Why write zeros when one can use metadata?

By: Maynard Handley (name99.delete@this.name99.org), August 19, 2018 11:12 am
Room: Moderated Discussions
Paul A. Clayton (paaronclayton.delete@this.gmail.com) on August 18, 2018 6:19 pm wrote:
> Maynard Handley (name99.delete@this.name99.org) on August 18, 2018 4:22 pm wrote:
> [snip]
> > Oops left out one more idea. Once you have a machinery to handle remote atomics AND you own
> > the entire stack, you can do some crazy things. For example consider the problem of zeroing
> > an entire page. Obviously doing this on the main CPU sucks, but doing it on a secondary thread
> > (and thus the machinery of callbacks and waits) also sucks. But what about this?
> > - you extend the cache protocol to include invalidate messages
> > that describe not just a single line but an address
> > plus a count of lines (say up to 256 or so, whatever is necessary to cover a page seems about right)
> > - you offer an atomic remote zero (or remote fill) instruction that is given a line address
> > plus count. The first thing it does is flush the line from all other caches, then it marks
> > the set of lines locally as in some sort of intermediate state, then, fast as possible, it
> > starts zeroing each line and, once zeroed, it goes back to a standard "valid" state.
>
> All zero cache lines (and pages) compress extremely well. I think it
> makes more sense to use metadata to indicate such compression (at page
> granularity one could steal a state in PTEs, which further avoids excess
> cost in metadata fetching) and avoid memory read entirely. The lines can
> be zero filled around the first write.
>

What you say is true, of course, but it's orthogonal to the aspect I wanted to highlight, the use of "remote atomics" for more than just atomic operations. It fits more into the "compressed L3" suggestion.
Zeroing is simply an especially simple example, but the other real world examples (memfill, and then memcopy) build on top of it, and are usually less amenable to simple compression. (memfill maybe, depending on your line compressor; memcopy likely not).
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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
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        It's marketing materialdmcq2018/08/17 05:23 AM
          It's marketing materialAndrei Frumusanu2018/08/17 07:25 AM
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          It's marketing materialGroo2018/08/17 01:44 PM
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                Just by way of clarification AM2018/08/18 12:49 PM
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                    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
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                                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
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                          Remote atomicsMichael S2018/08/19 01:58 PM
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                                  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
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                              Remote atomicsanon2018/08/20 12:59 AM
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                                        Fitting time slices to execution phasesLinus Torvalds2018/08/21 04:26 PM
                                      Fitting time slices to execution phasesTravis2018/08/21 04:21 PM
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                                          Fitting time slices to execution phasesanon2018/08/22 06:52 PM
                                    Fitting time slices to execution phasesTravis2018/08/21 04:37 PM
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                                      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
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                                              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
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                                      rseq: holy grail rwlock?Linus Torvalds2018/08/21 05:10 PM
                                        rseq: holy grail rwlock?Travis2018/08/21 06:21 PM
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  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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