-bitAnon2012/08/19 07:16 PM
      New Article: ARM Goes 64-bitSteve2012/08/30 07:51 AM
  Scalar vs Vector registersRobert David Graham2012/08/19 05:19 PM
    Scalar vs Vector registersDavid Kanter2012/08/19 05:29 PM
  New Article: ARM Goes 64-bitBaserock ARM servers2012/08/21 04:13 PM
    Baserock ARM serversSysanon2012/08/21 04:14 PM
    A-15 virtualization and LPAE?Paul A. Clayton2012/08/21 06:13 PM
      A-15 virtualization and LPAE?Anon2012/08/21 07:13 PM
        Half-depth advantages?Paul A. Clayton2012/08/21 08:42 PM
          Half-depth advantages?Anon2012/08/22 03:33 PM
            Thanks for the information (NT)Paul A. Clayton2012/08/22 04:04 PM
      A-15 virtualization and LPAE?C. Ladisch2012/08/23 11:12 AM
        A-15 virtualization and LPAE?Paul2012/08/23 03:17 PM
        Excessive pessimismPaul A. Clayton2012/08/23 04:08 PM
          Excessive pessimismDavid Kanter2012/08/23 05:05 PM
    New Article: ARM Goes 64-bitMichael S2012/08/22 07:12 AM
      BTW, Baserock==product, Codethink==company (NT)Paul A. Clayton2012/08/22 08:56 AM
  New Article: ARM Goes 64-bitReinoud Zandijk2012/08/21 11:27 PM
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