A15/Octa scheduling problem..

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), May 13, 2013 2:19 pm
Room: Moderated Discussions
Anon (no.delete@this.thanks.com) on May 12, 2013 4:31 pm wrote:
> There is a complex patch on its away apparently, but not the for kernel
> currently used, so it may be a while before we see this fixed.

Quite frankly, the fact that the Exynos 5 currently only works in "either or" configuration almost certainly means that there is something fundamentally wrong with the hardware design, to the point where no amount of "complex patches" can fix it.

And no, I have absolutely no information about the Exynos to actually back that up. But quite frankly, the easiest way by far to do big.little would be to just treat it as CPU hotplug, which Linux has supported for a long time. At which time mixing things should be trivial from a software standpoint.

The fact that that isn't how people do it makes me go: "the hardware is buggered".

The most likely explanation for the buggery is that cache coherency doesn't work between little.big cores. So each cluster of four CPU's is cache-coherent within the cluster, but not across clusters. That is, afaik, what nVidia does for their 4+1 "vSMP" too.

And non-cache-coherent clusters are a bitch to handle. The easiest way to do it is to basically turn off one cluster, flush the caches on it, and turn on another one. Which would explain the reports of "S Lag" on the Exynos 5 octa. Switching between clusters is going to result in user-noticeable latencies.

I understand why you'd do this as a hardware designer - it makes your job oh so much easier. But I don't understand people who think it's actually a good idea in the bigger picture.

I now return you to your regularly scheduled excuse-fest from Wilco about how ARM did it right, and why cache coherency is a bad idea.

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TopicPosted ByDate
Haswell Graphicsyah2013/05/02 09:14 AM
  Haswell GraphicsLinus Torvalds2013/05/02 10:38 AM
    Haswell GraphicsEmil Briggs2013/05/03 05:51 AM
      4K displaysDoug S2013/05/03 02:51 PM
        4K displaysEmil Briggs2013/05/03 05:01 PM
    Haswell GraphicsGreg2013/05/03 07:45 AM
      Haswell Graphicsanon2013/05/05 12:32 AM
    Haswell GraphicsRobert Williams2013/05/03 04:16 PM
      Haswell GraphicsWilco2013/05/04 08:39 PM
        Haswell GraphicsDavid Kanter2013/05/04 10:29 PM
          Haswell Graphicsanon2013/05/05 12:35 AM
            Haswell GraphicsMichael S2013/05/05 03:20 AM
            Haswell GraphicsWilco2013/05/05 06:23 PM
          Haswell Graphicsx2013/05/05 04:18 AM
            Haswell GraphicsDavid Kanter2013/05/05 08:53 AM
              Haswell GraphicsMichael S2013/05/05 09:32 AM
                Haswell GraphicsDoug S2013/05/05 01:29 PM
              Haswell Graphicsnone2013/05/05 02:30 PM
          Haswell GraphicsWilco2013/05/05 06:06 PM
            A15, not in phonesDavid Kanter2013/05/06 12:23 AM
              A15, not in phonesnone2013/05/06 02:10 AM
              A15, not in phonesWilco2013/05/06 05:53 AM
                A15, not in phonesDoug S2013/05/06 11:32 AM
                  A15, not in phonesWilco2013/05/09 02:08 AM
                    A15, not in phonesMichael S2013/05/09 02:33 AM
                      links above invisible, sorryMichael S2013/05/09 02:37 AM
                        links above invisible, sorryWilco2013/05/12 10:20 AM
                          A15/Octa scheduling problem..Anon2013/05/12 04:31 PM
                            A15/Octa scheduling problem..none2013/05/12 11:26 PM
                              A15/Octa scheduling problem..David Kanter2013/05/13 01:00 AM
                                A15/Octa scheduling problem..none2013/05/13 02:36 AM
                            A15/Octa scheduling problem..Ricardo B2013/05/13 08:42 AM
                            A15/Octa scheduling problem..Linus Torvalds2013/05/13 02:19 PM
                              A15/Octa scheduling problem..Anon2013/05/13 03:06 PM
                                A15/Octa scheduling problem..Linus Torvalds2013/05/13 04:46 PM
                                  A15/Octa scheduling problem..Ricardo B2013/05/13 05:12 PM
                                  A15/Octa scheduling problem..Gabriele Svelto2013/05/14 01:28 AM
                                    Cache Coherent DMA on ARM A9Konrad Schwarz2013/05/14 07:18 AM
                                A15/Octa scheduling problem..Gabriele Svelto2013/05/14 01:25 AM
                                  A15/Octa scheduling problem..Anon2013/05/14 04:44 PM
                                    A15/Octa scheduling problem..Gabriele Svelto2013/05/15 12:24 AM
                            A15/Octa scheduling problem..Ricardo B2013/05/14 08:56 AM
                              A15/Octa scheduling problem..Exophase2013/05/14 11:21 AM
                    A15, not in phonesDoug S2013/05/09 08:02 PM
              A15, not in phonesRichardC2013/05/06 09:12 AM
                A15, not in phonesDavid Kanter2013/05/18 10:47 AM
                  Example claims that 20nm cost/transistor not happyMark Roulo2013/05/18 12:27 PM
                  A15, not in phonesExophase2013/05/18 02:41 PM
                    A15, not in phonesMichael S2013/05/18 03:20 PM
                      A15, not in phonesExophase2013/05/18 07:40 PM
                        A15, not in phonesMichael S2013/05/19 03:56 AM
                          A15, not in phonesmpx2013/05/21 01:31 AM
                            A15, not in phonesMichael S2013/05/21 09:21 AM
                    A15, not in phonesaaron spink2013/05/18 05:50 PM
                      A15, not in phonesWilco2013/05/19 06:20 AM
                        A15, not in phonesaaron spink2013/05/19 11:21 AM
                  A15, not in phonesWilco2013/05/19 06:39 AM
                    A15, not in phonesExophase2013/05/19 10:13 AM
                      A15, not in phonesWilco2013/05/19 01:02 PM
                        A15, not in phonesDavid Kanter2013/05/21 08:53 AM
                          A15, not in phonesWilco2013/05/21 04:55 PM
                    A15, not in phonesDavid Kanter2013/05/20 07:08 PM
                      A15, not in phonesDoug S2013/05/21 09:49 AM
                        A15, not in phonesnone2013/05/21 01:53 PM
                          A15, not in phonesDoug S2013/05/21 08:39 PM
                            A15, not in phonesDavid Kanter2013/05/21 11:44 PM
                      A15, not in phonesnone2013/05/21 01:36 PM
                      A15, not in phonesWilco2013/05/21 06:16 PM
                        A15, not in phonesMichael S2013/05/22 02:42 AM
                          A15, not in phonesGabriele Svelto2013/05/22 02:59 AM
                          A15, not in phonesWilco2013/05/22 04:40 AM
                            A15, not in phonesMichael S2013/05/22 07:21 AM
                              Why M0?N Monson2013/05/22 12:01 PM
                                Why M0?Dan Fay2013/05/23 01:07 PM
                                  Why 8-bit MCUs might be chosenPaul A. Clayton2013/05/23 02:10 PM
                                    Why 8-bit MCUs might be chosenDan Fay2013/05/24 06:06 AM
                                      Why 8-bit MCUs might be chosenRicardo B2013/05/24 07:15 AM
                                    Why 8-bit MCUs might be chosenDavid Hess2013/05/24 07:02 AM
                                    Why 8-bit MCUs might be chosenj2013/05/24 07:53 AM
                                      Thanks for the feedbackPaul A. Clayton2013/05/24 08:36 AM
                                    Why 8-bit MCUs might be chosenFelix2013/05/24 08:24 PM
                                      Why 8-bit MCUs might be chosenDavid Hess2013/05/27 09:32 PM
                                  Why 8-bit?N Monson2013/05/23 11:11 PM
                                    Why 8-bit?Dan Fay2013/05/24 06:00 AM
                                      Why 8-bit?Michael S2013/05/24 06:41 AM
                                      Why 8-bit?51 is fun2013/05/24 08:33 AM
                                        Why 8-bit?Michael S2013/05/24 08:45 AM
                                        Why 8-bit?Louis Gerbarg2013/05/25 06:50 AM
                                    8051 not the sanest 8-bitterPaul A. Clayton2013/05/24 06:17 AM
                                      8051 not the sanest 8-bitterMichael S2013/05/24 06:53 AM
                                        8051 not the sanest 8-bitterDavid Hess2013/05/27 10:32 PM
                          A15, not in phonesExophase2013/05/22 07:03 AM
                        A15, not in phonesDavid Kanter2013/05/22 07:32 AM
                          A15, not in phonesExophase2013/05/22 08:20 AM
                            A15, not in phonesDavid Kanter2013/05/22 11:36 AM
                              A15, not in phonesnone2013/05/23 01:52 PM
                  A15, not in phonesVincent Diepeveen2013/05/21 09:30 AM
            Haswell GraphicsMichael S2013/05/06 12:31 AM
            Haswell GraphicsDoug S2013/05/06 12:52 AM
  Haswell GraphicsDoug S2013/05/02 10:51 AM
  Haswell GraphicsAlberto2013/05/02 10:51 AM
    Haswell Graphicssomeone2013/05/02 11:14 AM
      But...Alberto2013/05/03 12:49 PM
        Tied off to stop the bleedingsomeone2013/05/04 12:21 PM
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