Remote atomics

By: dmcq (dmcq.delete@this.fano.co.uk), August 19, 2018 2:33 pm
Room: Moderated Discussions
Michael S (already5chosen.delete@this.yahoo.com) on August 19, 2018 1:58 pm wrote:
> matthew (nobody.delete@this.example.com) on August 19, 2018 12:51 pm wrote:
> > Travis (travis.downs.delete@this.gmail.com) on August 19, 2018 11:36 am wrote:
> > > Maynard Handley (name99.delete@this.name99.org) on August 18, 2018 2:42 pm wrote:
> > >
> > > > compressed pages), or remote atomics (to speed up common atomic operations like ARC increments);
> > >
> > > We've been over this before, but remote atomics make no sense for the vast majority of
> > > code. A "remote atomic" necessarily has a latency of whatever shared thing is remote and
> > > doing the increment, so in the range of 10s of clocks for something like L3, or in the
> > > 100s of clocks if it happens in memory (or a bit less if in the memory controller).
> > >
> > > Existing atomic operations are almost totally local operations, that work
> > > at "core speed", and don't require any coherence traffic at all (except when
> > > another core wants the line, just applies to non-atomics as well).
> > >
> > > The overwhelming majority of the time when something line is operated on atomically, the
> > > last core to use that line was the same core. Making this case fast is really important.
> > >
> > > Remote atomics is like throwing out the best case which almost always happens in favor of always worst-case
> > > behavior, just to make that worst-case behavior a little better. Now every atomic access in your
> > > program has to go out over long-latency, power sucking busses, and the core has to wait forever to
> > > get the result back (and it almost always does need the result, as in your ARC example).
> > >
> > > It just makes no sense as an implementation of existing atomics.
> > >
> > > Now there are some specific sharing patterns where it may work, but it would have to be a new instruction.
> >
> > I think there's a good counter-example to your point, which is the Itanium McKinley implementation
> > of fetchadd. If the line is in L1, the CPU does it. If it's not, the core tells the
> > L2 to do it.
>
> Are you sure?
> Do you have a proof link?

I believe that is the way ARM intends for the remote atomics to work. I believe I saw something discussing the problem that the atomic variable wouldn't be pulled into the local cache, so what will happen is if the atomic is held by another core it will stay there until such time as another variable in the cache line is accessed.

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TopicPosted ByDate
ARM turns to a god and a heroAM2018/08/16 09:32 AM
  ARM turns to a god and a heroMaynard Handley2018/08/16 09:41 AM
    ARM turns to a god and a heroDoug S2018/08/16 11:11 AM
    ARM turns to a god and a heroGeoff Langdale2018/08/16 11:59 PM
      ARM turns to a god and a herodmcq2018/08/17 05:12 AM
  ARM is somewhat misleadingAdrian2018/08/16 11:56 PM
    It's marketing materialGabriele Svelto2018/08/17 01:00 AM
      It's marketing materialMichael S2018/08/17 03:13 AM
        It's marketing materialdmcq2018/08/17 05:23 AM
          It's marketing materialAndrei Frumusanu2018/08/17 07:25 AM
        It's marketing materialLinus Torvalds2018/08/17 11:20 AM
          It's marketing materialGroo2018/08/17 01:44 PM
            It's marketing materialDoug S2018/08/17 02:14 PM
          promises and deliveriesAM2018/08/17 02:32 PM
            promises and deliveriesPassing Through2018/08/17 03:02 PM
              Just by way of clarification Passing Through2018/08/17 03:15 PM
                Just by way of clarification AM2018/08/18 12:49 PM
                  Just by way of clarification Passing Through2018/08/18 01:34 PM
                    This ain't the nineties any longerPassing Through2018/08/18 01:54 PM
                      This ain't the nineties any longerMaynard Handley2018/08/18 02:50 PM
                        This ain't the nineties any longerPassing Through2018/08/18 03:57 PM
                          This ain't the nineties any longerPassing Through2018/09/06 02:42 PM
                            This ain't the nineties any longerMaynard Handley2018/09/07 04:10 PM
                              This ain't the nineties any longerPassing Through2018/09/07 04:48 PM
                                This ain't the nineties any longerMaynard Handley2018/09/07 05:22 PM
                Just by way of clarification Wilco2018/08/18 01:26 PM
                  Just by way of clarification Passing Through2018/08/18 01:39 PM
                  Just by way of clarification none2018/08/18 10:52 PM
                    Just by way of clarification dmcq2018/08/19 08:32 AM
                      Just by way of clarification none2018/08/19 08:54 AM
                        Just by way of clarification dmcq2018/08/19 11:24 AM
                          Just by way of clarification none2018/08/19 11:52 AM
                  Just by way of clarification Gabriele Svelto2018/08/19 06:41 AM
                    Just by way of clarification Passing Through2018/08/19 09:25 AM
                      Whiteboards at Gatwick airport anyone? Passing Through2018/08/20 04:24 AM
          It's marketing materialMichael S2018/08/18 11:12 AM
          It's marketing materialBrett2018/08/18 05:22 PM
            It's marketing materialBrett2018/08/18 05:33 PM
              It's marketing materialAdrian2018/08/19 01:21 AM
        A76AM2018/08/17 02:45 PM
          A76Michael S2018/08/18 11:20 AM
            A76AM2018/08/18 12:39 PM
              A76Michael S2018/08/18 12:49 PM
                A76AM2018/08/18 01:06 PM
                  A76Doug S2018/08/18 01:43 PM
                    A76Maynard Handley2018/08/18 02:42 PM
                      A76Maynard Handley2018/08/18 04:22 PM
                        Why write zeros when one can use metadata?Paul A. Clayton2018/08/18 06:19 PM
                          Why write zeros when one can use metadata?Maynard Handley2018/08/19 11:12 AM
                            Dictionary compress might apply to memcopyPaul A. Clayton2018/08/19 01:45 PM
                        Instructions for zeroingKonrad Schwarz2018/08/30 06:37 AM
                          Instructions for zeroingMaynard Handley2018/08/30 08:41 AM
                          Instructions for zeroingAdrian2018/08/30 11:37 AM
                            dcbz -> dcbzl (was: Instructions for zeroing)hobold2018/08/31 01:50 AM
                              dcbz -> dcbzl (was: Instructions for zeroing)dmcq2018/09/01 05:28 AM
                      A76Travis2018/08/19 11:36 AM
                        A76Maynard Handley2018/08/19 12:22 PM
                          A76Travis2018/08/19 02:07 PM
                            A76Maynard Handley2018/08/19 06:24 PM
                        Remote atomicsmatthew2018/08/19 12:51 PM
                          Remote atomicsMichael S2018/08/19 01:58 PM
                            Remote atomicsmatthew2018/08/19 02:32 PM
                              Remote atomicsMichael S2018/08/19 02:36 PM
                                Remote atomicsmatthew2018/08/19 02:48 PM
                                  Remote atomicsMichael S2018/08/19 03:16 PM
                                    Remote atomicsRicardo B2018/08/20 10:05 AM
                            Remote atomicsdmcq2018/08/19 02:33 PM
                          Remote atomicsTravis2018/08/19 02:32 PM
                            Remote atomicsMichael S2018/08/19 02:46 PM
                              Remote atomicsTravis2018/08/19 05:35 PM
                                Remote atomicsMichael S2018/08/20 03:29 AM
                            Remote atomicsmatthew2018/08/19 07:58 PM
                              Remote atomicsanon2018/08/20 12:59 AM
                                Remote atomicsTravis2018/08/20 10:26 AM
                              Remote atomicsTravis2018/08/20 09:57 AM
                              Remote atomicsLinus Torvalds2018/08/20 04:29 PM
                                Fitting time slices to execution phasesPaul A. Clayton2018/08/21 09:09 AM
                                  Fitting time slices to execution phasesLinus Torvalds2018/08/21 02:34 PM
                                    Fitting time slices to execution phasesLinus Torvalds2018/08/21 03:31 PM
                                      Fitting time slices to execution phasesGabriele Svelto2018/08/21 03:54 PM
                                        Fitting time slices to execution phasesLinus Torvalds2018/08/21 04:26 PM
                                      Fitting time slices to execution phasesTravis2018/08/21 04:21 PM
                                        Fitting time slices to execution phasesLinus Torvalds2018/08/21 04:39 PM
                                          Fitting time slices to execution phasesTravis2018/08/21 04:59 PM
                                            Fitting time slices to execution phasesLinus Torvalds2018/08/21 05:13 PM
                                      Fitting time slices to execution phasesanon2018/08/21 04:27 PM
                                        Fitting time slices to execution phasesLinus Torvalds2018/08/21 06:02 PM
                                          Fitting time slices to execution phasesEtienne2018/08/22 02:28 AM
                                        Fitting time slices to execution phasesGabriele Svelto2018/08/22 03:07 PM
                                          Fitting time slices to execution phasesTravis2018/08/22 04:00 PM
                                          Fitting time slices to execution phasesanon2018/08/22 06:52 PM
                                    Fitting time slices to execution phasesTravis2018/08/21 04:37 PM
                                    Is preventing misuse that complex?Paul A. Clayton2018/08/23 05:42 AM
                                      Is preventing misuse that complex?Linus Torvalds2018/08/23 12:46 PM
                                        Is preventing misuse that complex?Travis2018/08/23 01:29 PM
                                          Is preventing misuse that complex?Travis2018/08/23 01:33 PM
                                            Is preventing misuse that complex?Jeff S.2018/08/24 07:57 AM
                                              Is preventing misuse that complex?Travis2018/08/24 08:47 AM
                                          Is preventing misuse that complex?Linus Torvalds2018/08/23 02:30 PM
                                            Is preventing misuse that complex?Travis2018/08/23 03:11 PM
                                              Is preventing misuse that complex?Linus Torvalds2018/08/24 01:00 PM
                                                Is preventing misuse that complex?Gabriele Svelto2018/08/24 01:25 PM
                                                  Is preventing misuse that complex?Linus Torvalds2018/08/24 01:33 PM
                                  Fitting time slices to execution phasesTravis2018/08/21 03:54 PM
                                rseq: holy grail rwlock?Travis2018/08/21 03:18 PM
                                  rseq: holy grail rwlock?Linus Torvalds2018/08/21 03:59 PM
                                    rseq: holy grail rwlock?Travis2018/08/21 04:27 PM
                                      rseq: holy grail rwlock?Linus Torvalds2018/08/21 05:10 PM
                                        rseq: holy grail rwlock?Travis2018/08/21 06:21 PM
                  ARM design housesMichael S2018/08/21 05:07 AM
                    ARM design housesWilco2018/08/22 12:38 PM
                      ARM design housesMichael S2018/08/22 02:21 PM
                        ARM design housesWilco2018/08/22 03:23 PM
                          ARM design housesMichael S2018/08/29 01:58 AM
                            Qualcomm's core naming scheme really, really sucksHeikki Kultala2018/08/29 02:19 AM
                A76Maynard Handley2018/08/18 02:07 PM
                  A76Michael S2018/08/18 02:32 PM
                    A76Maynard Handley2018/08/18 02:52 PM
                      A76Michael S2018/08/18 03:04 PM
    ARM is somewhat misleadingjuanrga2018/08/17 01:20 AM
    Surprised??Alberto2018/08/17 01:52 AM
      Surprised??Alberto2018/08/17 02:10 AM
      Surprised??none2018/08/17 02:46 AM
      Garbage talkAndrei Frumusanu2018/08/17 07:30 AM
        Garbage talkMichael S2018/08/17 07:43 AM
          Garbage talkAndrei Frumusanu2018/08/17 09:51 AM
            Garbage talkMichael S2018/08/18 11:29 AM
        Garbage talkAdrian2018/08/17 08:28 AM
          Garbage talkAlberto2018/08/17 09:20 AM
          Garbage talkAndrei Frumusanu2018/08/17 09:48 AM
            Garbage talkAdrian2018/08/17 10:17 AM
              Garbage talkAndrei Frumusanu2018/08/17 10:36 AM
                Garbage talkAdrian2018/08/17 02:53 PM
                  Garbage talkAndrei Frumusanu2018/08/18 12:17 AM
        More like a religion he?? ARM has an easy life :)Alberto2018/08/17 09:13 AM
          More like a religion he?? ARM has an easy life :)Andrei Frumusanu2018/08/17 09:34 AM
            More like a religion he?? ARM has an easy life :)Alberto2018/08/17 10:03 AM
              More like a religion he?? ARM has an easy life :)Andrei Frumusanu2018/08/17 10:43 AM
              More like a religion he?? ARM has an easy life :)Doug S2018/08/17 02:17 PM
              15W phone SoCsAM2018/08/17 03:04 PM
          More like a religion he?? ARM has an easy life :)Maynard Handley2018/08/17 12:29 PM
  my future stuff will be better than your old stuff, hey I'm a god at last (NT)Eric Bron2018/08/18 03:34 AM
    my future stuff will be better than your old stuff, hey I'm a god at lastnone2018/08/18 08:34 AM
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