Avoiding ping pong

By: ook (ook.delete@this.oook.com), December 11, 2014 3:31 am
Room: Moderated Discussions
Linus Torvalds (torvalds.delete@this.linux-foundation.org) on December 8, 2014 1:34 pm wrote:
> You won't get scaling for much longer, and current trends are actually for lower power anyway. So what's the
> upside of pushing the whole parallelism snake-oil? We know that we need fairly complex OoO CPU's anyway, because
> people want reasonable performance and it turns out OoO is actually more efficient than slow in-order.
>

I agree in most parts but OoO can be made even more efficient. Future CPUs will likely still be based on OoO but my take would be that increasingly clever compilers add additional hints to the executables that allow the CPU to save power.

Among others I'm thinking of these types of hints:
* resize the (active) instruction window dynamically. If there is not much ILP to gain, don't waste energy searching for it in vain.
* define independent code strands. Present parallel work on the silver tablet.
* define how an efficient loop schedule would look like. Don't waste a couple of loop iterations with a bad schedule.
* flag that a piece of code is not on the critical path to a barrier. Don't waste power for no gain.


Without those hints the CPU would run just as well as today.
Also it might be a good idea to let the CPU evaluate if the hints were worthwhile and ignore them if not. Otherwise old hints in legacy code might actually decrease performance.
Or a versioning system for the hints makes sense and let the CPU decide whether hints originally written for microarchitecture A still make sense on microarchitecture B.
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ARMv8 getting atomic operationsdmcq2014/12/02 05:32 PM
  ARMv8 getting atomic operationsMaynard Handley2014/12/02 07:33 PM
    ARMv8 getting atomic operationsDoug S2014/12/02 10:30 PM
      ARMv8 getting atomic operationsdmcq2014/12/03 03:16 AM
      ARMv8 getting atomic operationsMaynard Handley2014/12/03 09:20 AM
      ARMv8 getting atomic operationsBrett2014/12/03 04:46 PM
    ARMv8 getting atomic operationsAndreas2014/12/03 06:51 AM
      ARMv8 getting atomic operationsLinus Torvalds2014/12/03 11:15 AM
        ARMv8 getting atomic operationsanon2014/12/03 05:08 PM
          Guaranteed transactionsPaul A. Clayton2014/12/03 08:04 PM
            Guaranteed transactionsanon2014/12/03 08:38 PM
              Avoiding ping pongPaul A. Clayton2014/12/04 09:11 AM
                Avoiding ping ponganon2014/12/04 10:15 AM
                  OoO window is limitedPaul A. Clayton2014/12/04 01:06 PM
                Avoiding ping pongAaron Spink2014/12/04 12:01 PM
                  Avoiding ping pongKonrad Schwarz2014/12/04 01:10 PM
                    Avoiding ping pongAaron Spink2014/12/04 02:31 PM
                    Avoiding ping pongGabriele Svelto2014/12/04 02:49 PM
                      Avoiding ping pongKonrad Schwarz2014/12/04 11:08 PM
                        Avoiding ping pongGabriele Svelto2014/12/05 12:04 AM
                          Avoiding ping pongEric Bron nli2014/12/05 02:28 AM
                            Avoiding ping pongKonrad Schwarz2014/12/05 03:37 AM
                              Avoiding ping pongEric Bron nli2014/12/05 04:23 AM
                                Avoiding ping pongKlimax2014/12/05 05:47 AM
                                  Avoiding ping pongEric Bron2014/12/05 06:24 AM
                              Avoiding ping pongGabriele Svelto2014/12/05 10:38 AM
                                Avoiding ping pongKonrad Schwarz2014/12/07 02:28 PM
                                  Avoiding ping pongGabriele Svelto2014/12/08 07:10 PM
                                    Avoiding ping pongKonrad Schwarz2014/12/09 05:12 AM
                                      Avoiding ping pongGabriele Svelto2014/12/09 07:31 AM
                                        Avoiding ping ponganon2014/12/09 11:24 PM
                            Avoiding ping pongGabriele Svelto2014/12/05 10:17 AM
                              Avoiding ping pongEric Bron2014/12/05 10:32 AM
                                Avoiding ping pongGabriele Svelto2014/12/05 12:45 PM
                                  Avoiding ping pongEric Bron2014/12/06 02:20 AM
                                    Avoiding ping pongnksingh2014/12/06 03:42 AM
                                      Avoiding ping pongEric Bron2014/12/06 04:04 AM
                                        Avoiding ping pongGiGNiC2014/12/06 06:27 AM
                                          Avoiding ping pongEric Bron nli2014/12/06 06:44 AM
                                          Avoiding ping pongEric Bron2014/12/06 07:07 AM
                                            Avoiding ping pongnksingh2014/12/07 04:06 PM
                                              Avoiding ping pongEric Bron2014/12/08 04:17 AM
                                                Avoiding ping pongGiGNiC2014/12/08 11:53 AM
                                                Avoiding ping pongnksingh2014/12/08 05:53 PM
                                                  Avoiding ping pongEric Bron2014/12/09 01:33 AM
                                    Avoiding ping pongdmsc2014/12/06 04:12 AM
                                      Avoiding ping pongEric Bron2014/12/06 04:25 AM
                                        Avoiding ping pongKlimax2014/12/06 05:49 AM
                                          Avoiding ping pongrwessel2014/12/07 02:34 AM
                                        Avoiding ping pongdmsc2014/12/06 07:39 AM
                                        Avoiding ping pongKonrad Schwarz2014/12/07 02:37 PM
                                          Avoiding ping pongMichael S2014/12/07 04:37 PM
                                            Avoiding ping pongKonrad Schwarz2014/12/08 04:35 AM
                          Avoiding ping pongKonrad Schwarz2014/12/05 03:30 AM
                        Avoiding ping pongLinus Torvalds2014/12/05 12:58 PM
                          Avoiding ping pongEric Bron2014/12/06 02:42 AM
                            Avoiding ping pongnksingh2014/12/06 03:51 AM
                              Avoiding ping pongEric Bron2014/12/06 04:08 AM
                            Avoiding ping pongLinus Torvalds2014/12/06 01:25 PM
                              Avoiding ping pongnksingh2014/12/07 03:26 PM
                                Avoiding ping pongEric Bron2014/12/08 04:35 AM
                                  Avoiding ping pongBrett2014/12/08 10:00 AM
                                    Avoiding ping pongEric Bron2014/12/08 10:48 AM
                                    Avoiding ping pongrwessel2014/12/08 12:52 PM
                                      Avoiding ping pongBrett2014/12/08 01:58 PM
                                      Avoiding ping pongDoug S2014/12/08 02:04 PM
                              Avoiding ping pongJouni Osmala2014/12/08 02:45 AM
                                Avoiding ping ponganon2014/12/08 05:44 AM
                                  Avoiding ping pongJouni Osmala2014/12/08 01:10 PM
                                    Avoiding ping pongLinus Torvalds2014/12/08 01:34 PM
                                      Avoiding ping pongJouni Osmala2014/12/08 03:47 PM
                                        Avoiding ping pongLinus Torvalds2014/12/08 08:08 PM
                                          Avoiding ping pongGabriele Svelto2014/12/09 07:48 AM
                                            Avoiding ping pongMaynard Handley2014/12/09 11:41 AM
                                              Avoiding ping pongPatrick Chase2014/12/09 01:06 PM
                                              Avoiding ping pongGabriele Svelto2014/12/09 01:52 PM
                                                Avoiding ping pongPatrick Chase2014/12/09 02:08 PM
                                            Why read RWT or Reddit when you can get journalists to do it for you?Rob Thorpe2015/01/02 08:20 AM
                                              Why read RWT or Reddit when you can get journalists to do it for you?juanrga2015/01/02 11:21 AM
                                                Why read RWT or Reddit when you can get journalists to do it for you?EduardoS2015/01/02 11:37 AM
                                                  Why read RWT or Reddit when you can get journalists to do it for you?juanrga2015/01/03 12:00 PM
                                                Why read RWT or Reddit when you can get journalists to do it for you?Eric Bron nli2015/01/02 02:28 PM
                                                  Why read RWT or Reddit when you can get journalists to do it for you?juanrga2015/01/03 12:02 PM
                                                    Why read RWT or Reddit when you can get journalists to do it for you?Michael S2015/01/03 12:36 PM
                                                      Why read RWT or Reddit when you can get journalists to do it for you?juanrga2015/01/03 01:11 PM
                                                        Why read RWT or Reddit when you can get journalists to do it for you?Michael S2015/01/03 01:30 PM
                                                          Why read RWT or Reddit when you can get journalists to do it for you?Eric Bron2015/01/03 02:57 PM
                                                            KNL cacheDavid Kanter2015/01/03 07:36 PM
                                                              KNL cacheEric Bron2015/01/04 03:34 AM
                                                                KNL cacheMichael S2015/01/04 04:11 AM
                                                                  KNL cacheEric Bron2015/01/04 04:57 AM
                                                                    KNL cacheMichael S2015/01/04 05:21 AM
                                                                      KNL cacheEric Bron2015/01/04 05:58 AM
                                                          Why read RWT or Reddit when you can get journalists to do it for you?juanrga2015/01/07 05:47 AM
                                                            Why read RWT or Reddit when you can get journalists to do it for you?Michael S2015/01/07 08:27 AM
                                                              Manycores vs multicoresjuanrga2015/01/10 04:10 PM
                                                                Manycores vs multicoresAaron Spink2015/01/10 05:32 PM
                                                                  Manycores vs multicoresjuanrga2015/01/10 06:32 PM
                                                                    Manycores vs multicoresExophase2015/01/10 06:49 PM
                                                                      Manycores vs multicoresjuanrga2015/01/10 08:21 PM
                                                                        Manycores vs multicoresExophase2015/01/10 08:51 PM
                                                                        Manycores vs multicoresAaron Spink2015/01/10 09:03 PM
                                                                    Manycores vs multicoresAaron Spink2015/01/10 07:21 PM
                                                                      Manycores vs multicoresjuanrga2015/01/10 08:25 PM
                                                                        Manycores vs multicoresAaron Spink2015/01/10 09:11 PM
                                                                          Manycores vs multicoresJouni Osmala2015/01/11 04:50 AM
                                                                            Manycores vs multicoresjuanrga2015/01/11 08:58 AM
                                                                            Manycores vs multicorescoppice2015/01/12 10:01 PM
                                                                              Manycores vs multicoresJouni Osmala2015/01/13 04:38 AM
                                                                        Manycores vs multicoresanon2015/01/11 03:19 AM
                                                                      Manycores vs multicoresMichael S2015/01/11 05:44 AM
                                                                        Manycores vs multicoresAaron Spink2015/01/11 05:55 PM
                                                                          Manycores vs multicoresMichael S2015/01/12 04:41 AM
                                                                            Manycores vs multicoresEric Bron2015/01/12 06:29 AM
                                                                              Manycores vs multicoresEric Bron2015/01/12 06:30 AM
                                                        Why read RWT or Reddit when you can get journalists to do it for you?Eric Bron2015/01/03 02:54 PM
                                                          Why read RWT or Reddit when you can get journalists to do it for you?juanrga2015/01/07 05:48 AM
                                                            Why read RWT or Reddit when you can get journalists to do it for you?Eric Bron2015/01/07 07:41 AM
                                                              Manycores vs multicoresjuanrga2015/01/10 04:14 PM
                                                    Why read RWT or Reddit when you can get journalists to do it for you?Eric Bron2015/01/03 02:42 PM
                                                      Why read RWT or Reddit when you can get journalists to do it for you?juanrga2015/01/07 06:03 AM
                                                        Why read RWT or Reddit when you can get journalists to do it for you?Eric Bron2015/01/07 07:45 AM
                                                        Why read RWT or Reddit when you can get journalists to do it for you?Linus Torvalds2015/01/08 03:09 PM
                                                          Pink unicorns for salejuanrga2015/01/10 05:09 PM
                                                        Intentionally picking a competitors slow part is cheating ...Mark Roulo2015/01/08 06:37 PM
                                                          Intentionally picking a competitors slow part is cheating ...coppice2015/01/08 11:38 PM
                                                            Intentionally picking a competitors slow part is cheating ...Mark Roulo2015/01/09 09:13 AM
                                                              Intentionally picking a competitors slow part is cheating ...Anon2015/01/10 02:00 AM
                                                              Intentionally picking a competitors slow part is cheating ...David Hess2015/01/11 01:03 PM
                                                            Intentionally picking a competitors slow part is cheating ...someone2015/01/09 10:31 AM
                                                              Intentionally picking a competitors slow part is cheating ...coppice2015/01/12 09:45 PM
                                                                Intentionally picking a competitors slow part is cheating ...coppice2015/01/12 09:47 PM
                                                                  Intentionally picking a competitors slow part is cheating ...Michael S2015/01/13 07:53 AM
                                                                    Intentionally picking a competitors slow part is cheating ...coppice2015/01/13 09:44 AM
                                                                      Intentionally picking a competitors slow part is cheating ...Michael S2015/01/13 10:01 AM
                                                                        Intentionally picking a competitors slow part is cheating ...coppice2015/01/13 08:35 PM
                                                                      Core sizesjuanrga2015/01/13 12:28 PM
                                                          NVIDIA'S FIRST CPU IS A WINNER (Linley Gwennap)juanrga2015/01/10 04:34 PM
                                                        Why read RWT or Reddit when you can get journalists to do it for you?Patrick Chase2015/01/08 07:02 PM
                                                        Why read RWT or Reddit when you can get journalists to do it for you?coppice2015/01/08 10:18 PM
                                                          Why read RWT or Reddit when you can get journalists to do it for you?Patrick Chase2015/01/09 11:54 AM
                                                            Why read RWT or Reddit when you can get journalists to do it for you?Mark Roulo2015/01/09 12:59 PM
                                                              Why read RWT or Reddit when you can get journalists to do it for you?Patrick Chase2015/01/09 03:20 PM
                                                                Why read RWT or Reddit when you can get journalists to do it for you?Eric Bron2015/01/09 03:30 PM
                                                            Alternatives to OOOE (again)juanrga2015/01/10 04:50 PM
                                                              Alternatives to OOOE (again)David Kanter2015/01/11 12:10 AM
                                                                Alternatives to OOOE (again)juanrga2015/01/11 08:30 AM
                                                            Why read RWT or Reddit when you can get journalists to do it for you?Gabriele Svelto2015/01/11 12:53 AM
                                              Why read RWT or Reddit when you can get journalists to do it for you?Fake Linus Torvalds2015/01/03 12:14 PM
                                                Why read RWT or Reddit when you can get journalists to do it for you?Rob Thorpe2015/01/03 08:25 PM
                                          Avoiding ping pongMaynard Handley2014/12/09 11:33 AM
                                            Avoiding ping pongPatrick Chase2014/12/09 01:54 PM
                                              Avoiding ping pongMaynard Handley2014/12/09 06:56 PM
                                      Avoiding ping pongSalvatore De Dominicis2014/12/09 08:51 AM
                                        Avoiding ping pongPatrick Chase2014/12/09 02:00 PM
                                      Avoiding ping pongook2014/12/11 03:31 AM
                                      Avoiding ping pongArt Scott2014/12/19 10:19 PM
                                        Avoiding ping pongEric Bron nli2014/12/20 04:05 AM
                                      What about specialization?Troll?2015/01/02 07:55 AM
                                        What about specialization?Ungo2015/01/04 03:27 PM
                                      Avoiding ping pongfewwef2015/01/05 08:16 PM
                                      Avoiding ping pongV.Krishn2015/01/08 06:11 AM
                                    Avoiding ping pongGabriele Svelto2014/12/08 07:32 PM
                                    Avoiding ping ponganon2014/12/08 11:37 PM
                            Avoiding ping pongKonrad Schwarz2014/12/10 06:23 AM
                              Avoiding ping pongLinus Torvalds2014/12/10 11:56 AM
                          Object reference lockingDavid W2014/12/08 11:36 PM
                            Object reference lockingPatrick Chase2014/12/09 04:52 PM
                              Object reference lockingDavid W2014/12/11 05:18 AM
                    ISA != interface for "most programmers"Paul A. Clayton2014/12/04 03:34 PM
                      ISA != interface for "most programmers"rwessel2014/12/04 07:50 PM
                  Interesting! (exporting hot lines/cache-aware ISA); "Please sir, I want some more" (NT)Paul A. Clayton2014/12/04 02:26 PM
                  Avoiding ping pongMichael S2014/12/06 03:48 PM
          ARMv8 getting atomic operationsLinus Torvalds2014/12/04 12:05 PM
            LL/SC idiom recognition is not admitting RMW superiorityPaul A. Clayton2014/12/04 02:34 PM
            ARMv8 getting atomic operationsanon2014/12/04 10:17 PM
    ARMv8 getting atomic operationsPatrick Chase2014/12/03 12:09 PM
  limited ordernksingh2014/12/04 10:17 PM
    I didn't understand this either. (NT)Konrad Schwarz2014/12/04 10:32 PM
    limited orderdmcq2014/12/05 02:13 AM
    limited orderbakaneko2014/12/05 09:11 AM
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