To elaborate

By: Travis Downs (travis.downs.delete@this.gmail.com), July 13, 2020 6:03 am
Room: Moderated Discussions
David Kanter (dkanter.delete@this.realworldtech.com) on July 12, 2020 8:22 pm wrote:
> David Kanter (dkanter.delete@this.realworldtech.com) on July 12, 2020 8:20 pm wrote:
> > Travis Downs (travis.downs.delete@this.gmail.com) on July 12, 2020 8:03 pm wrote:
> > > David Kanter (dkanter.delete@this.realworldtech.com) on July 12, 2020 6:13 pm wrote:
> > > > I did some analysis a while back that might useful to share here.
> > > >
> > > > 8.0 mm2 SKL core
> > > > 0.9 mm2 AVX512
> > > > 2.0 mm2 1MB L2$
> > > > 2.4 mm2 1.375MB L3$
> > > > 0.4 mm2 Snoop filter
> > > > 2.0 mm2 caching and home agent
> > > > 2.2 mm2 FIVR, PLLs
> > > > 0.4 mm2 Mystery block
> > > >
> > > > 18.0 mm2 Total SKL-SP tile
> > > >
> > > > So AVX512 is about 5% of the tile area, the tiles are 72% of the total area of SKL-SP.
> > > >
> > > > If you removed AVX512 you'd save 28mm2 for the whole chip, which would let you add at most 2 tiles.
> > > >
> > > > In that vein, it seems like a pretty reasonable trade-off.
> > >
> > > Hmm, you read my mind, I was just wandering over to see if someone had done something like this.
> >
> > A long time ago I did the analysis for a ton of this stuff.
> >
> > > Do you have anything for BDW?
> >
> > Nope.
> >
> > > Another way of approaching the full cost of AVX-512 would be to look
> > > at the BDW-SKL delta. Obviously there were other changes in SKL that may have increased area, but
> > > not that many, and BDW is the same process so this could upper-bound the AVX2->AVX-512 cost.
> >
> > There were a lot of changes in BDW-->SKL. I suspect this analysis wouldn't be as fruitful.
>
> The other thing to consider is that many of the SKL microarchitectural
> choices might be tied to AVX512. E.g., banking/porting in the L1D.

That's sort of the the point. You can measure pretty effectively the size of the vector units and register files (that's how you got your numbers, right), but others might argue that supporting AVX-512 has knock-on effects all over the chip. For example, they might say the load-store path has to be 4x as large (vs 128-bit) and so on.

So the idea is that since BDW is the last chip to be clearly a 256-bit design, you can compare BDW and SKL to get a kind of upper bound on the cost of 256-512, including the types of needed-for-512 architectural changes you mention.

Yeah, SKL has some other changes vs BDW, not related to 512, but not *all that many*. Anyway, these push the error in one direction: making 512 cost look larger than it is, so it's a useful upper bound (because you can say "even under the very conservative assumption that 100% of the added area was 512 support...).

< Previous Post in ThreadNext Post in Thread >
TopicPosted ByDate
Skylake-SP area breakdownDavid Kanter2020/07/12 05:13 PM
  Skylake-SP area breakdownanon22020/07/12 06:01 PM
    Skylake-SP area breakdownTravis Downs2020/07/12 07:02 PM
      Skylake-SP area breakdownanon2020/07/12 07:44 PM
  Skylake-SP area breakdownTravis Downs2020/07/12 07:03 PM
    Skylake-SP area breakdownDavid Kanter2020/07/12 07:20 PM
      To elaborateDavid Kanter2020/07/12 07:22 PM
        To elaborateTravis Downs2020/07/13 06:03 AM
          To elaborateAnon2020/07/13 06:36 AM
            To elaborateAdrian2020/07/13 12:45 PM
              To elaborateAnon2020/07/13 01:06 PM
                To elaborateChester2020/07/13 07:30 PM
  Alternatives ImplementationsKyle Siefring2020/07/13 05:02 PM
    Alternatives ImplementationsTravis Downs2020/07/13 07:41 PM
    Alternatives ImplementationsMaynard Handley2020/07/13 09:37 PM
      Alternatives ImplementationsDoug S2020/07/13 10:25 PM
        Mask costsDavid Kanter2020/07/14 07:13 AM
        Alternatives Implementationstarlinian2020/07/14 07:22 AM
          Alternatives ImplementationsDoug S2020/07/14 09:03 AM
          Alternatives ImplementationsMaynard Handley2020/07/14 09:12 AM
        Alternatives ImplementationsMaynard Handley2020/07/14 09:10 AM
          Alternatives ImplementationsDoug S2020/07/14 09:47 AM
            Alternatives ImplementationsBrett2020/07/14 12:38 PM
            Alternatives Implementationstarlinian2020/07/14 01:30 PM
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