Somthing That Pass Me By - GT/s?

Discussion in 'PC Industry' started by sir doris, Oct 27, 2009.

  1. sir doris

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    I noticed the speed of the i7's QPI is measured in GT/s as opposed to MB/s, can anyone tell me what this magical new (to me at least) unit of measurement means?
     
  2. Albuquerque

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    GigaTransactions per second. Each transaction can be 20 bits, so on the 4.8 GT/s boards that's a 12GB/s serial connection between each endpoint on the bus.

    I'm not really sure why they went to a packet-based measuring system. Hopefully someone else knows the inside scoop on that...
     
  3. sir doris

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    Thanks, that's enough information to keep me happy :)
     
  4. Albuquerque

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    I haven't gone digging on this really, but part of me wonders if there's room in this communication link for packet (or, transaction) fragmentation. Say, we know the maximum size of each transaction can be 20 bits. Maybe it can work like some sort of packed VLIW system where individual commands can be packed together to make more use of the bus? Of course, that also assumes that there will be the other cases where you can't pack enough together and thus you get transactions that use all 20 bits.

    That seems fairly unlikely, to be honest. I mean, 20 bits isn't much data... But I still can't figure out any real reason they've gone from measuring in bytes to transactions.
     
  5. hoom

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    Hyper Transport has been measured in GT for a while & isn't QPI the intel version of that?
    Not surprising they would use same terminology.
     
  6. Thowllly

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    Maybe Intel calls it gigatransactions per second, but in general it means gigatransfers per second. It's been in use for many years already. Mhz became an inaccurate measurement with the introduction of DDR signalling. A 1 Ghz DDR link transfers data at 2GT/s, so the clockrate is measured in Ghz and the signaling rate is measured in GT/s.

    It's inaccurate to use the unit Ghz for signaling rate, because you need both a low to high edge and a high to low edge to get 1 hz, so a 2GT/s link runs at most at 1Ghz, only the clock might run at 2Ghz if DDR signaling is not used.

    For serial links Gbits/second can be more accurate than GT/s, serial links using 8b/10b encoding or similar have a lower bitrate than the transferrate. A 3GT/s sata link runs at 2.4Gbit/s.
     
  7. hoom

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    Yes, I think thats the main reason they use that terminology: 3GT/s sounds better to marketing types than the more meaningful 2.4Gbit/s
     
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