memcpy - instruction cracking vs DMA

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), October 3, 2021 10:48 am
Room: Moderated Discussions
Brendan (btrotter.delete@this.gmail.com) on October 2, 2021 4:53 pm wrote:
>
> The DMA that Linus seems to be thinking of (e.g. a dedicated "Intel 8237" chip sitting on a
> distant legacy/ISA bus) mostly got superseded/replaced by bus mastering functionality built
> directly into almost every device;

No, I'm very much thinking of modern DMA - on-die and cache coherent.

Because even when it is close-by and cache coherent, it is just stupid and wrong for CPU memory copies.

The memory accesses need to be done by the memory unit itself, in order to not screw up data that is in the local L1 cache - which is not at all uncommon.

In fact, you want it even closer than the L1D$ - you want it to interact with the actual store buffers. You want the memory copies to be visible and part of the OoO machinery, so that you can do the re-ordering, do the store buffer snooping, etc. They should use all the byte lane shifting and masking logic that is very much already there.

I call it a "CPU memory copy logic", and claim that it should be driven by CPU instructions, and that is should become part of the uop stream and schedule with the instructions around it. So it needs to be right there in the very core of the CPU, by the same memory unit that handles all the other loads and stores.

Nobody sane calls that "DMA" unless you just want to make some mental gymnastics.

Any actual DMA engine that is not directly tied to the CPU memory unit is much too far away, and cannot be sanely ordered wrt all the other memory ops around the memcpy that will be right there.

A very common memory copy pattern is to literally copy a data structure, and then modify the result. We're talking tens - maybe hundreds - of bytes, that can be done with one or two byte shift/mask buffer movements that should be scheduled exactly like any random read or write. Except it isn't a fixed-size read, and it doesn't go through a register file.

Linus
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Armv8.8-A and Armv9.3-Aanonymou52021/09/16 03:25 PM
  Armv8.8-A and Armv9.3-ADoug S2021/09/16 10:57 PM
    Armv8.8-A and Armv9.3-ABrett2021/09/16 11:32 PM
      Armv8.8-A and Armv9.3-Aanon2021/09/16 11:55 PM
        Armv8.8-A and Armv9.3-Anone2021/09/17 12:51 AM
      Armv8.8-A and Armv9.3-AJörn Engel2021/09/17 05:42 AM
        Armv8.8-A and Armv9.3-AMichael S2021/09/17 07:48 AM
          Armv8.8-A and Armv9.3-AJörn Engel2021/09/18 02:01 PM
            Armv8.8-A and Armv9.3-AMichael S2021/09/18 03:58 PM
              microbenchmark resultsMichael S2021/09/19 04:46 PM
                microbenchmark source codeMichael S2021/09/19 04:58 PM
                  microbenchmark source code-.-2021/09/20 04:49 PM
                    microbenchmark source codeMichael S2021/09/21 10:17 AM
                      microbenchmark source code-.-2021/09/21 04:33 PM
                        microbenchmark source codeMichael S2021/09/21 06:05 PM
                microbenchmark resultsAnon2021/09/19 05:32 PM
                  microbenchmark resultsJörn Engel2021/09/19 08:46 PM
                    microbenchmark resultsdmcq2021/09/20 02:19 AM
                      microbenchmark resultsMichael S2021/09/20 05:12 AM
                      microbenchmark results-.-2021/09/20 04:44 PM
                        microbenchmark resultsMichael S2021/09/21 10:23 AM
                          microbenchmark results-.-2021/09/21 04:35 PM
                            microbenchmark resultsAndrey2021/09/21 05:25 PM
                              I agree (NT)Michael S2021/09/21 06:07 PM
                              microbenchmark results-.-2021/09/22 05:56 PM
                                microbenchmark resultsMichael S2021/09/23 06:11 AM
                                  microbenchmark resultsdmcq2021/09/23 07:53 AM
                                  microbenchmark resultsAndrey2021/09/23 10:20 AM
                                microbenchmark resultsAndrey2021/09/23 10:11 AM
                                  microbenchmark results-.-2021/09/23 08:01 PM
                                    microbenchmark resultsSimon Farnsworth2021/09/24 02:47 AM
                                      microbenchmark results-.-2021/09/24 06:00 PM
                                    microbenchmark resultsAndrey2021/09/24 08:29 AM
                                      microbenchmark resultsdmcq2021/09/24 01:05 PM
                                        microbenchmark resultsDoug S2021/09/24 02:12 PM
                                          microbenchmark results---2021/09/24 07:06 PM
                                            microbenchmark resultsDoug S2021/09/24 11:46 PM
                                              microbenchmark results---2021/09/25 09:56 AM
                                                microbenchmark resultsJukka Larja2021/09/26 02:01 AM
                                                microbenchmark resultsDoug S2021/09/26 09:41 AM
                                                  microbenchmark resultsdmcq2021/09/26 01:37 PM
                                                    microbenchmark resultsDoug S2021/09/27 10:32 AM
                                                      microbenchmark resultsdmcq2021/09/28 07:56 AM
                                              microbenchmark resultsDummond D. Slow2021/09/25 12:49 PM
                                                microbenchmark resultsBrett2021/09/25 03:31 PM
                                              microbenchmark resultsdmcq2021/09/25 12:51 PM
                                                microbenchmark resultsDoug S2021/09/26 09:45 AM
                                            microbenchmark resultsRichard S2021/09/25 01:51 AM
                                              microbenchmark resultsDummond D. Slow2021/09/25 12:52 PM
                                                microbenchmark results---2021/09/25 03:04 PM
                                      SVE alignment with non power-of-2 widths-.-2021/09/24 06:10 PM
                                        SVE alignment with non power-of-2 widthsAndrey2021/09/25 04:46 AM
                                          SVE alignment with non power-of-2 widths-.-2021/09/25 05:35 PM
                                          SVE alignment with non power-of-2 widthsKevin G2021/09/27 09:46 AM
                                            SVE alignment with non power-of-2 widths-.-2021/09/27 09:06 PM
                                              SVE alignment with non power-of-2 widthsJukka Larja2021/09/28 06:37 AM
                                                SVE alignment with non power-of-2 widthsAndrey2021/09/28 12:12 PM
                                                  SVE alignment with non power-of-2 widthsdmcq2021/09/28 02:29 PM
                                                SVE alignment with non power-of-2 widths-.-2021/09/28 06:37 PM
                                                  SVE alignment with non power-of-2 widthsJukka Larja2021/09/29 06:50 AM
                    microbenchmark results---2021/09/20 07:11 AM
                    microbenchmark resultsJörn Engel2021/09/23 05:10 AM
                      microbenchmark resultsMichael S2021/09/23 05:55 AM
                        microbenchmark resultsJörn Engel2021/09/23 09:24 AM
                          microbenchmark resultsRoyi2021/09/26 04:25 PM
                      microbenchmark resultsdmcq2021/09/23 10:42 AM
                        microbenchmark results---2021/09/23 11:53 AM
                      microbenchmark resultsanon22021/09/23 02:40 PM
                microbenchmark results: Zen 3Adrian2021/09/22 01:57 AM
                  microbenchmark results: Zen 3Adrian2021/09/22 02:08 AM
                    microbenchmark results: Zen 3Michael S2021/09/22 05:48 AM
                      microbenchmark results: Zen 3Adrian2021/09/22 06:05 AM
        Armv8.8-A and Armv9.3-AKonrad Schwarz2021/09/28 05:45 AM
    Armv8.8-A and Armv9.3-ALinus Torvalds2021/09/17 08:59 AM
      Armv8.8-A and Armv9.3-ADoug S2021/09/17 11:35 AM
        Armv8.8-A and Armv9.3-Anksingh2021/09/17 12:23 PM
          Armv8.8-A and Armv9.3-ADoug S2021/09/17 02:35 PM
            Armv8.8-A and Armv9.3-AKonrad Schwarz2021/10/15 06:23 AM
              Armv8.8-A and Armv9.3-Arwessel2021/10/15 06:49 AM
        Armv8.8-A and Armv9.3-AAdrian2021/09/17 11:07 PM
          Armv8.8-A and Armv9.3-ADoug S2021/09/18 07:34 AM
            Armv8.8-A and Armv9.3-AAdrian2021/09/18 07:38 AM
      Armv8.8-A and Armv9.3-Ablaine2021/09/18 10:37 AM
      Armv8.8-A and Armv9.3-ABrett2021/09/19 01:06 PM
        Armv8.8-A and Armv9.3-Admcq2021/09/19 01:36 PM
        Armv8.8-A and Armv9.3-ADoug S2021/09/19 06:07 PM
          Armv8.8-A and Armv9.3-A - movesdmcq2021/09/28 09:54 AM
            Armv8.8-A and Armv9.3-A - movesDoug S2021/09/28 01:57 PM
              Armv8.8-A and Armv9.3-A - movesdmcq2021/09/28 02:21 PM
                Armv8.8-A and Armv9.3-A - movesNoSpammer2021/09/29 03:53 AM
                  Armv8.8-A and Armv9.3-A - movesrwessel2021/09/29 06:55 AM
                    Armv8.8-A and Armv9.3-A - movesdmcq2021/09/29 07:53 AM
                      Armv8.8-A and Armv9.3-A - movesrwessel2021/09/29 11:35 AM
                        Armv8.8-A and Armv9.3-A - movesdmcq2021/09/29 01:44 PM
                          Armv8.8-A and Armv9.3-A - movesrwessel2021/09/29 01:58 PM
                            Armv8.8-A and Armv9.3-A - movesdmcq2021/09/29 03:52 PM
                              Armv8.8-A and Armv9.3-A - movesrwessel2021/09/29 06:36 PM
                              Armv8.8-A and Armv9.3-A - movesAndrey2021/09/29 07:58 PM
                    Armv8.8-A and Armv9.3-A - movesDoug S2021/09/29 10:10 AM
                      Armv8.8-A and Armv9.3-A - movesrwessel2021/09/29 11:30 AM
                        Armv8.8-A and Armv9.3-A - movesDoug S2021/09/29 10:02 PM
                          Armv8.8-A and Armv9.3-A - movesrwessel2021/09/29 11:22 PM
                            Armv8.8-A and Armv9.3-A - movesMark Roulo2021/09/30 07:37 AM
                              Armv8.8-A and Armv9.3-A - movesrwessel2021/09/30 08:02 AM
                                Did they publish a full description? (NT)Michael S2021/09/30 08:12 AM
                                  Did they publish a full description?rwessel2021/09/30 09:18 AM
                                    Did they publish a full description?Michael S2021/09/30 10:24 AM
                                      Did they publish a full description?rwessel2021/09/30 10:42 AM
                                    Did they publish a full description?Adrian2021/10/01 12:22 AM
                                  Do we even okiw it's three instructions per move?Carson2021/09/30 10:28 PM
                                    Do we even okiw it's three instructions per move?Adrian2021/10/01 12:27 AM
                                    Do we even okiw it's three instructions per move?rwessel2021/10/01 04:19 AM
                            Armv8.8-A and Armv9.3-A - movesDoug S2021/09/30 09:48 AM
                              Armv8.8-A and Armv9.3-A - movesrwessel2021/09/30 10:39 AM
                                Armv8.8-A and Armv9.3-A - movesDoug S2021/09/30 02:56 PM
                                  Armv8.8-A and Armv9.3-A - movesrwessel2021/09/30 05:20 PM
                                    Armv8.8-A and Armv9.3-A - movesdmcq2021/10/01 04:38 AM
                                      Armv8.8-A and Armv9.3-A - movesMichael S2021/10/01 05:04 AM
                                        Armv8.8-A and Armv9.3-A - movesLinus Torvalds2021/10/01 11:01 AM
                                          memcpy - instruction cracking vs DMArpg2021/10/02 02:51 AM
                                            memcpy - instruction cracking vs DMAAdrian2021/10/02 03:45 AM
                                              memcpy - instruction cracking vs DMADoug S2021/10/02 09:47 AM
                                                memcpy - instruction cracking vs DMAAdrian2021/10/02 10:15 AM
                                                memcpy - instruction cracking vs DMArwessel2021/10/02 11:37 AM
                                                  memcpy - instruction cracking vs DMADoug S2021/10/02 06:49 PM
                                            memcpy - instruction cracking vs DMALinus Torvalds2021/10/02 10:43 AM
                                              memcpy - instruction cracking vs DMAdmcq2021/10/02 11:32 AM
                                              memcpy - instruction cracking vs DMABrett2021/10/02 11:45 AM
                                              memcpy - instruction cracking vs DMA---2021/10/02 03:03 PM
                                                memcpy - instruction cracking vs DMA---2021/10/02 03:12 PM
                                                  Moving copy to DRAM doesn't help for small copiesMark Roulo2021/10/02 03:59 PM
                                                    Moving copy to DRAM doesn't help for small copies---2021/10/02 07:32 PM
                                                      Moving copy to DRAM doesn't help for small copiesMichael S2021/10/03 01:40 AM
                                                        Moving copy to DRAM doesn't help for small copiesDoug S2021/10/03 10:09 AM
                                                          Moving copy to DRAM doesn't help for small copiesrwessel2021/10/03 10:51 AM
                                                          Moving copy to DRAM doesn't help for small copiesLinus Torvalds2021/10/03 11:09 AM
                                                            How about environments such as Java?Mark Roulo2021/10/03 12:41 PM
                                                              How about environments such as Java?rwessel2021/10/03 12:49 PM
                                                                How about environments such as Java?Mark Roulo2021/10/03 01:22 PM
                                                              How about environments such as Java?anon22021/10/03 07:58 PM
                                                                How about environments such as Java?Etienne Lorrain2021/10/04 05:08 AM
                                                                  Apart from "It depends" there is no short answer. (NT)Michael S2021/10/04 05:30 AM
                                                                  How about environments such as Java?Andrey2021/10/04 06:04 AM
                                                                  How about environments such as Java?anon22021/10/04 06:32 AM
                                                                How about environments such as Java?Mark Roulo2021/10/04 07:31 AM
                                                                How about environments such as Java?---2021/10/04 09:41 AM
                                                                  How about environments such as Java?Doug S2021/10/04 10:23 AM
                                                                    How about environments such as Java?Andrey2021/10/04 12:14 PM
                                                                      How about environments such as Java?Doug S2021/10/04 01:20 PM
                                                                  How about environments such as Java?anon22021/10/04 02:23 PM
                                                                  How about environments such as Java?rwessel2021/10/04 04:54 PM
                                                            Moving copy to DRAM doesn't help for small copiesJörn Engel2021/10/04 05:52 AM
                                                            Early software zeroing !=== early hardware zeroingPaul A. Clayton2021/10/05 11:19 AM
                                                              Early software zeroing !=== early hardware zeroingDoug S2021/10/05 12:21 PM
                                                memcpy - instruction cracking vs DMABrendan2021/10/02 04:53 PM
                                                  memcpy - instruction cracking vs DMALinus Torvalds2021/10/03 10:48 AM
                                                    memcpy - instruction cracking vs DMAdmcq2021/10/03 01:54 PM
                                              memcpy - instruction cracking vs DMAYuhong Bao2021/10/03 01:30 AM
                                                memcpy - instruction cracking vs DMADavid Hess2021/10/05 05:19 PM
                                                  memcpy - instruction cracking vs DMAAdrian2021/10/05 11:28 PM
                                                    memcpy - instruction cracking vs DMAEtienne Lorrain2021/10/06 02:24 AM
                                                    memcpy - instruction cracking vs DMArwessel2021/10/06 03:38 AM
                                                      memcpy - instruction cracking vs DMAAdrian2021/10/06 04:04 AM
                                                        memcpy - instruction cracking vs DMArwessel2021/10/06 05:59 AM
                                                    memcpy - instruction cracking vs DMA---2021/10/06 09:07 AM
                                                      memcpy - instruction cracking vs DMAAndrey2021/10/06 02:59 PM
                                                    memcpy - instruction cracking vs DMAgallier22021/10/06 11:06 PM
                                                      memcpy - instruction cracking vs DMAAdrian2021/10/06 11:59 PM
                                              memcpy - instruction cracking vs DMAMichael S2021/10/03 01:51 AM
                                                memcpy - instruction cracking vs DMArwessel2021/10/03 05:06 AM
                                                  memcpy - instruction cracking vs DMAMichael S2021/10/03 05:24 AM
                                                    memcpy - instruction cracking vs DMAMatt Sayler2021/10/03 08:02 AM
                                                    memcpy - instruction cracking vs DMADoug S2021/10/03 10:14 AM
                                      Armv8.8-A and Armv9.3-A - movesrwessel2021/10/01 05:10 AM
                                        Armv8.8-A and Armv9.3-A - movesEtienne Lorrain2021/10/01 07:55 AM
                                          Armv8.8-A and Armv9.3-A - movesrwessel2021/10/01 08:14 AM
                                            Armv8.8-A and Armv9.3-A - movesDoug S2021/10/01 11:17 AM
                                              Armv8.8-A and Armv9.3-A - movesrwessel2021/10/02 04:57 AM
  Armv8.8-A and Armv9.3-Anone2021/10/13 06:06 AM
    Armv8.8-A and Armv9.3-AAdrian2021/10/13 06:22 AM
      Armv8.8-A and Armv9.3-ADoug S2021/10/13 09:01 AM
        Armv8.8-A and Armv9.3-Admcq2021/10/13 10:17 AM
          Armv8.8-A and Armv9.3-Anone2021/10/13 10:26 PM
            Armv8.8-A and Armv9.3-Admcq2021/10/14 08:22 AM
    Armv8.8-A and Armv9.3-Arwessel2021/10/14 09:01 AM
      Armv8.8-A and Armv9.3-AAnon2021/10/14 11:08 AM
        Armv8.8-A and Armv9.3-AMichael S2021/10/14 01:25 PM
      Armv8.8-A and Armv9.3-ADoug S2021/10/14 11:18 AM
        Armv8.8-A and Armv9.3-Arwessel2021/10/14 07:07 PM
          Armv8.8-A and Armv9.3-ADoug S2021/10/14 10:23 PM
            Armv8.8-A and Armv9.3-Admcq2021/10/15 01:41 AM
              Armv8.8-A and Armv9.3-AGabriele Svelto2021/10/15 05:07 AM
            Armv8.8-A and Armv9.3-Arwessel2021/10/15 04:49 AM
              Armv8.8-A and Armv9.3-ADoug S2021/10/15 10:44 AM
                Armv8.8-A and Armv9.3-Ame2021/10/15 06:34 PM
                  Armv8.8-A and Armv9.3-ADoug S2021/10/16 09:47 AM
                    Armv8.8-A and Armv9.3-Ame2021/10/17 05:19 AM
                      Armv8.8-A and Armv9.3-ADoug S2021/10/17 10:17 AM
                        Armv8.8-A and Armv9.3-Ame2021/10/17 12:31 PM
                          Armv8.8-A and Armv9.3-ADoug S2021/10/17 01:33 PM
                            Armv8.8-A and Armv9.3-AzArchJon2021/10/18 10:35 AM
                              Armv8.8-A and Armv9.3-ADoug S2021/10/18 02:35 PM
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