Sunny Cove wide

By: Travis Downs (travis.downs.delete@this.gmail.com), December 13, 2018 9:50 am
Room: Moderated Discussions
Jeff S. (fakity.delete@this.fake.com) on December 13, 2018 8:33 am wrote:
> Travis Downs (travis.downs.delete@this.gmail.com) on December 13, 2018 7:42 am wrote:
> > No, you are right, it's 48 KiB.
>
> > Interesting that they are moving to a non-power of two size. If we assume it's still
> > indexed in a straightforward manner I guess this also eliminate of all sorts of power-of-two
> > conflicts that occured when lines are accessed with some 2^n stride.
>
> Yeah, I think modulo 4 kiB (64 sets of 64B lines) virtual indexing in an extremely safe bet. Making
> 8->12 way set CAMs seems drastically more straightforward to me than wrapping lines modulo 6 kiB.
> The latter feels borderline intractable given the need to sanely handle doubly-mapped physical
> pages, which would no longer be constrained to individual sets. (and you can't really kick the
> can to the general architecture and declare pages are all now 6 kiB or whatever)

You should ignore it, it was a brief moment of nonsense where I thought you'd need both 12-way and 96 sets for a 48 KiB VIPT cache. Of course not: it will just be 12-way (perhaps with some "pseudo" in front now to simplify the CAM as you point out).

>
> I don't really follow your point about maintaining VI eliminating 2^n B stride aliasing though. Anything
> with a virtual stride of 2^(12 + x) bytes should still hit the same set, regardless of way count,
> right? There is no intrinsic page coloring constraint I see here that would make L2/L3 (with typically
> 32 or 64 kiB wide ways and hence physical stride aliasing) do anything differently.

Note that was the nonsense world where I thought you needed 96 sets, i.e., "modulo 6 KiB" mapping. In that case you have a "mod 3" in the indexing so while it doesn't eliminate aliasing it reduces the effect by 3 by spreading out power-of-two strides out over 3 more sets. So things with a virtual stride of 2^(12 + x) can live in 12 * 3 = 36 ways before they overflow.

Of course that was all in my nonsense world so it doesn't matter.

>
> And some further architectural speculation:
> Both Zen and some of IBM's newer chips are moving to u-tagged L1D, which trades a minor hit rate penalty in exchange
> for drastically reducing CAM power and complexity. I would have guessed that AMD would have been the first to
> go with >32 kiB L1Ds since that architecture is inherently friendlier to (relatively) cheaply adding ways to
> sets. Intel going to 48 kiB makes me strongly suspect they've moved to the same approach, especially since Sunny
> Cove sounds like it will be in chip segments all the way down to mobile, and there was no mention of "up to 48
> kiB L1D", and doing ever-wider traditional VIPT CAMs doesn't feel amenable to super low power.

< Previous Post in ThreadNext Post in Thread >
TopicPosted ByDate
Sunny Cove wideSeni2018/12/12 02:58 PM
  Sunny Cove wideTravis Downs2018/12/12 09:25 PM
    Sunny Cove wideJeff S.2018/12/12 10:26 PM
      Sunny Cove wideTravis Downs2018/12/13 08:42 AM
        Sunny Cove wideanon2018/12/13 09:09 AM
          Sunny Cove wideTravis Downs2018/12/13 09:30 AM
            Sunny Cove wideJeff S.2018/12/13 09:40 AM
              Sunny Cove wideTravis Downs2018/12/13 09:55 AM
                Sunny Cove wideJeff S.2018/12/13 12:41 PM
                  Sunny Cove wideTravis Downs2018/12/13 02:03 PM
                  Non-power-of-two set sizesPaul A. Clayton2018/12/15 07:30 PM
        Sunny Cove wideJeff S.2018/12/13 09:33 AM
          Sunny Cove wideTravis Downs2018/12/13 09:50 AM
          What is "u-tagged"?G Adair2018/12/13 09:54 PM
            What is "u-tagged"?Travis Downs2018/12/13 11:22 PM
            What is "u-tagged"?Jeff S.2018/12/14 08:48 AM
              What is "u-tagged"?anon2018/12/14 08:51 PM
                What is "u-tagged"?Jeff S.2018/12/14 10:23 PM
                  What is "u-tagged"?anon2018/12/15 05:37 AM
                    What is "u-tagged"?anon2018/12/15 08:06 AM
                      What is "u-tagged"?Travis Downs2018/12/15 09:52 AM
                        What is "u-tagged"?anon2018/12/16 08:26 AM
                          What is "u-tagged"?Anon2018/12/18 04:25 AM
    Sunny Cove wideSeni2018/12/13 03:33 AM
      Sunny Cove wideKevin G2018/12/13 08:37 AM
        Sunny Cove wideTravis Downs2018/12/13 09:17 AM
          Sunny Cove wideKevin G2018/12/17 10:09 AM
            Sunny Cove wideTravis Downs2018/12/18 03:14 PM
              Sunny Cove wideKevin G2018/12/19 12:02 PM
      Sunny Cove wideTravis Downs2018/12/13 08:51 AM
        Sunny Cove wideMaynard Handley2018/12/13 11:25 AM
          Sunny Cove wideTravis Downs2018/12/13 12:23 PM
            Sunny Cove wideanon2018/12/13 02:01 PM
              Sunny Cove wideTravis Downs2018/12/13 02:22 PM
                Sunny Cove wideanon2018/12/13 04:51 PM
                  Sunny Cove wideTravis Downs2018/12/13 05:36 PM
                    Sunny Cove wideanon2018/12/14 03:57 AM
                      Sunny Cove wideLinus Torvalds2018/12/14 01:54 PM
                        Sunny Cove wideanon2018/12/14 04:25 PM
                          Sunny Cove wideLinus Torvalds2018/12/14 06:46 PM
                            Sunny Cove wideanon2018/12/15 02:57 AM
                              Sunny Cove wideanon2018/12/15 05:59 AM
                                Sunny Cove wideanon2018/12/15 06:59 AM
                                  Sunny Cove wideanon2018/12/15 07:03 AM
                                Sunny Cove widea_different_anon2018/12/15 07:45 AM
                              Sunny Cove wideSeni2018/12/15 06:25 AM
                                Sunny Cove wideanon2018/12/15 07:02 AM
                                  Sunny Cove wideLinus Torvalds2018/12/15 10:52 AM
                                    Sunny Cove wideanon2018/12/15 11:13 AM
                                      Sunny Cove wideTravis Downs2018/12/16 11:15 AM
                                      Sunny Cove wideanon2018/12/17 12:42 AM
                                        how many anons here? (NT)Michael S2018/12/17 02:46 AM
                      Sunny Cove wideTravis Downs2018/12/15 10:08 AM
                        Sunny Cove wideanon2018/12/15 10:55 AM
                          Sunny Cove wideTravis Downs2018/12/16 09:19 AM
                            Sunny Cove wideanon2018/12/16 10:37 AM
                              Sunny Cove wideTravis Downs2018/12/16 10:57 AM
                                Sunny Cove wideanon2018/12/16 12:04 PM
                                  Sunny Cove wideTravis Downs2018/12/16 07:51 PM
                          Sunny Cove wideTravis Downs2018/12/16 11:32 AM
        Sunny Cove wideSeni2018/12/13 04:20 PM
          Fair enough! (NT)Travis Downs2018/12/13 04:43 PM
  Sunny Cove wide-.-2018/12/13 04:37 AM
    Sunny Cove wideanon2018/12/13 09:06 AM
      Sunny Cove wideTravis Downs2018/12/13 09:39 AM
        Sunny Cove wideanon2018/12/13 12:09 PM
          Sunny Cove wideTravis Downs2018/12/13 12:27 PM
            Sunny Cove wideanon2018/12/13 01:11 PM
    Sunny Cove wideTravis Downs2018/12/13 09:23 AM
      Sunny Cove wideanonymous22018/12/13 03:20 PM
        Sunny Cove wideTravis Downs2018/12/13 05:00 PM
          Sunny Cove wideanon³2018/12/13 10:34 PM
            Sunny Cove wideTravis Downs2018/12/16 07:53 PM
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