Bridges, Wells - positive

By: Michael S (already5chosen.delete@this.yahoo.com), December 21, 2017 10:00 am
Room: Moderated Discussions
Travis (travis.downs.delete@this.gmail.com) on December 21, 2017 8:35 am wrote:
> Michael S (already5chosen.delete@this.yahoo.com) on December 21, 2017 1:52 am wrote:
> > Micahel S (already5chosen.delete@this.yahoo.com) on December 20, 2017 2:53 pm wrote:
> > > Bridges? Wells?
> >
> > Yes, Bridges and Wells show the same behavior.
> > 13.5 cycles per iteration on Ivy Bridge (i7-3770)
> > 14 cycles per iteration on Haswell (E3-1271 v3)
>
> Huh - those are the values for the loop with 2 stores?

Yes.

> What do you get for 1 store?

5.5 or 6.

> How
> many iterations are you averaging over?

100M iterations total. So, for 64K buffers, I run the measurement ~100K time

> If you print out the results for shorter loops
> I wonder if you see the bimodal behavior: i.e., if 13.5 is the average between (longish)
> periods of ~9 cycles and periods of ~18 cycles (that's what I see on Skylake).

I am not *that* interested.

>
> >
> > So my first thought (associativity conflict, due to SKL L2 having fewer ways than L1D) proves wrong.
> >
> > However I still think that for some reason approximately 30-50% of the stores that
> > shell be going to L2 end up in main memory and most of the rest goes to LLC.
> >
> > Unlike me, you like to read performance counters. What do they say?
>
> They show that the loops are bottlenecked on store buffer entries, which makes sense: the stores are committing
> slowly so the bottleneck as observed by the core will always be "SB full". It doesn't tell us much.
>
> The counters dealing with hits and misses mostly tell the expected story: the expected number
> of references to L2 and essentially no requests L3 or DRAM, so does not appear anything is
> going to those higher levels (and indeed the results are probably "too fast" for that).

No, not too fast.
On my Haswell I see that LLC is capable to store a cache line approximately every 7.5 clocks.

But if the counters say that it is not happening then I have to believe the counters.

>
> You also find that while there are many fine-grained and interesting counters for all sorts of load
> stalls, stores definitely get the short end of the stick, so in general you don't have very much
> visibilities into stores. At the L2 level there are l2_rqsts.all_rfo, and l2_rqsts.rfo_miss and l2_rqsts.rfo_hit,
> but this apparently aren't triggered by stores to lines already modified in L2, so they are close
> to zero (maybe they trigger only when the state had to be changed from something else to M in L2,
> or maybe they are counting RFO requests from other cores that probe this L2). You only see the stores
> indirectly though l2_rqsts.references and it has the expected number.
>
>

< Previous Post in ThreadNext Post in Thread >
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
Reply to this Topic
Name:
Email:
Topic:
Body: No Text
How do you spell purple?