ARM A73 benchmarks-latency

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), March 16, 2017 2:01 pm
Room: Moderated Discussions
Gabriele Svelto (gabriele.svelto.delete@this.gmail.com) on March 16, 2017 3:45 am wrote:
>
> If TLB performance wasn't important then why so much resources were spent in improving them in the last few
> years? We've got improvements in the speed of TLBs (dual walkers, faster reload and invalidation), in their
> size (larger first level TLBs, second-level TLBs) and effectiveness (larger pages, tagging, nested paging).

There's a lot of other subtle things too, like whether the CPU caches the TLB lookups in the regular data caches, and how well it handles TLB misses in the pipeline.

For example, maybe you have an fairly out-of-order memory model, and you can reorder loads and stores - but your memory pipeline might need the physical address to be able to do that. So a TLB miss may be more than just having to look things up in page tables: it may end up basically serializing your memory operations, and causing your pipeline to stall entirely, and turn what should be an out-of-order machine effectively into a slow in-order one.

And in-order is particularly bad when it means that you don't get to overlap the memory accesses.

It's fairly easy to see a memory pipeline that works well when things are nice and in the TLB, and just falls entirely flat on its face when even just some of the memory accesses miss in the TLB.

Software TLB filling shows this problem very very clearly, but honestly, hardware walking in no way guarantees it doesn't happen. An overly simple hardware walker will have the exact same issues that a SW walker has.

Linus
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TopicPosted ByDate
ARM A73 benchmarksSymmetry03/14/17 06:24 AM
  ARM A73 benchmarksPer Hesselgren03/14/17 07:18 AM
    ARM A73 benchmarks-latencyPer Hesselgren03/14/17 08:58 AM
      ARM A73 benchmarks-latencySymmetry03/14/17 10:12 AM
        ARM A73 benchmarks-latencyPer Hesselgren03/14/17 03:54 PM
          ARM A73 benchmarks-latencyWilco03/15/17 01:45 AM
            ARM A73 benchmarks-latencyPer Hesselgren03/15/17 02:57 AM
              ARM A73 benchmarks-latencyPer Hesselgren03/15/17 03:00 AM
                ARM A73 benchmarks-latencyPer Hesselgren03/15/17 03:01 AM
                  clickable linkMichael S03/15/17 04:05 AM
            ARM A73 benchmarks-latencyLinus Torvalds03/15/17 10:05 AM
              ARM A73 benchmarks-latencyIreland03/15/17 05:02 PM
              ARM A73 benchmarks-latencyGabriele Svelto03/16/17 03:45 AM
                ARM A73 benchmarks-latencyLinus Torvalds03/16/17 02:01 PM
                  lmbench is horribly brokenWilco03/16/17 04:57 PM
                    lmbench is horribly brokenLinus Torvalds03/16/17 06:49 PM
                      lmbench is horribly brokenLinus Torvalds03/17/17 01:10 PM
                        lmbench is horribly brokenLinus Torvalds03/17/17 01:52 PM
                        lmbench is horribly brokenExophase03/17/17 02:31 PM
                          lmbench is horribly brokenGabriele Svelto03/17/17 03:20 PM
                          lmbench is horribly brokenLinus Torvalds03/17/17 05:56 PM
                            lmbench is horribly brokenExophase03/17/17 06:21 PM
                              lmbench is horribly brokenLinus Torvalds03/17/17 06:43 PM
                                lmbench is horribly brokenIreland03/17/17 07:37 PM
                                  lmbench is horribly brokenbakaneko03/18/17 11:17 AM
                                    lmbench is horribly brokenIreland03/18/17 12:23 PM
                                      lmbench is horribly brokenanon03/18/17 07:35 PM
                                      lmbench is horribly brokenbakaneko03/21/17 08:08 AM
                                        lmbench is horribly brokenIreland03/21/17 03:14 PM
                                lmbench is horribly brokenGabriele Svelto03/18/17 04:01 PM
                                  accessing dram RichardC03/18/17 06:33 PM
                                lmbench is horribly brokenExophase03/18/17 04:26 PM
                                  lmbench is horribly brokenWilco03/18/17 05:40 PM
                                    benchmarking reality?Anon03/19/17 02:29 PM
                                    lmbench is horribly brokenLinus Torvalds03/19/17 04:25 PM
                                      mea culpa (lmbench is horribly broken)Linus Torvalds03/19/17 06:05 PM
                                        mea culpa (lmbench is horribly broken)Bill Broadley03/21/17 01:41 AM
                                          mea culpa (lmbench is horribly broken)Linus Torvalds03/21/17 09:01 AM
                                            mea culpa (lmbench is horribly broken)Linus Torvalds03/21/17 11:14 AM
                                            mea culpa (lmbench is horribly broken)Linus Torvalds03/21/17 05:03 PM
                                              mea culpa (lmbench is horribly broken)Etienne03/22/17 04:37 AM
                                              mea culpa (lmbench is horribly broken)Tim McCaffrey03/22/17 08:54 AM
                                                mea culpa (lmbench is horribly broken)Tim McCaffrey03/22/17 09:34 AM
                                                mea culpa (lmbench is horribly broken)Linus Torvalds03/22/17 10:35 AM
                                                  mea culpa (lmbench is horribly broken)Ireland03/22/17 12:11 PM
                                                    mea culpa (lmbench is horribly broken)Ireland03/22/17 12:26 PM
                                                    mea culpa (lmbench is horribly broken)rwessel03/22/17 03:03 PM
                                                      mea culpa (lmbench is horribly broken)Ireland03/22/17 03:35 PM
                                                  mea culpa (lmbench is horribly broken)Linus Torvalds03/22/17 01:35 PM
                                                    mea culpa (lmbench is horribly broken)Gabriele Svelto03/23/17 08:05 AM
                                                      mea culpa (lmbench is horribly broken)Linus Torvalds03/23/17 10:43 AM
                                                        mea culpa (lmbench is horribly broken)Gabriele Svelto03/23/17 01:56 PM
                                                          mea culpa (lmbench is horribly broken)Ireland03/23/17 02:36 PM
                                                  mea culpa (lmbench is horribly broken)Travis03/22/17 01:38 PM
                                              mea culpa (lmbench is horribly broken)anon03/22/17 07:22 PM
                                                mea culpa (lmbench is horribly broken)Travis03/22/17 08:57 PM
                                                  mea culpa (lmbench is horribly broken)anon03/23/17 12:44 AM
                                                    mea culpa (lmbench is horribly broken)Michael S03/23/17 05:59 PM
                                                      mea culpa (lmbench is horribly broken)Travis03/23/17 09:03 PM
                                                    power8 numbersoctoploid03/24/17 11:47 PM
                                                      power8 numbers stride=128octoploid03/25/17 04:36 AM
                                                        power8 numbers stride=128Linus Torvalds03/25/17 10:50 AM
                                                          power8 numbers stride=128Gabriele Svelto03/25/17 11:27 PM
                                              mea culpa (lmbench is horribly broken)anon03/23/17 01:14 AM
                                                mea culpa (lmbench is horribly broken)Linus Torvalds03/23/17 11:22 AM
                                                  Thank you. Associativity misses explain it.anon03/23/17 10:48 PM
                                                    Thank you. Associativity misses explain it.Linus Torvalds03/24/17 01:26 PM
                                                      Thank you. Associativity misses explain it.Travis03/24/17 10:01 PM
                                                        thanks should read "but if it is any TYPE of mix" (NT)Travis03/24/17 10:02 PM
                                                        Thank you. Associativity misses explain it.Linus Torvalds03/25/17 12:10 PM
                                                          Thank you. Associativity misses explain it.Travis03/25/17 04:08 PM
                                                            Thank you. Associativity misses explain it.Linus Torvalds03/26/17 10:27 AM
                                  lmbench is horribly brokenLinus Torvalds03/19/17 03:51 PM
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