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Thread: Hypertransport vs QPI

  1. #1

    Default Hypertransport vs QPI

    So I was looking at this wiki entry about Device Bandwidth and I didn't understand something.

    Quote Originally Posted by wiki
    HyperTransport (800 MHz, 16-pair) 25,600 Mbit/s 3,200 MB/s
    HyperTransport (1 GHz, 16-pair) 32,000 Mbit/s 4,000 MB/s
    PCI Express 1.0 (x16 link)[28] 32,000 Mbit/s 4,000 MB/s
    PCI Express 2.0 (x8 link)[29] 32,000 Mbit/s 4,000 MB/s
    PCI-X QDR 34,133 Mbit/s 4,266 MB/s
    AGP 8x 64-bit 34,133 Mbit/s 4,266 MB/s
    PCI Express (x32 link)[28] 64,000 Mbit/s 8,000 MB/s
    PCI Express 2.0 (x16 link)[29] 64,000 Mbit/s 8,000 MB/s
    QuickPath Interconnect (2.4 GHz) 76,800 Mbit/s 9,600 MB/s
    QuickPath Interconnect (3.2 GHz) 102,400 Mbit/s 12,800 MB/s
    PCI Express 2.0 (x32 link)[29] 128,000 Mbit/s 16,000 MB/s
    HyperTransport (2.8 GHz, 32-pair) 179,200 Mbit/s 22,400 MB/s
    HyperTransport 3.1 (3.2 GHz, 32-pair) 409,600 Mbit/s 51,200 MB/s
    Why is AMD stuggling against Intel when HT is faster than QPI? Is there something about this table that I am not understanding?
    Last edited by grishkafool; 05-29-2009 at 08:43 AM.
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  2. #2

    Default Re: Hypertransport vs QPI

    Quote Originally Posted by grishkafool View Post
    So I was looking at this wiki entry about Device Bandwidth and I didn't understand something.

    Why is AMD stuggling against Intel when HT is faster than QPI? Is there something about this table that I am not understanding?
    Yes, there is something your not understanding. Processing power's bandwidth needs. In fact, neither AMD nor Intel has processing cores capable of coming even close to saturating HTT or QPI. It doesn't matter who has the fastest bus speeds when neither company is filling their respective bus type up. In fact, the QPI numbers look way low. Consider that QPI is how the CPU is talking to memory, and triple channel memory goes WAY over those numbers. Something is amiss with that Wiki.

    Intel claims MUCH higher bandwidth. I trust Intel more then I trust Wiki's.

    "Intel® QuickPath Interconnect (QPI) – This is the technology that has replaced the front side bus used in previous generation Xeon® processors. Our previous generation architecture had a bandwidth of 21 GB/s vs. the QPI bandwidth of 46.1 GB/s. This is a speedup of 2.2X, very impressive. For applications that require lots of I/O this is huge. It’s like going from a country back road to an expressway! "

    The Server Room Blog : Tags : qpi

    That shows that wiki is broken and wrong (as if that's a surprise :p ). Also, 46.1Gb a second is WAY more then any modern CPU can fill up. Even counting triple 10.6Gb/s per channel memory links, the CPU has enough left over bandwidth on it's bus to do whatever it needs to. If Hypertransport provides more bandwidth then even that, it won't matter. AMD's core's themselves are no match against Core i7 core for core. With both architectures having way more bandwidth then either can use, it's down to who has the faster chips to eat up some of that over-kill bus speed.

    Edit: I know where the confusion comes in, it's the 4.8(2.4Ghz) or 6.4(3.2Ghz) GT/s rating of QPI based on the concept of "Gigatransfers". But the final numbers translate to much higher bandwidth then the wiki alludes to because it leaves out several clock multipliers along the way.
    Last edited by moshpit; 05-31-2009 at 10:57 PM.
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