Avoiding ping pong

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), December 8, 2014 1:34 pm
Room: Moderated Discussions
Jouni Osmala (josmala.delete@this.cc.hut.fi) on December 8, 2014 1:10 pm wrote:
>
> I'm assuming that 90+% of programs already run fast enough and they don't matter for this.
> Its all about asking question in what use current computers are too slow , and can you parallerize
> that or are those cases already parallel. And I'm assuming you can parallerize atleast 10%
> of those times where user waits CPU for long enough to actually notice it.

What's the advantage?

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.

The whole "let's parallelize" thing is a huge waste of everybody's time. There's this huge body of "knowledge" that parallel is somehow more efficient, and that whole huge body is pure and utter garbage. Big caches are efficient. Parallel stupid small cores without caches are horrible unless you have a very specific load that is hugely regular (ie graphics).

Nobody is ever going to go backwards from where we are today. Those complex OoO cores aren't going away. Scaling isn't going to continue forever, and people want mobility, so the crazies talking about scaling to hundreds of cores are just that - crazy. Why give them an ounce of credibility?

Where the hell do you envision that those magical parallel algorithms would be used?

The only place where parallelism matters is in graphics or on the server side, where we already largely have it. Pushing it anywhere else is just pointless.

So give up on parallelism already. It's not going to happen. End users are fine with roughly on the order of four cores, and you can't fit any more anyway without using too much energy to be practical in that space. And nobody sane would make the cores smaller and weaker in order to fit more of them - the only reason to make them smaller and weaker is because you want to go even further down in power use, so you'd still not have lots of those weak cores.

So the whole argument that people should parallelise their code is fundamentally flawed. It rests on incorrect assumptions. It's a fad that has been going on too long.

Parallel code makes sense in the few cases I mentioned, where we already largely have it covered, because in the server space, people have been parallel for a long time.

It does not necessarily make sense elsewhere. Even in completely new areas that we don't do today because you cant' afford it. If you want to do low-power ubiquotous computer vision etc, I can pretty much guarantee that you're not going to do it with code on a GP CPU. You're likely not even going to do it on a GPU because even that is too expensive (power wise), but with specialized hardware, probably based on some neural network model.

Give it up. The whole "parallel computing is the future" is a bunch of crock.

Linus
< Previous Post in ThreadNext Post in Thread >
TopicPosted ByDate
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
Reply to this Topic
Name:
Email:
Topic:
Body: No Text
How do you spell green?