How about environments such as Java?

By: Mark Roulo (nothanks.delete@this.xxx.com), October 3, 2021 11:41 am
Room: Moderated Discussions
Linus Torvalds (torvalds.delete@this.linux-foundation.org) on October 3, 2021 11:09 am wrote:
> Doug S (foo.delete@this.bar.bar) on October 3, 2021 10:09 am wrote:
> >
> > Zeroing has room for optimization, both since you will often zero more than one page at a time and
> > because zeroes are rarely read before they are overwritten - so you want that activity to occur outside
> > of the cache.
>
> No you don't, actually.
>
> People have tried various pre-zeroing schemes over and over
> and over again, and it's always been a loss in the end.
>
> Why? Caches work, and they grow over time. And basically every single time you zero
> something, you are doing so because you're going to access much of the end result -
> even if it's just to overwrite it with final data - in the not too distant future.
>
> Pre-zeroing and doing it at a DRAM or memory controller level is always going to be the wrong answer.
> It's going to mean that when you access it, you're now going to take that very expensive cache miss.
>
> Yes, you can always find benchmarks where pre-zeroing is great, because you can
> pick the benchmark where you have just the right working set size, and you can time
> the memory operations to when they are most effective for that benchmark.
>
> And then on real loads it won't work at all. In fact, even on the benchmark it will be a loss on
> other microarchitecures with bigger caches - so you're basically pessimising for the future.
>
> So what you want to do is to zero your memory basically as late as possible, just before it gets used. That
> way the data will be close when it is accessed. Even if it's accessed just for writing the actual new data on
> top - a lot of zeroing is for initialization and security reasons, and to make for consistent behavior - it
> will be at least already dirty and exclusive in your caches, which is exactly what you want for a write.
>
> So for big sparse arrays (or huge initial allocations), you may actually be much better
> off allocating them with something like a "mmap()" interface for anonymous memory (pick
> whatever non-unix equivalent), and just telling the system that you will need this much
> memory, but then depend on demand-paging to zero the pages for you before use.
>
> Yes, you'll then take the page faults dynamically, but it might well end up
> much better than pre-zeroing big buffers that you won't use for a while.
>
> As a rule of thumb, you never ever want to move memory accesses closer to DRAM,
> unless you have been explicitly told "I don't want this data any more" (or you have
> some really good detection of "this working set won't fit in any caches").
>
> DRAM is just too far away, and caches are too effective - and you
> very seldom know how much cache you have on a software level.
>
> Side note: that detection of "this working set won't fit in any caches" may well be
> about the CPU knowing the size of a memory copy or memory clear operation ahead of time,
> and taking those kinds of very explicit hints into account. Which is just another reason
> you should have memory copy support in hardware, and not do it in software.
>
> Linus

How confident are you about this for languages such as Java? Especially Java running in a server context with sophisticated multi-threaded GC?

This seems like the sort of environment where zero-ing the data to be allocated later might be a win, especially if the data could be zero-d when nothing else needed the DRAM bandwidth.

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Armv8.8-A and Armv9.3-Aanonymou52021/09/16 02:25 PM
  Armv8.8-A and Armv9.3-ADoug S2021/09/16 09:57 PM
    Armv8.8-A and Armv9.3-ABrett2021/09/16 10:32 PM
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      Armv8.8-A and Armv9.3-AJörn Engel2021/09/17 04:42 AM
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          Armv8.8-A and Armv9.3-AJörn Engel2021/09/18 01:01 PM
            Armv8.8-A and Armv9.3-AMichael S2021/09/18 02:58 PM
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                      microbenchmark source code-.-2021/09/21 03:33 PM
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                      microbenchmark resultsMichael S2021/09/20 04:12 AM
                      microbenchmark results-.-2021/09/20 03:44 PM
                        microbenchmark resultsMichael S2021/09/21 09:23 AM
                          microbenchmark results-.-2021/09/21 03:35 PM
                            microbenchmark resultsAndrey2021/09/21 04:25 PM
                              I agree (NT)Michael S2021/09/21 05:07 PM
                              microbenchmark results-.-2021/09/22 04:56 PM
                                microbenchmark resultsMichael S2021/09/23 05:11 AM
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                                  microbenchmark resultsAndrey2021/09/23 09:20 AM
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                                  microbenchmark results-.-2021/09/23 07:01 PM
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                                      microbenchmark results-.-2021/09/24 05:00 PM
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                                      microbenchmark resultsdmcq2021/09/24 12:05 PM
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                                              microbenchmark results---2021/09/25 08:56 AM
                                                microbenchmark resultsJukka Larja2021/09/26 01:01 AM
                                                microbenchmark resultsDoug S2021/09/26 08:41 AM
                                                  microbenchmark resultsdmcq2021/09/26 12:37 PM
                                                    microbenchmark resultsDoug S2021/09/27 09:32 AM
                                                      microbenchmark resultsdmcq2021/09/28 06:56 AM
                                              microbenchmark resultsDummond D. Slow2021/09/25 11:49 AM
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                                              microbenchmark resultsdmcq2021/09/25 11:51 AM
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                                            microbenchmark resultsRichard S2021/09/25 12:51 AM
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                                          SVE alignment with non power-of-2 widths-.-2021/09/25 04:35 PM
                                          SVE alignment with non power-of-2 widthsKevin G2021/09/27 08:46 AM
                                            SVE alignment with non power-of-2 widths-.-2021/09/27 08:06 PM
                                              SVE alignment with non power-of-2 widthsJukka Larja2021/09/28 05:37 AM
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                    microbenchmark resultsJörn Engel2021/09/23 04:10 AM
                      microbenchmark resultsMichael S2021/09/23 04:55 AM
                        microbenchmark resultsJörn Engel2021/09/23 08:24 AM
                          microbenchmark resultsRoyi2021/09/26 03:25 PM
                      microbenchmark resultsdmcq2021/09/23 09:42 AM
                        microbenchmark results---2021/09/23 10:53 AM
                      microbenchmark resultsanon22021/09/23 01:40 PM
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                  microbenchmark results: Zen 3Adrian2021/09/22 01:08 AM
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            Armv8.8-A and Armv9.3-A - movesDoug S2021/09/28 12:57 PM
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                                  Did they publish a full description?rwessel2021/09/30 08:18 AM
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                                      Did they publish a full description?rwessel2021/09/30 09:42 AM
                                    Did they publish a full description?Adrian2021/09/30 11:22 PM
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                                    Do we even okiw it's three instructions per move?Adrian2021/09/30 11:27 PM
                                    Do we even okiw it's three instructions per move?rwessel2021/10/01 03:19 AM
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                              Armv8.8-A and Armv9.3-A - movesrwessel2021/09/30 09:39 AM
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                                                memcpy - instruction cracking vs DMA---2021/10/02 02:12 PM
                                                  Moving copy to DRAM doesn't help for small copiesMark Roulo2021/10/02 02:59 PM
                                                    Moving copy to DRAM doesn't help for small copies---2021/10/02 06:32 PM
                                                      Moving copy to DRAM doesn't help for small copiesMichael S2021/10/03 12:40 AM
                                                        Moving copy to DRAM doesn't help for small copiesDoug S2021/10/03 09:09 AM
                                                          Moving copy to DRAM doesn't help for small copiesrwessel2021/10/03 09:51 AM
                                                          Moving copy to DRAM doesn't help for small copiesLinus Torvalds2021/10/03 10:09 AM
                                                            How about environments such as Java?Mark Roulo2021/10/03 11:41 AM
                                                              How about environments such as Java?rwessel2021/10/03 11:49 AM
                                                                How about environments such as Java?Mark Roulo2021/10/03 12:22 PM
                                                              How about environments such as Java?anon22021/10/03 06:58 PM
                                                                How about environments such as Java?Etienne Lorrain2021/10/04 04:08 AM
                                                                  Apart from "It depends" there is no short answer. (NT)Michael S2021/10/04 04:30 AM
                                                                  How about environments such as Java?Andrey2021/10/04 05:04 AM
                                                                  How about environments such as Java?anon22021/10/04 05:32 AM
                                                                How about environments such as Java?Mark Roulo2021/10/04 06:31 AM
                                                                How about environments such as Java?---2021/10/04 08:41 AM
                                                                  How about environments such as Java?Doug S2021/10/04 09:23 AM
                                                                    How about environments such as Java?Andrey2021/10/04 11:14 AM
                                                                      How about environments such as Java?Doug S2021/10/04 12:20 PM
                                                                  How about environments such as Java?anon22021/10/04 01:23 PM
                                                                  How about environments such as Java?rwessel2021/10/04 03:54 PM
                                                            Moving copy to DRAM doesn't help for small copiesJörn Engel2021/10/04 04:52 AM
                                                            Early software zeroing !=== early hardware zeroingPaul A. Clayton2021/10/05 10:19 AM
                                                              Early software zeroing !=== early hardware zeroingDoug S2021/10/05 11:21 AM
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                                              memcpy - instruction cracking vs DMAMichael S2021/10/03 12:51 AM
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                                        Armv8.8-A and Armv9.3-A - movesEtienne Lorrain2021/10/01 06:55 AM
                                          Armv8.8-A and Armv9.3-A - movesrwessel2021/10/01 07:14 AM
                                            Armv8.8-A and Armv9.3-A - movesDoug S2021/10/01 10:17 AM
                                              Armv8.8-A and Armv9.3-A - movesrwessel2021/10/02 03:57 AM
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                          Armv8.8-A and Armv9.3-ADoug S2021/10/17 12:33 PM
                            Armv8.8-A and Armv9.3-AzArchJon2021/10/18 09:35 AM
                              Armv8.8-A and Armv9.3-ADoug S2021/10/18 01:35 PM
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