5-6 wide core, why no mention from Intel?

By: David Kanter (dkanter.delete@this.realworldtech.com), October 1, 2015 3:49 pm
Room: Moderated Discussions
Maynard Handley (name99.delete@this.name99.org) on October 1, 2015 1:57 pm wrote:
> David Kanter (dkanter.delete@this.realworldtech.com) on October 1, 2015 10:01 am wrote:
> > Wouter Tinus (wouter.tinus.delete@this.gmail.com) on September 30, 2015 3:14 pm wrote:
> > > It seems easy to argue that Skylake is a 5-wide or even 6-wide machine.
> > >
> > > - 5 wide decode
> > > - 6 wide allocation/decoder queue
> > > - 6 wide ROB
> > > - 8 wide issue
> > > - 8 wide retire (4/thread)
> > >
> > > Though Haswell already added extra two extra issue ports, this the first real increase in width
> > > since the introduction of Merom back in 2006. Yet they didn't even bother to mention it at IDF :(
> >
> > Actually, I think Sandy Bridge and Haswell were more significant.
> >
> > It's nice to have more ALUs, but what really matters are the load/store units. Having 10 ALUs with 1 LD/ST
> > unit is really pointless, except on code with insanely high compute:memory ratios (which isn't most code).
> >
> > For a general purpose CPU, I'd focus on getting the load/store right first, then focus on the ALUs.
>
> I'm missing your point here, David. Is this sarcasm, or a dig at another CPU?
> Hasn't Intel had 2 load/1 store per cycle since, what, Sandy Bridge?

My above statement was serious. It is certainly negative commentary on many CPUs, which have crappy load/store units. But it wasn't a dig at any in particular. I could point out a number of CPUs with crappy memory hierarchies, starting with the P4 and Bulldozer.

> (FWIW I agree with you that load/store matters. I suspect that's a bottleneck Apple will tackle in the future
> moving from their current 2 loads or 1 load/1 store [I don't think they support 2 store/cycle];

2 stores/cycle is pretty expensive, and I suspect there is lower hanging fruit for Apple. Also, stores are very expensive in terms of coherency/consistency/ordering.

> but since they're more concerned with power than Intel getting to that point may require their
> swapping out the traditional style load-store queues (associative and so expensive) with the
> sort of "indexed" queues that have been suggested as one component of kilo-instruction class
> machines. This would be a venture into somewhat uncharted territory [I don't think anyone has
> commercialized these ideas yet] so I suspect they won't go there until they have to.

What kind of indexed queues? Honestly, getting away from cams in the load/store unit seems damn hard when you want low latency. If you want store forwarding, you simply have to do something like a cam check.

> Of course Apple [and ARM in general] have the advantage of load-store pair which isn't
> perfect (eg it's not going to help your vector throughput) but certainly helps in a large
> set of common cases, and so reduces the pressure to amp up to Intel's 2+1 load/store.)

Uh, LDP and STP mean its more likely that you want multiple load/store units. I'm not 100% sure of the semantics and benefits.

For example, what happens if the pair loads target different pages? You'd need to do two separate translations through the TLB.

David
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TopicPosted ByDate
Update to Intel Optimization ManualSHK2015/09/29 05:38 AM
  gather speedEric Bron2015/09/29 09:43 AM
    gather speedGabriele Svelto2015/09/29 12:00 PM
  Update to Intel Optimization ManualTim McCaffrey2015/09/29 11:18 AM
    Update to Intel Optimization ManualSHK2015/09/29 12:04 PM
      Update to Intel Optimization ManualAnon2015/09/29 02:23 PM
    Update to Intel Optimization Manualnone2015/09/29 10:31 PM
      Update to Intel Optimization ManualMichael S2015/09/30 04:24 AM
    Update to Intel Optimization ManualMichael S2015/09/30 04:30 AM
      Update to Intel Optimization ManualTim McCaffrey2015/09/30 10:01 AM
  5-6 wide core, why no mention from Intel?Wouter Tinus2015/09/30 02:14 PM
    5-6 wide core, why no mention from Intel?Maynard Handley2015/09/30 03:30 PM
      5-6 wide core, why no mention from Intel?Alberto2015/10/01 12:13 AM
        5-6 wide core, why no mention from Intel?anon2015/10/01 02:21 AM
          5-6 wide core, why no mention from Intel?Alberto2015/10/01 04:41 AM
            5-6 wide core, why no mention from Intel?anon2015/10/01 05:27 AM
              5-6 wide core, why no mention from Intel?Alberto2015/10/01 08:33 AM
                5-6 wide core, why no mention from Intel?juanrga2015/10/01 10:24 AM
        5-6 wide core, why no mention from Intel?Maynard Handley2015/10/01 08:57 AM
    5-6 wide core, why no mention from Intel?juanrga2015/10/01 03:59 AM
      5-6 wide core, why no mention from Intel?Wouter Tinus2015/10/01 02:48 PM
        5-6 wide core, why no mention from Intel?juanrga2015/10/03 03:17 AM
          5-6 wide core, why no mention from Intel?Wouter Tinus2015/10/03 11:19 AM
            Are you kidding? (NT)juanrga2015/10/04 05:30 AM
              Are you kidding?Wouter Tinus2015/10/04 03:18 PM
                Are you kidding?juanrga2015/10/05 09:46 AM
                  Are you kidding?David Kanter2015/10/05 11:24 AM
                    Are you kidding?anon2015/10/05 09:26 PM
                    Are you kidding?Linus Torvalds2015/10/07 04:49 AM
                      Are you kidding?juanrga2015/10/07 10:46 AM
                        Are you kidding?anon2015/10/07 06:21 PM
                  Are you kidding?Wouter Tinus2015/10/05 01:25 PM
                    Are you kidding?juanrga2015/10/06 10:17 AM
                      Are you kidding?Stubabe2015/10/07 12:17 AM
                        Are you kidding?juanrga2015/10/07 10:56 AM
                          Amazing...Wouter Tinus2015/10/07 11:31 AM
                            Amazing...juanrga2015/10/07 03:45 PM
                          Are you kidding?Stubabe2015/10/07 11:57 AM
                            Are you kidding?juanrga2015/10/07 03:59 PM
                          Are you kidding?Wilco2015/10/07 02:07 PM
                            Are you kidding?juanrga2015/10/07 04:33 PM
      5-6 wide core, why no mention from Intel?Eric Bron2015/10/04 04:18 AM
    5-6 wide core, why no mention from Intel?David Kanter2015/10/01 09:01 AM
      Optimal number and kind of execution unitsjuanrga2015/10/01 10:50 AM
        Optimal number and kind of execution unitsPatrick Chase2015/10/01 04:38 PM
          Optimal number and kind of execution unitsI.S.T.2015/10/01 05:10 PM
            Optimal number and kind of execution unitsPatrick Chase2015/10/01 11:39 PM
          Optimal number and kind of execution unitsExophase2015/10/01 10:11 PM
          Optimal number and kind of execution unitsjuanrga2015/10/02 05:14 AM
      LD/ST unitsSHK2015/10/01 11:11 AM
        LD/ST unitsDavid Kanter2015/10/01 12:54 PM
          LD/ST unitsSHK2015/10/02 04:55 AM
            LD/ST unitsJukka Larja2015/10/02 09:49 PM
        LD/ST unitsMaynard Handley2015/10/01 01:01 PM
          LD/ST unitsanon2015/10/01 09:54 PM
      5-6 wide core, why no mention from Intel?Maynard Handley2015/10/01 12:57 PM
        5-6 wide core, why no mention from Intel?David Kanter2015/10/01 03:49 PM
          5-6 wide core, why no mention from Intel?Maynard Handley2015/10/01 06:21 PM
          5-6 wide core, why no mention from Intel?Exophase2015/10/01 10:07 PM
            5-6 wide core, why no mention from Intel?Maynard Handley2015/10/02 12:10 AM
              5-6 wide core, why no mention from Intel?Megol2015/10/02 03:39 AM
                5-6 wide core, why no mention from Intel?Michael S2015/10/02 04:27 AM
                5-6 wide core, why no mention from Intel?Maynard Handley2015/10/02 09:37 AM
                  5-6 wide core, why no mention from Intel?noko2015/10/02 05:19 PM
              5-6 wide core, why no mention from Intel?Exophase2015/10/02 06:43 AM
                5-6 wide core, why no mention from Intel?Maynard Handley2015/10/02 09:45 AM
                  5-6 wide core, why no mention from Intel?Exophase2015/10/02 10:23 AM
          5-6 wide core, why no mention from Intel?Wilco2015/10/02 12:48 PM
            5-6 wide core, why no mention from Intel?Maynard Handley2015/10/02 01:25 PM
              5-6 wide core, why no mention from Intel?Wilco2015/10/02 02:26 PM
              5-6 wide core, why no mention from Intel?noko2015/10/02 05:45 PM
                5-6 wide core, why no mention from Intel?Maynard Handley2015/10/02 06:54 PM
            5-6 wide core, why no mention from Intel?David Kanter2015/10/02 01:59 PM
              5-6 wide core, why no mention from Intel?Wilco2015/10/02 02:59 PM
                5-6 wide core, why no mention from Intel?David Kanter2015/10/02 03:15 PM
                  5-6 wide core, why no mention from Intel?Wilco2015/10/02 04:06 PM
                    LDP/STP usage in AArch64 for 403.gccnone2015/10/03 01:04 AM
                      LDP/STP usage in AArch64 for 403.gccWilco2015/10/03 03:02 AM
                        LDP/STP usage in AArch64 for 403.gccnone2015/10/03 03:11 AM
                          LDP/STP usage in AArch64 for 403.gccWilco2015/10/03 03:37 AM
                            LDP/STP usage in AArch64 for 403.gccnone2015/10/03 04:37 AM
                              LDP/STP usage in AArch64 for 403.gccWilco2015/10/03 05:26 AM
                  5-6 wide core, why no mention from Intel?Maynard Handley2015/10/02 04:24 PM
              5-6 wide core, why no mention from Intel?Maynard Handley2015/10/02 03:07 PM
  Update to Intel Optimization Manualanon2015/09/30 04:43 PM
  Update to Intel Optimization ManualPatrick Chase2015/09/30 09:44 PM
    Update to Intel Optimization Manualanon2015/09/30 10:49 PM
    Update to Intel Optimization Manualnone2015/09/30 10:50 PM
    Update to Intel Optimization ManualDavid Kanter2015/10/01 12:52 PM
      Update to Intel Optimization ManualPatrick Chase2015/10/01 04:16 PM
        Update to Intel Optimization Manualanon2015/10/01 10:45 PM
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