Avoiding ping pong

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), December 10, 2014 11:56 am
Room: Moderated Discussions
Konrad Schwarz (no.spam.delete@this.no.spam) on December 10, 2014 6:23 am wrote:
>
> And this really clinches the argument: to use reference counts, you need acyclic (directed) graphs.

Note that for cyclic graphs, you'd still often use atomics, just not generally reference counts (or quite possibly in addition to reference counts - with the reference count being used to track usage separate from the internal graph usage).

Think of the usual trivial traversal model: walk the graph, do something to each entry, and set a flag that you've done it (exactly because of cycles). Now, think about what happens in threaded environments. Yeah, that "do something and set a flag" is suddenly very much racy.

And for exactly the same reason as for reference counts, using an external lock is problematic. So you tend to want to have lockless models - ie atomics. Instead of an atomic increment, it might be an atomic compare-and-exchange ("set state to 'in progress', but only if nobody else already did so).

(Combine with "use sequence number instead of simple flag", in order to avoid things like "have to do a second traversal just to clear the flag again to do the next round")

And to make things more interesting still, it's not usually a "one or the other" situation, it's often a combination of things. I already mentioned using reference counts in addition to that whole "have I handled this already" flag/sequence count. But you then generally have other models too, including straight traditional locking on a higher level (maybe you want to traverse the graph concurrently, but you do not want to have multiple different traversals active at the same time).

And you can do various other fancy tricks. In Linux, we introduced this notion of a "reflock" that is both a reference count and a lock, and has very specific semantic guarantees (reference counts can be incremented atomically without taking the lock, but only if nobody else holds the lock etc).

And yes, you can often hide this from the user. A garbage collector is obviously doing exactly that kind of graph traversal (and often uses sequence counts not just to avoid that dual traversal, but for more fundamental reasons: to avoid a certain class of races you may well want to delay actual freeing, and use the generational data of the sequence count to free things only after they haven't been reachable for two or more cycles).

Of course, once you get into more complex situations, you do end up often effectively wanting LL/SC rather than just a simple unconditional atomic RMW instruction. So I'm absolutely not against the LL/SC model, I just think that you do want both, and I think having ARM64 add atomic rmw operations is actually a good thing.

Linus
< Previous Post in ThreadNext Post in Thread >
TopicPosted ByDate
ARMv8 getting atomic operationsdmcq12/02/14 05:32 PM
  ARMv8 getting atomic operationsMaynard Handley12/02/14 07:33 PM
    ARMv8 getting atomic operationsDoug S12/02/14 10:30 PM
      ARMv8 getting atomic operationsdmcq12/03/14 03:16 AM
      ARMv8 getting atomic operationsMaynard Handley12/03/14 09:20 AM
      ARMv8 getting atomic operationsBrett12/03/14 04:46 PM
    ARMv8 getting atomic operationsAndreas12/03/14 06:51 AM
      ARMv8 getting atomic operationsLinus Torvalds12/03/14 11:15 AM
        ARMv8 getting atomic operationsanon12/03/14 05:08 PM
          Guaranteed transactionsPaul A. Clayton12/03/14 08:04 PM
            Guaranteed transactionsanon12/03/14 08:38 PM
              Avoiding ping pongPaul A. Clayton12/04/14 09:11 AM
                Avoiding ping ponganon12/04/14 10:15 AM
                  OoO window is limitedPaul A. Clayton12/04/14 01:06 PM
                Avoiding ping pongAaron Spink12/04/14 12:01 PM
                  Avoiding ping pongKonrad Schwarz12/04/14 01:10 PM
                    Avoiding ping pongAaron Spink12/04/14 02:31 PM
                    Avoiding ping pongGabriele Svelto12/04/14 02:49 PM
                      Avoiding ping pongKonrad Schwarz12/04/14 11:08 PM
                        Avoiding ping pongGabriele Svelto12/05/14 12:04 AM
                          Avoiding ping pongEric Bron nli12/05/14 02:28 AM
                            Avoiding ping pongKonrad Schwarz12/05/14 03:37 AM
                              Avoiding ping pongEric Bron nli12/05/14 04:23 AM
                                Avoiding ping pongKlimax12/05/14 05:47 AM
                                  Avoiding ping pongEric Bron12/05/14 06:24 AM
                              Avoiding ping pongGabriele Svelto12/05/14 10:38 AM
                                Avoiding ping pongKonrad Schwarz12/07/14 02:28 PM
                                  Avoiding ping pongGabriele Svelto12/08/14 07:10 PM
                                    Avoiding ping pongKonrad Schwarz12/09/14 05:12 AM
                                      Avoiding ping pongGabriele Svelto12/09/14 07:31 AM
                                        Avoiding ping ponganon12/09/14 11:24 PM
                            Avoiding ping pongGabriele Svelto12/05/14 10:17 AM
                              Avoiding ping pongEric Bron12/05/14 10:32 AM
                                Avoiding ping pongGabriele Svelto12/05/14 12:45 PM
                                  Avoiding ping pongEric Bron12/06/14 02:20 AM
                                    Avoiding ping pongnksingh12/06/14 03:42 AM
                                      Avoiding ping pongEric Bron12/06/14 04:04 AM
                                        Avoiding ping pongGiGNiC12/06/14 06:27 AM
                                          Avoiding ping pongEric Bron nli12/06/14 06:44 AM
                                          Avoiding ping pongEric Bron12/06/14 07:07 AM
                                            Avoiding ping pongnksingh12/07/14 04:06 PM
                                              Avoiding ping pongEric Bron12/08/14 04:17 AM
                                                Avoiding ping pongGiGNiC12/08/14 11:53 AM
                                                Avoiding ping pongnksingh12/08/14 05:53 PM
                                                  Avoiding ping pongEric Bron12/09/14 01:33 AM
                                    Avoiding ping pongdmsc12/06/14 04:12 AM
                                      Avoiding ping pongEric Bron12/06/14 04:25 AM
                                        Avoiding ping pongKlimax12/06/14 05:49 AM
                                          Avoiding ping pongrwessel12/07/14 02:34 AM
                                        Avoiding ping pongdmsc12/06/14 07:39 AM
                                        Avoiding ping pongKonrad Schwarz12/07/14 02:37 PM
                                          Avoiding ping pongMichael S12/07/14 04:37 PM
                                            Avoiding ping pongKonrad Schwarz12/08/14 04:35 AM
                          Avoiding ping pongKonrad Schwarz12/05/14 03:30 AM
                        Avoiding ping pongLinus Torvalds12/05/14 12:58 PM
                          Avoiding ping pongEric Bron12/06/14 02:42 AM
                            Avoiding ping pongnksingh12/06/14 03:51 AM
                              Avoiding ping pongEric Bron12/06/14 04:08 AM
                            Avoiding ping pongLinus Torvalds12/06/14 01:25 PM
                              Avoiding ping pongnksingh12/07/14 03:26 PM
                                Avoiding ping pongEric Bron12/08/14 04:35 AM
                                  Avoiding ping pongBrett12/08/14 10:00 AM
                                    Avoiding ping pongEric Bron12/08/14 10:48 AM
                                    Avoiding ping pongrwessel12/08/14 12:52 PM
                                      Avoiding ping pongBrett12/08/14 01:58 PM
                                      Avoiding ping pongDoug S12/08/14 02:04 PM
                              Avoiding ping pongJouni Osmala12/08/14 02:45 AM
                                Avoiding ping ponganon12/08/14 05:44 AM
                                  Avoiding ping pongJouni Osmala12/08/14 01:10 PM
                                    Avoiding ping pongLinus Torvalds12/08/14 01:34 PM
                                      Avoiding ping pongJouni Osmala12/08/14 03:47 PM
                                        Avoiding ping pongLinus Torvalds12/08/14 08:08 PM
                                          Avoiding ping pongGabriele Svelto12/09/14 07:48 AM
                                            Avoiding ping pongMaynard Handley12/09/14 11:41 AM
                                              Avoiding ping pongPatrick Chase12/09/14 01:06 PM
                                              Avoiding ping pongGabriele Svelto12/09/14 01:52 PM
                                                Avoiding ping pongPatrick Chase12/09/14 02:08 PM
                                            Why read RWT or Reddit when you can get journalists to do it for you?Rob Thorpe01/02/15 08:20 AM
                                              Why read RWT or Reddit when you can get journalists to do it for you?juanrga01/02/15 11:21 AM
                                                Why read RWT or Reddit when you can get journalists to do it for you?EduardoS01/02/15 11:37 AM
                                                  Why read RWT or Reddit when you can get journalists to do it for you?juanrga01/03/15 12:00 PM
                                                Why read RWT or Reddit when you can get journalists to do it for you?Eric Bron nli01/02/15 02:28 PM
                                                  Why read RWT or Reddit when you can get journalists to do it for you?juanrga01/03/15 12:02 PM
                                                    Why read RWT or Reddit when you can get journalists to do it for you?Michael S01/03/15 12:36 PM
                                                      Why read RWT or Reddit when you can get journalists to do it for you?juanrga01/03/15 01:11 PM
                                                        Why read RWT or Reddit when you can get journalists to do it for you?Michael S01/03/15 01:30 PM
                                                          Why read RWT or Reddit when you can get journalists to do it for you?Eric Bron01/03/15 02:57 PM
                                                            KNL cacheDavid Kanter01/03/15 07:36 PM
                                                              KNL cacheEric Bron01/04/15 03:34 AM
                                                                KNL cacheMichael S01/04/15 04:11 AM
                                                                  KNL cacheEric Bron01/04/15 04:57 AM
                                                                    KNL cacheMichael S01/04/15 05:21 AM
                                                                      KNL cacheEric Bron01/04/15 05:58 AM
                                                          Why read RWT or Reddit when you can get journalists to do it for you?juanrga01/07/15 05:47 AM
                                                            Why read RWT or Reddit when you can get journalists to do it for you?Michael S01/07/15 08:27 AM
                                                              Manycores vs multicoresjuanrga01/10/15 04:10 PM
                                                                Manycores vs multicoresAaron Spink01/10/15 05:32 PM
                                                                  Manycores vs multicoresjuanrga01/10/15 06:32 PM
                                                                    Manycores vs multicoresExophase01/10/15 06:49 PM
                                                                      Manycores vs multicoresjuanrga01/10/15 08:21 PM
                                                                        Manycores vs multicoresExophase01/10/15 08:51 PM
                                                                        Manycores vs multicoresAaron Spink01/10/15 09:03 PM
                                                                    Manycores vs multicoresAaron Spink01/10/15 07:21 PM
                                                                      Manycores vs multicoresjuanrga01/10/15 08:25 PM
                                                                        Manycores vs multicoresAaron Spink01/10/15 09:11 PM
                                                                          Manycores vs multicoresJouni Osmala01/11/15 04:50 AM
                                                                            Manycores vs multicoresjuanrga01/11/15 08:58 AM
                                                                            Manycores vs multicorescoppice01/12/15 10:01 PM
                                                                              Manycores vs multicoresJouni Osmala01/13/15 04:38 AM
                                                                        Manycores vs multicoresanon01/11/15 03:19 AM
                                                                      Manycores vs multicoresMichael S01/11/15 05:44 AM
                                                                        Manycores vs multicoresAaron Spink01/11/15 05:55 PM
                                                                          Manycores vs multicoresMichael S01/12/15 04:41 AM
                                                                            Manycores vs multicoresEric Bron01/12/15 06:29 AM
                                                                              Manycores vs multicoresEric Bron01/12/15 06:30 AM
                                                        Why read RWT or Reddit when you can get journalists to do it for you?Eric Bron01/03/15 02:54 PM
                                                          Why read RWT or Reddit when you can get journalists to do it for you?juanrga01/07/15 05:48 AM
                                                            Why read RWT or Reddit when you can get journalists to do it for you?Eric Bron01/07/15 07:41 AM
                                                              Manycores vs multicoresjuanrga01/10/15 04:14 PM
                                                    Why read RWT or Reddit when you can get journalists to do it for you?Eric Bron01/03/15 02:42 PM
                                                      Why read RWT or Reddit when you can get journalists to do it for you?juanrga01/07/15 06:03 AM
                                                        Why read RWT or Reddit when you can get journalists to do it for you?Eric Bron01/07/15 07:45 AM
                                                        Why read RWT or Reddit when you can get journalists to do it for you?Linus Torvalds01/08/15 03:09 PM
                                                          Pink unicorns for salejuanrga01/10/15 05:09 PM
                                                        Intentionally picking a competitors slow part is cheating ...Mark Roulo01/08/15 06:37 PM
                                                          Intentionally picking a competitors slow part is cheating ...coppice01/08/15 11:38 PM
                                                            Intentionally picking a competitors slow part is cheating ...Mark Roulo01/09/15 09:13 AM
                                                              Intentionally picking a competitors slow part is cheating ...Anon01/10/15 02:00 AM
                                                              Intentionally picking a competitors slow part is cheating ...David Hess01/11/15 01:03 PM
                                                            Intentionally picking a competitors slow part is cheating ...someone01/09/15 10:31 AM
                                                              Intentionally picking a competitors slow part is cheating ...coppice01/12/15 09:45 PM
                                                                Intentionally picking a competitors slow part is cheating ...coppice01/12/15 09:47 PM
                                                                  Intentionally picking a competitors slow part is cheating ...Michael S01/13/15 07:53 AM
                                                                    Intentionally picking a competitors slow part is cheating ...coppice01/13/15 09:44 AM
                                                                      Intentionally picking a competitors slow part is cheating ...Michael S01/13/15 10:01 AM
                                                                        Intentionally picking a competitors slow part is cheating ...coppice01/13/15 08:35 PM
                                                                      Core sizesjuanrga01/13/15 12:28 PM
                                                          NVIDIA'S FIRST CPU IS A WINNER (Linley Gwennap)juanrga01/10/15 04:34 PM
                                                        Why read RWT or Reddit when you can get journalists to do it for you?Patrick Chase01/08/15 07:02 PM
                                                        Why read RWT or Reddit when you can get journalists to do it for you?coppice01/08/15 10:18 PM
                                                          Why read RWT or Reddit when you can get journalists to do it for you?Patrick Chase01/09/15 11:54 AM
                                                            Why read RWT or Reddit when you can get journalists to do it for you?Mark Roulo01/09/15 12:59 PM
                                                              Why read RWT or Reddit when you can get journalists to do it for you?Patrick Chase01/09/15 03:20 PM
                                                                Why read RWT or Reddit when you can get journalists to do it for you?Eric Bron01/09/15 03:30 PM
                                                            Alternatives to OOOE (again)juanrga01/10/15 04:50 PM
                                                              Alternatives to OOOE (again)David Kanter01/11/15 12:10 AM
                                                                Alternatives to OOOE (again)juanrga01/11/15 08:30 AM
                                                            Why read RWT or Reddit when you can get journalists to do it for you?Gabriele Svelto01/11/15 12:53 AM
                                              Why read RWT or Reddit when you can get journalists to do it for you?Fake Linus Torvalds01/03/15 12:14 PM
                                                Why read RWT or Reddit when you can get journalists to do it for you?Rob Thorpe01/03/15 08:25 PM
                                          Avoiding ping pongMaynard Handley12/09/14 11:33 AM
                                            Avoiding ping pongPatrick Chase12/09/14 01:54 PM
                                              Avoiding ping pongMaynard Handley12/09/14 06:56 PM
                                      Avoiding ping pongSalvatore De Dominicis12/09/14 08:51 AM
                                        Avoiding ping pongPatrick Chase12/09/14 02:00 PM
                                      Avoiding ping pongook12/11/14 03:31 AM
                                      Avoiding ping pongArt Scott12/19/14 10:19 PM
                                        Avoiding ping pongEric Bron nli12/20/14 04:05 AM
                                      What about specialization?Troll?01/02/15 07:55 AM
                                        What about specialization?Ungo01/04/15 03:27 PM
                                      Avoiding ping pongfewwef01/05/15 08:16 PM
                                      Avoiding ping pongV.Krishn01/08/15 06:11 AM
                                    Avoiding ping pongGabriele Svelto12/08/14 07:32 PM
                                    Avoiding ping ponganon12/08/14 11:37 PM
                            Avoiding ping pongKonrad Schwarz12/10/14 06:23 AM
                              Avoiding ping pongLinus Torvalds12/10/14 11:56 AM
                          Object reference lockingDavid W12/08/14 11:36 PM
                            Object reference lockingPatrick Chase12/09/14 04:52 PM
                              Object reference lockingDavid W12/11/14 05:18 AM
                    ISA != interface for "most programmers"Paul A. Clayton12/04/14 03:34 PM
                      ISA != interface for "most programmers"rwessel12/04/14 07:50 PM
                  Interesting! (exporting hot lines/cache-aware ISA); "Please sir, I want some more" (NT)Paul A. Clayton12/04/14 02:26 PM
                  Avoiding ping pongMichael S12/06/14 03:48 PM
          ARMv8 getting atomic operationsLinus Torvalds12/04/14 12:05 PM
            LL/SC idiom recognition is not admitting RMW superiorityPaul A. Clayton12/04/14 02:34 PM
            ARMv8 getting atomic operationsanon12/04/14 10:17 PM
    ARMv8 getting atomic operationsPatrick Chase12/03/14 12:09 PM
  limited ordernksingh12/04/14 10:17 PM
    I didn't understand this either. (NT)Konrad Schwarz12/04/14 10:32 PM
    limited orderdmcq12/05/14 02:13 AM
    limited orderbakaneko12/05/14 09:11 AM
Reply to this Topic
Name:
Email:
Topic:
Body: No Text
How do you spell green?