Infinity Fabric = PCIe

By: Aaron Spink (, March 16, 2017 11:21 pm
Room: Moderated Discussions
Michael S ( on March 16, 2017 2:45 pm wrote:
> Third, I don't see how, at lower protocol layers and assuming the same transfer rate, QPI could
> have lower latency than HT3. The two are rather similar. When there are differences, like
> in check sequences, QPI appears to have higher algorithmic delays. In return, it potentially
> has higher reliability. I say potentially, because I don't believe that bit errors rate at
> which the difference matters occur in real life. I believe that in real life the BER is either
> so low that both HT3 and QPI work fine or so high that both of them do not survive.
> I don't know what happens at higher protocol layers. They are not public.
Well, first of all, they doesn't have the same GT/s. QPI should pretty much always have a signficant transfer rate advantage over HT for a given dB loss budget.

I don't know what check sequences you think QPI has higher algorithmic delays for, can you give and example?

BER is always somewhat of a conundrum on the design side of things. I know, and the electrical guys generally know, that the realistic BER is significantly better than any BER that they will ever commit to. This is a result of the realities of both what can be controlled by who and what can be reasonably measured for a given part. AKA, if we are only testing to say 10e-15, then we can only rely on 10e-15. As far as what can survive, QPI can survive with a realistically extremely high BER while losing minimal performance. It can also contain errors that do happen on the link for a wide variety of reasons. That's largely due to the per flit error containment in QPI. In stark contrast, HT as far as I've seen it defined has little to no containment but merely halt and catch fire functionality relying on periodic CRC checking that is effectively independent of flow control and therefore protocol control.
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    Infinity Fabric = PCIeslacker2017/03/15 02:59 PM
      Infinity Fabric = PCIeMichael S2017/03/15 04:19 PM
        Infinity Fabric = PCIeIreland2017/03/15 04:49 PM
        Infinity Fabric = PCIeRicardo B2017/03/15 10:25 PM
        Infinity Fabric = PCIeslacker2017/03/15 10:45 PM
          Infinity Fabric = PCIeRicardo B2017/03/15 11:02 PM
            Infinity Fabric = PCIeAaron Spink2017/03/16 01:08 AM
              Infinity Fabric = PCIeRicardo B2017/03/16 05:30 AM
                Infinity Fabric = PCIeAaron Spink2017/03/16 10:58 AM
        Infinity Fabric = PCIeGabriele Svelto2017/03/16 03:48 AM
          Latency vs. bandwidthDavid Kanter2017/03/16 08:04 AM
      Infinity Fabric = PCIeGroo2017/03/15 05:35 PM
        Infinity Fabric = PCIeslacker2017/03/15 11:11 PM
          Infinity Fabric = PCIeAdrian2017/03/16 02:51 AM
            Infinity Fabric = PCIeMichael S2017/03/16 04:32 AM
              Infinity Fabric = PCIeAdrian2017/03/16 07:58 AM
                Infinity Fabric = PCIeRicardo B2017/03/16 09:16 AM
                  Infinity Fabric = PCIeMichael S2017/03/16 09:54 AM
                    Infinity Fabric = PCIeRicardo B2017/03/16 10:07 AM
                      Infinity Fabric = PCIeMichael S2017/03/16 02:33 PM
                        Infinity Fabric = PCIeGabriele Svelto2017/03/17 03:35 AM
                          Infinity Fabric = PCIeAaron Spink2017/03/17 10:25 AM
                            Thanks, your posts on this thread have been extremely informative (NT)Gabriele Svelto2017/03/17 01:08 PM
                            Infinity Fabric = PCIeRicardo B2017/03/17 06:08 PM
                              Infinity Fabric = PCIeAaron Spink2017/03/17 06:27 PM
                              Quickpath papers - circuits and implementationslacker2017/03/18 06:50 PM
                                Quickpath papers - circuits and implementationRicardo B2017/03/19 08:07 AM
                    Infinity Fabric = PCIeAaron Spink2017/03/16 11:11 AM
                      Infinity Fabric = PCIeMichael S2017/03/16 02:46 PM
                        Infinity Fabric = PCIeRicardo B2017/03/16 03:07 PM
                          Infinity Fabric = PCIeMichael S2017/03/16 05:13 PM
                            Infinity Fabric = PCIeAaron Spink2017/03/16 11:04 PM
                            Infinity Fabric = PCIeRicardo B2017/03/16 11:30 PM
                              Infinity Fabric = PCIeMichael S2017/03/17 07:24 AM
                                Infinity Fabric = PCIeRicardo B2017/03/17 06:07 PM
              Infinity Fabric = PCIeDavid Kanter2017/03/16 08:36 AM
                Infinity Fabric = PCIeMichael S2017/03/16 08:50 AM
                  Infinity Fabric = PCIeAaron Spink2017/03/16 11:24 AM
                    Infinity Fabric = PCIeMichael S2017/03/16 02:45 PM
                      Infinity Fabric = PCIeAaron Spink2017/03/16 11:21 PM
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