Virtually tagged L1-caches

By: --- (---.delete@this.redheron.com), September 11, 2021 9:53 am
Room: Moderated Discussions
sr (nobody.delete@this.nowhere.com) on September 11, 2021 1:36 am wrote:
> --- (---.delete@this.redheron.com) on September 8, 2021 2:28 pm wrote:
>
> > Finally I cannot figure out how Apple bypasses D- way prediction, but they don't seem to need
> > it -- at least even the most random stream of addresses (in terms of hitting different ways)
> > that I could generate never seemed to cause even a hiccup in terms of invalid way prediction.
> > I have a few hypotheses, but no strong leads, and no patents suggest an explanation.
> > On the I- side it's easier -- way info is stored in various appropriate places,
> > like, for example, the Next Fetch Predictor -- and this is described in patents.
>
> That' way L1-cache tags need to be in program address space. With physical tags it's impossible to do any
> kind of way prediction with tag bits but with linear address space you could use tag bits itself to do "way
> prediction". AMD is documented how they do it. It's a "way prediction" because it isn't traditional way prediction
> where first is compared predicted way and then rest - what they do is to make tags so short that they can
> compare all ways effectively. So way prediction newer missed, it's either L1 hit or miss.
>

I understand what you are saying. There are multiple ways in which this sort of thing can be done, ranging from placing a way table in the TLB (D2D cache) to using partial tag comparisons (which Apple definitely does for the L2, as in this patent: https://patents.google.com/patent/US10157137B1
to mapping packing of lines into the rows of a subarray.

What I have not established is any particular claim as to how Apple does it in the L1D, just that whatever they do, they manage to hide it from visible code. Standard partial tag comparison is a probabilistic thing so it should occasionally fail and generate a timing glitch. I never saw such -- but Apple have a very nice, lightweight, Replay+Speculative Scheduling mechanism, so with rare enough glitches, they can probably hide such a glitch.

Compare this to throughput based on address patterns, where you can, with carefully crafted patterns, very definitely see the falloff in throughput with different TLB access requirements, requests that all route to one half of the cache, requests separated by varying numbers of cache lines, and so on.
< Previous Post in ThreadNext Post in Thread >
TopicPosted ByDate
POWER10 SAP SD benchmarkanon22021/09/06 03:36 PM
  POWER10 SAP SD benchmarkDaniel B2021/09/07 02:31 AM
    "Cores" (and SPEC)Rayla2021/09/07 07:51 AM
      "Cores" (and SPEC)anon2021/09/07 03:56 PM
  POWER10 SAP SD benchmarkAnon2021/09/07 03:24 PM
    POWER10 SAP SD benchmarkAnon2021/09/07 03:27 PM
  Virtually tagged L1-cachessr2021/09/08 05:49 AM
    Virtually tagged L1-cachesdmcq2021/09/08 08:22 AM
      Virtually tagged L1-cachessr2021/09/08 08:56 AM
      Virtually tagged L1-cachesHugo Décharnes2021/09/08 08:58 AM
        Virtually tagged L1-cachessr2021/09/08 10:09 AM
          Virtually tagged L1-cachesHugo Décharnes2021/09/08 10:46 AM
            Virtually tagged L1-cachessr2021/09/08 11:35 AM
              Virtually tagged L1-cachesHugo Décharnes2021/09/08 12:23 PM
                Virtually tagged L1-cachessr2021/09/08 12:40 PM
                  Virtually tagged L1-cachesanon2021/09/09 03:16 AM
                    Virtually tagged L1-cachesKonrad Schwarz2021/09/10 05:19 AM
                      Virtually tagged L1-cachesHugo Décharnes2021/09/10 06:59 AM
                        Virtually tagged L1-cachesanon2021/09/14 03:17 AM
                          Virtually tagged L1-cachesdmcq2021/09/14 09:34 AM
                            Or use a PLB (NT)Paul A. Clayton2021/09/14 09:45 AM
                              Or use a PLBLinus Torvalds2021/09/14 03:27 PM
                                Or use a PLBanon2021/09/15 12:15 AM
                                  Or use a PLBMichael S2021/09/15 03:21 AM
                                    Or use a PLBdmcq2021/09/15 03:42 PM
                                      Or use a PLBKonrad Schwarz2021/09/16 04:24 AM
                                        Or use a PLBMichael S2021/09/16 10:13 AM
                                          Or use a PLB---2021/09/16 01:02 PM
                                  PLB referencePaul A. Clayton2021/09/18 02:35 PM
                                    PLB referenceMichael S2021/09/18 04:14 PM
                                      Demand paging/translation orthogonalPaul A. Clayton2021/09/19 07:33 AM
                                        Demand paging/translation orthogonalMichael S2021/09/19 09:10 AM
                                      PLB referenceCarson2021/09/20 10:19 PM
                                    PLB referencesr2021/09/20 06:02 AM
                                      PLB referenceMichael S2021/09/20 07:03 AM
                                        PLB referenceLinus Torvalds2021/09/20 12:10 PM
                                  Or use a PLBsr2021/09/20 04:32 AM
                              Or use a PLBsr2021/09/21 09:36 AM
                                Or use a PLBLinus Torvalds2021/09/21 10:04 AM
                                  Or use a PLBsr2021/09/21 10:48 AM
                                    Or use a PLBLinus Torvalds2021/09/21 01:55 PM
                                      Or use a PLBsr2021/09/22 06:55 AM
                                        Or use a PLBrwessel2021/09/22 07:09 AM
                                        Or use a PLBLinus Torvalds2021/09/22 11:50 AM
                                          Or use a PLBsr2021/09/22 01:00 PM
                                            Or use a PLBdmcq2021/09/22 04:07 PM
                                            Or use a PLBEtienne Lorrain2021/09/23 08:50 AM
                                          Or use a PLBanon22021/09/22 04:09 PM
                                            Or use a PLBdmcq2021/09/23 02:35 AM
                                          Or use a PLB2021/09/23 09:37 AM
                                            Or use a PLBLinus Torvalds2021/09/23 12:01 PM
                                              Or use a PLBgpd2021/09/24 03:59 AM
                                                Or use a PLBLinus Torvalds2021/09/24 10:45 AM
                                                  Or use a PLBdmcq2021/09/24 12:43 PM
                                                  Or use a PLBsr2021/09/25 10:19 AM
                                                    Or use a PLBLinus Torvalds2021/09/25 10:44 AM
                                                      Or use a PLBsr2021/09/25 11:11 AM
                                                        Or use a PLBLinus Torvalds2021/09/25 11:31 AM
                                                          Or use a PLBsr2021/09/25 11:52 AM
                                                            Or use a PLBLinus Torvalds2021/09/25 12:05 PM
                                                              Or use a PLBsr2021/09/25 12:23 PM
                                                                Or use a PLBrwessel2021/09/25 03:29 PM
                                                                  Or use a PLBsr2021/10/01 12:22 AM
                                                                    Or use a PLBrwessel2021/10/01 06:19 AM
                                                                      Or use a PLBDavid Hess2021/10/01 10:35 AM
                                                                        Or use a PLBrwessel2021/10/02 04:47 AM
                                                                      Or use a PLBsr2021/10/02 11:16 AM
                                                                        Or use a PLBrwessel2021/10/02 11:53 AM
                                                          Or use a PLBLinus Torvalds2021/09/25 11:57 AM
                                                            Or use a PLBsr2021/09/25 12:07 PM
                                                              Or use a PLBLinus Torvalds2021/09/25 12:21 PM
                                                                Or use a PLBsr2021/09/25 12:40 PM
                                                                  Or use a PLBnksingh2021/09/27 09:07 AM
                                                          Or use a PLB2021/09/27 09:02 AM
                                                            Or use a PLBLinus Torvalds2021/09/27 10:20 AM
                                                              Or use a PLBLinus Torvalds2021/09/27 12:58 PM
                                                                Or use a PLBdmcq2021/09/28 10:59 AM
                                              Or use a PLBsr2021/09/25 10:34 AM
                                                Or use a PLBrwessel2021/09/25 03:44 PM
                                                  Or use a PLBsr2021/10/01 01:04 AM
                                                    Or use a PLBrwessel2021/10/01 06:33 AM
                                                      I386 segmentation highlightssr2021/10/04 07:53 AM
                                                        I386 segmentation highlightsAdrian2021/10/04 09:53 AM
                                                          I386 segmentation highlightssr2021/10/04 10:19 AM
                                                        I386 segmentation highlightsrwessel2021/10/04 04:57 PM
                                                          I386 segmentation highlightssr2021/10/05 11:16 AM
                                                            I386 segmentation highlightsMichael S2021/10/05 12:27 PM
                                                            I386 segmentation highlightsrwessel2021/10/05 04:20 PM
                                                Or use a PLBJohnG2021/09/25 10:18 PM
                                              Or use a PLB2021/09/27 07:37 AM
                                                Or use a PLBHeikki Kultala2021/09/28 03:53 AM
                                                  Or use a PLBrwessel2021/09/28 07:29 AM
                                        Or use a PLBDavid Hess2021/09/23 06:00 PM
                                          Or use a PLBAdrian2021/09/24 01:21 AM
                                            Or use a PLBdmcq2021/09/25 12:41 PM
                                        Or use a PLBblaine2021/09/26 11:19 PM
                                          Or use a PLBDavid Hess2021/09/27 11:35 AM
                                            Or use a PLBblaine2021/09/27 05:19 PM
                                            Or use a PLBAdrian2021/09/27 10:40 PM
                                              Or use a PLBAdrian2021/09/27 10:59 PM
                                                Or use a PLBdmcq2021/09/28 07:45 AM
                                              Or use a PLBrwessel2021/09/28 07:45 AM
                                              Or use a PLBDavid Hess2021/09/28 12:50 PM
                                                Or use a PLBEtienne Lorrain2021/09/30 01:25 AM
                                                  Or use a PLBDavid Hess2021/10/01 10:40 AM
                                  MMU privilegessr2021/09/21 11:07 AM
                                    MMU privilegesLinus Torvalds2021/09/21 01:49 PM
                            Virtually tagged L1-cachesKonrad Schwarz2021/09/16 04:18 AM
                          Virtually tagged L1-cachesCarson2021/09/16 01:12 PM
                            Virtually tagged L1-cachesanon22021/09/16 05:16 PM
                              Virtually tagged L1-cachesrwessel2021/09/16 06:29 PM
                          Virtually tagged L1-cachessr2021/09/20 04:20 AM
              Virtually tagged L1-caches---2021/09/08 02:28 PM
                Virtually tagged L1-cachesanonymou52021/09/08 08:28 PM
                  Virtually tagged L1-cachesanonymou52021/09/08 08:34 PM
                  Virtually tagged L1-caches---2021/09/09 10:14 AM
                    Virtually tagged L1-cachesanonymou52021/09/09 10:44 PM
                Multi-threading?David Kanter2021/09/09 09:32 PM
                  Multi-threading?---2021/09/10 09:19 AM
                Virtually tagged L1-cachessr2021/09/11 01:19 AM
                Virtually tagged L1-cachessr2021/09/11 01:36 AM
                  Virtually tagged L1-caches---2021/09/11 09:53 AM
                    Virtually tagged L1-cachessr2021/09/12 12:43 AM
                      Virtually tagged L1-cachesLinus Torvalds2021/09/12 11:10 AM
                        Virtually tagged L1-cachessr2021/09/12 11:57 AM
                          Virtually tagged L1-cachesdmcq2021/09/13 08:31 AM
                            Virtually tagged L1-cachessr2021/09/20 04:11 AM
            Virtually tagged L1-cachessr2021/09/11 02:49 AM
      Virtually tagged L1-cachesLinus Torvalds2021/09/08 12:34 PM
        Virtually tagged L1-cachesdmcq2021/09/09 02:46 AM
          Virtually tagged L1-cachesdmcq2021/09/09 02:58 AM
          Virtually tagged L1-cachessr2021/09/11 01:29 AM
            Virtually tagged L1-cachesdmcq2021/09/11 08:59 AM
              Virtually tagged L1-cachessr2021/09/12 12:57 AM
                Virtually tagged L1-cachesdmcq2021/09/12 08:44 AM
                  Virtually tagged L1-cachessr2021/09/12 09:48 AM
                    Virtually tagged L1-cachesdmcq2021/09/12 01:22 PM
                      Virtually tagged L1-cachessr2021/09/20 04:40 AM
    Where do you see this information? (NT)anon22021/09/09 02:45 AM
      Where do you see this information?sr2021/09/11 01:40 AM
        Where do you see this information?anon22021/09/11 01:53 AM
          Where do you see this information?sr2021/09/11 02:08 AM
            Thank you (NT)anon22021/09/11 04:31 PM
Reply to this Topic
Name:
Email:
Topic:
Body: No Text
How do you spell avocado?