Ring vs mesh LLC bandwidth

By: Travis Downs (travis.downs.delete@this.gmail.com), February 22, 2021 11:40 pm
Room: Moderated Discussions
I recall reading several times was that one of the primary motivations for moving from a ring topology (used for everything Sandy Bridge up to Broadwell, and Skylake client) to a mesh (used in SKX and later server chips) was because the ring was a bandwidth limiter as the core counts increased (this was also the reason for the dual ring design on the high core count chips of later generations).

However, this paper directly compares the LLC bandwidth for Broadwell and Cascade Lake, and finds that in terms of absolute bandwidth per core and bandwidth scaling, Broadwell seems to do better, especially for reads (see 4.2 "L3 Scalability"):


BDW vs CLX LLC Bandwidth

Note how CLX L3 bandwidth tails off fairly heavily at high core counts for each test.

What gives?

CLX comes out looking mostly fine on the actual applications evaluated in the rest of the paper, especially due to might high memory bandwidth, but I was surprised to the see the primary purported benefit of the mesh not being at all evident here (the opposite, really).
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TopicPosted ByDate
Ring vs mesh LLC bandwidthTravis Downs2021/02/22 11:40 PM
  Ring vs mesh LLC bandwidthGionatan Danti2021/02/23 02:42 AM
    Intel Mesh Whitepaperanon2021/02/23 09:30 AM
      Intel Mesh WhitepaperMichael S2021/02/23 10:10 AM
        Intel Mesh Whitepaperanonymou52021/02/23 02:48 PM
          Intel Mesh WhitepaperLinus Torvalds2021/02/23 03:02 PM
            Intel Mesh WhitepaperFoo_2021/02/23 04:09 PM
              Intel Mesh WhitepaperJeff S.2021/02/23 04:50 PM
              Intel Mesh WhitepaperLinus Torvalds2021/02/23 05:31 PM
                Sapphire Rapids Chiplet Rumorsanon2021/02/23 06:37 PM
                  Sapphire Rapids Chiplet Total SpeculationWes Felter2021/02/23 07:55 PM
                    Sapphire Rapids Chiplet Total SpeculationMichael S2021/02/24 03:18 AM
                      Sapphire Rapids Chiplet Total SpeculationAndrei F2021/02/24 04:09 AM
                        Sapphire Rapids Chiplet Total SpeculationJeff S.2021/02/24 10:04 AM
                          Sapphire Rapids Chiplet Total SpeculationAndrei F2021/02/24 04:00 PM
                            Sapphire Rapids Chiplet Total SpeculationJeff S.2021/02/24 05:25 PM
                              LinkAnon2021/02/24 05:40 PM
                Intel Mesh WhitepaperMichael S2021/02/24 03:04 AM
              Intel Mesh WhitepaperAndrei F2021/02/24 04:05 AM
            Intel Mesh WhitepaperJukka Larja2021/02/24 08:28 AM
              Intel Mesh WhitepaperMichael S2021/02/24 10:31 AM
                Intel Mesh WhitepaperJukka Larja2021/02/25 06:33 AM
              Intel Mesh WhitepaperKevin G2021/02/24 11:29 AM
                Intel Mesh WhitepaperJukka Larja2021/02/25 06:43 AM
                  Intel Mesh WhitepaperTim McCaffrey2021/02/25 12:30 PM
                    Intel Mesh WhitepaperAnon2021/02/25 01:24 PM
                  > 64 threads not expensivejuha2021/02/26 01:25 AM
                    > 64 threads not expensiveJukka Larja2021/02/27 01:38 PM
          Intel Mesh WhitepaperAaron Spink2021/02/24 03:10 PM
            Intel Mesh WhitepaperNitin Agrawal2021/02/25 02:58 AM
              Intel Mesh WhitepaperFoo_2021/02/25 03:27 AM
              Intel Mesh WhitepaperMichael S2021/02/25 05:17 AM
                Intel Mesh WhitepaperEmil Briggs2021/02/26 05:20 AM
                  Intel Mesh WhitepaperNitin Agrawal2021/02/26 08:37 AM
                    Intel Mesh WhitepaperEmil Briggs2021/02/26 10:50 AM
      Intel Mesh WhitepaperTravis Downs2021/02/23 11:21 PM
        Loaded vs idle latency/bandwidthanon2021/02/24 09:11 AM
          Loaded vs idle latency/bandwidthTravis Downs2021/02/24 01:15 PM
            s/have between/have better/ (NT)Travis Downs2021/02/24 01:16 PM
            Loaded vs idle latency/bandwidthanon2021/02/24 05:40 PM
              Loaded vs idle latency/bandwidthTravis Downs2021/02/27 12:14 AM
                Loaded vs idle latency/bandwidthGionatan Danti2021/03/01 05:09 AM
        Intel Mesh Whitepaperblaine2021/02/24 11:40 AM
  Ring vs mesh LLC bandwidthChester2021/02/24 12:22 PM
    Ring vs mesh LLC bandwidthTravis Downs2021/02/24 12:53 PM
      Ring vs mesh LLC bandwidthChester2021/02/25 10:15 AM
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