Cannot reproduce with microcode 0xc6

By: Travis Downs (travis.downs.delete@this.gmail.com), March 24, 2019 6:34 pm
Room: Moderated Discussions
Adrian (a.delete@this.acm.org) on February 28, 2019 9:54 am wrote:
> Travis Downs (travis.downs.delete@this.gmail.com) on February 27, 2019 7:25 am wrote:
> >
> > You can reverse the behavior, showing the max value by setting W_MAX envvar to 1, like so:
> >
> > W_MAX=1 ./offset-test.sh
> >
> > If you have the chance to run the CoffeeLake test again like this I would be very interested.
> >
>
>
> I have done the test again on the Coffee Lake.
>
> I have repeated it a few times without seeing anything except maybe a shift by
> about half cycle towards higher values. There was still no value over 10.
>
>
> Then I opened a file manager and I have started to do random clicks on directories or files.
>
> This caused immediately a few 18 cycle values, one precisely when I opened the file manager
> and a few others exactly when doing various clicks. Not all clicks had effects.
>
>
> So I assume that this happens due to some interaction with the cache activity
> of the other threads. I still wonder why with the old microcode it happened so
> reproducible at certain positions even without much activity on the computer.
>
>
>
> Like I have written in the other message, with W_MAX=1 and the old microcode there were some extra neighboring
> positions where the 18 cycle values appeared frequently, but not reproducible when the test was repeated.

Interesting. I saw a similar effect on Skylake (client) with the old microcodes: it usually ran fast, but in the specific case that I loaded all 4 CPUs with 'stress -c 4', the test would always run in slow mode. Loading only 3 CPUs only rarely triggered slow mode (probably when actually 4 CPUs were active due to some background activity).

The weird thing is that this only had to be true at the start of the test: once it started slow, every iteration would usually be slow even if I killed the extra load. The opposite was also true: if I started the load in the middle of the test it would stay fast. The effect also remained when I ran the test process at the highest priority of --rr 99, so it would always monopolize the CPU regardless of any other load.

I don't have any explanation for this effect, and whether it is hardware or OS related or what. I had some theory that if the CPU was not idle right as the test process started that it somehow would behave different, hence needing to load all 4 CPUs which guarantees no idle CPUs, but then I would expect a test where I loaded CPU 0, and the started with test with affinity forced to CPU 0 would also show the effect ... but it didn't.

Thanks for your help, Adrian!
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TopicPosted ByDate
Why does writing to non-sequential lines in L2 perform so poorly?Travis2017/12/20 02:44 PM
  Bridges? Wells? (NT)Micahel S2017/12/20 03:53 PM
    Bridges? Wells? (NT)Travis2017/12/20 04:46 PM
      That should say "huh"? (NT)Travis2017/12/20 04:46 PM
        That should say "huh"?Jeff S.2017/12/20 05:11 PM
          That should say "huh"?Travis2017/12/20 06:34 PM
    Bridges? Wells?Jeff S.2017/12/20 05:17 PM
      Bridges? Wells?Travis2017/12/20 06:37 PM
    Bridges, Wells - positiveMichael S2017/12/21 02:52 AM
      Bridges, Wells - positiveTravis2017/12/21 09:35 AM
        Bridges, Wells - positiveMichael S2017/12/21 10:00 AM
  Why does writing to non-sequential lines in L2 perform so poorly?Linus Torvalds2017/12/20 06:18 PM
    Why does writing to non-sequential lines in L2 perform so poorly?Travis2017/12/20 06:54 PM
      Why does writing to non-sequential lines in L2 perform so poorly?Linus Torvalds2017/12/21 12:12 PM
        Why does writing to non-sequential lines in L2 perform so poorly?anon2017/12/22 03:29 AM
          Why does writing to non-sequential lines in L2 perform so poorly?Linus Torvalds2017/12/22 01:16 PM
            Why does writing to non-sequential lines in L2 perform so poorly?Travis2017/12/23 08:48 PM
            Why does writing to non-sequential lines in L2 perform so poorly?Travis Downs2020/06/13 03:18 PM
              Why does writing to non-sequential lines in L2 perform so poorly?John D. McCalpin2020/06/18 12:50 PM
                Why does writing to non-sequential lines in L2 perform so poorly?Travis Downs2020/06/18 05:32 PM
                  Why does writing to non-sequential lines in L2 perform so poorly?Travis Downs2020/06/18 05:34 PM
    Why does writing to non-sequential lines in L2 perform so poorly?anon.12017/12/21 06:09 PM
      Why does writing to non-sequential lines in L2 perform so poorly?Linus Torvalds2017/12/22 01:20 PM
        Why does writing to non-sequential lines in L2 perform so poorly?Travis2017/12/24 02:09 PM
  Why does writing to non-sequential lines in L2 perform so poorly?Travis2017/12/20 08:52 PM
    Why does writing to non-sequential lines in L2 perform so poorly?Adrian2017/12/21 12:09 AM
      Why does writing to non-sequential lines in L2 perform so poorly?Travis2017/12/21 09:23 AM
    Why does writing to non-sequential lines in L2 perform so poorly?-.-2017/12/27 03:53 AM
      Why does writing to non-sequential lines in L2 perform so poorly?-.-2017/12/27 03:53 AM
        Why does writing to non-sequential lines in L2 perform so poorly?Travis2017/12/27 04:18 PM
  Why does writing to non-sequential lines in L2 perform so poorly?Etienne2017/12/21 02:36 AM
    Why does writing to non-sequential lines in L2 perform so poorly?Michael S2017/12/21 02:58 AM
      Why does writing to non-sequential lines in L2 perform so poorly?Travis2017/12/21 09:26 AM
        Michael ignore my last question - saw your other reply (NT)Travis2017/12/21 09:27 AM
  Why does writing to non-sequential lines in L2 perform so poorly?Nksingg2017/12/26 06:47 AM
    Why does writing to non-sequential lines in L2 perform so poorly?David Kanter2017/12/26 11:48 AM
    Why does writing to non-sequential lines in L2 perform so poorly?Travis2017/12/27 04:33 PM
  Cannot reproduce with microcode 0xc6Travis Downs2019/02/26 04:23 PM
    Cannot reproduce with microcode 0xc6Adrian2019/02/26 09:35 PM
    Cannot reproduce with microcode 0xc6Adrian2019/02/26 10:07 PM
    Cannot reproduce with microcode 0xc6Adrian2019/02/27 05:02 AM
      Cannot reproduce with microcode 0xc6Travis Downs2019/02/27 08:25 AM
        Cannot reproduce with microcode 0xc6Adrian2019/02/28 01:16 AM
          Cannot reproduce with microcode 0xc6Travis Downs2019/03/07 06:51 PM
        Cannot reproduce with microcode 0xc6Adrian2019/02/28 09:54 AM
          Cannot reproduce with microcode 0xc6Travis Downs2019/03/24 06:34 PM
    Cannot reproduce with microcode 0xc6Travis Downs2019/02/27 03:20 PM
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