Coherency: Forwarding and Owned

Article: The Common System Interface: Intel's Future Interconnect
By: Peter Gerdes (truepath.delete@this.infiniteinjury.org), August 29, 2007 2:11 pm
Room: Moderated Discussions
Thanks for the great article. I've been looking for this sort of info about CSI for awhile.

So I'm a bit unclear about how intel's M state compares with the O state from AMD. The difference is supposedly that when a copy of a cache line in state M is requested the cache line has to be written to memory but what does this really mean when you have integrated memory controllers?

If a processor has a modified cache line for memory it is controlling isn't this effectively equivalent to being in the O state? Presumably that processor is answering all requests about the memory it controls and it would be silly for it to physically interrogate the memory when it has the current memory state on hand. Now maybe there is something I'm missing but it seems the primary difference between having an O state and not is whether a modified cache line can be stored on any processor or is only held on the processor owning that address.

Anyway my speculation as to why intel didn't implement the O state is that they expect the cost of invalidating a read to be higher than the cost of doing an extra write. Certainly this seems plausible if the processors optimistically start using the first response they receive and must throw out that work if another proc invalidates the read. In fact with a NUMA aware OS it seems extremely likely that either the proc with the modified cache line or the one trying to read it control the corresponding memory, in which case no extra writes need to be done.

I feel I must be missing something here since it would be strange for AMD to implement the O optimization if that is primarily useful on systems with a clogged shared bus. Is it an issue related to cache size?

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New article - The Common System Interface: Intel's Future InterconnectDavid Kanter2007/08/28 01:16 AM
  New article - The Common System Interface: Intel's Future InterconnectVincent Diepeveen2007/08/28 05:02 AM
  New article - The Common System Interface: Intel's Future InterconnectRichard Cownie2007/08/28 10:28 AM
    New article - The Common System Interface: Intel's Future InterconnectVincent Diepeveen2007/08/31 11:44 AM
      New article - The Common System Interface: Intel's Future InterconnectRichard Cownie2007/08/31 08:53 PM
        New article - The Common System Interface: Intel's Future InterconnectVincent Diepeveen2007/09/01 02:21 AM
          Adding layers can simplify designPaul A. Clayton2007/09/01 07:39 AM
          New article - The Common System Interface: Intel's Future InterconnectMichael S2007/09/02 02:25 AM
        New article - The Common System Interface: Intel's Future InterconnectJonathan Kang2007/09/14 12:47 PM
    New article - The Common System Interface: Intel's Future InterconnectDavid Kanter2007/09/14 08:47 PM
  New article - The Common System Interface: Intel's Future InterconnectPaul2007/08/28 11:04 AM
    New article - The Common System Interface: Intel's Future InterconnectDavid Kanter2007/08/28 12:43 PM
      New article - The Common System Interface: Intel's Future InterconnectJoe Chang2007/08/28 06:17 PM
        New article - The Common System Interface: Intel's Future InterconnectJoe Chang2007/08/29 04:27 PM
  Thanks for the workWouter Tinus2007/08/28 12:33 PM
    Thanks for the workmac2007/08/29 12:44 PM
  New article - The Common System Interface: Intel's Future InterconnectHerbert Hum2007/08/28 01:22 PM
    ThanksDavid Kanter2007/08/28 04:13 PM
  Many thanks, very, very interesting! (NT)Cameron Jack2007/08/29 01:51 AM
  very nice article + memory ctrl integrationMarcin Niewiadomski2007/08/29 11:46 AM
    very nice article + memory ctrl integrationDavid Kanter2007/09/14 08:50 PM
      very nice article + memory ctrl integrationMarcin Niewiadomski2007/09/16 08:48 PM
  Coherency: Forwarding and OwnedPeter Gerdes2007/08/29 02:11 PM
    Coherency: Forwarding and OwnedDavid Kanter2007/08/29 06:29 PM
      Coherency: Forwarding and Ownednick2007/08/29 07:03 PM
        Coherency: Forwarding and OwnedDavid Kanter2007/08/29 11:08 PM
      Coherency: Forwarding and OwnedMichael S2007/08/30 01:17 AM
        Coherency: Forwarding and OwnedDavid Kanter2007/08/30 07:31 AM
      Coherency: Forwarding and OwnedPeter Gerdes2007/08/30 11:46 AM
        Coherency: Forwarding and OwnedDavid Kanter2007/08/30 01:46 PM
          Coherency: Forwarding and OwnedPeter Gerdes2007/08/30 07:03 PM
            Coherency: Forwarding and OwnedDavid Kanter2007/09/14 08:44 PM
              Node Interleaveunknown2007/09/15 03:14 AM
                Node InterleaveDavid Kanter2007/09/15 07:50 AM
                  Node InterleaveHoward Chu2007/09/16 12:14 PM
              Coherency: Forwarding and OwnedPeter Gerdes2007/09/16 12:50 PM
                Coherency: Forwarding and OwnedDavid Kanter2007/09/16 04:34 PM
                  Coherency: Forwarding and OwnedEduardoS2007/09/16 04:52 PM
                  Coherency: Forwarding and OwnedJonathan Kang2007/09/17 05:16 AM
                  Coherency: Forwarding and OwnedMatthias2007/09/17 06:59 AM
                    Coherency: Forwarding and Owned - additionMatthias2007/09/17 07:01 AM
                Coherency: Forwarding and Ownedanonymous2007/09/17 09:15 AM
                  Coherency: Forwarding and OwnedPeter Gerdes2007/09/17 12:44 PM
  New article - The Common System Interface: Intel's Future InterconnectMr. Camel2007/08/30 03:16 PM
    New article - The Common System Interface: Intel's Future InterconnectMichael S2007/08/31 01:11 AM
      New article - The Common System Interface: Intel's Future InterconnectMr. Camel2007/08/31 03:13 AM
        New article - The Common System Interface: Intel's Future InterconnectMichael S2007/08/31 03:24 AM
        New article - The Common System Interface: Intel's Future InterconnectDavid Kanter2007/08/31 05:39 AM
          New article - The Common System Interface: Intel's Future InterconnectMichael S2007/08/31 06:53 AM
      New article - The Common System Interface: Intel's Future InterconnectDavid Kanter2007/08/31 05:41 AM
        New article - The Common System Interface: Intel's Future InterconnectMichael S2007/08/31 06:36 AM
          New article - The Common System Interface: Intel's Future InterconnectMr. Camel2007/08/31 08:36 AM
  Thanks and excellent work!Jack A.2007/08/30 07:41 PM
  Lamport's TLAKonrad Schwarz2007/09/02 01:57 AM
    Lamport's TLADavid Kanter2007/09/02 07:55 PM
    Lamport's TLABrannon2007/09/03 07:12 AM
      Lamport's TLAKonrad Schwarz2007/09/18 10:21 AM
        Lamport's TLABrannon2007/09/18 01:58 PM
  New article - The Common System Interface: Intel's Future InterconnectJosé Javier Zarate2007/09/09 04:01 PM
  New article - The Common System Interface: Intel's Future InterconnectJonathan Kang2007/09/16 05:42 PM
    Remote prefetchDavid Kanter2007/09/17 07:51 AM
  New article - The Common System Interface: Intel's Future InterconnectJigal2007/09/22 02:39 PM
    New article - The Common System Interface: Intel's Future InterconnectDavid Kanter2007/09/22 04:35 PM
      New article - The Common System Interface: Intel's Future Interconnect8B/10B Latency2007/09/22 06:16 PM
        New article - The Common System Interface: Intel's Future Interconnectanon2007/09/22 08:05 PM
          New article - The Common System Interface: Intel's Future InterconnectDavid W. Hess2007/09/22 08:50 PM
            Clocking in CSIDavid Kanter2007/09/23 08:46 AM
              Hypertransport 3 AC CouplingDavid W. Hess2007/09/23 09:32 AM
                Hypertransport 3 AC Couplinganon2007/09/23 09:53 AM
                  Clocking lanesDavid Kanter2007/09/23 10:51 AM
        New article - The Common System Interface: Intel's Future InterconnectDavid Kanter2007/09/22 08:34 PM
          New article - The Common System Interface: Intel's Future InterconnectDavid W. Hess2007/09/22 09:10 PM
            New article - The Common System Interface: Intel's Future InterconnectJonathan Kang2007/09/25 07:15 AM
          New article - The Common System Interface: Intel's Future InterconnectMichael S2007/09/23 01:06 AM
            New article - The Common System Interface: Intel's Future InterconnectDavid W. Hess2007/09/23 03:41 AM
            Serialization delayDavid Kanter2007/09/23 08:57 AM
              Serialization delayMichael S2007/09/23 11:20 AM
                Serialization delayDavid Kanter2007/09/23 12:43 PM
                  Serialization delayMichael S2007/09/24 12:40 AM
                    Serialization delayMichael S2007/09/24 04:28 AM
                    Serialization delayAaron Spink2007/09/24 12:19 PM
                      Serialization delayMichael S2007/09/25 03:38 AM
                        Serialization delayJonathan Kang2007/09/25 08:10 AM
                          Serialization delayDavid W. Hess2007/09/26 12:22 AM
                        Serialization delayAaron Spink2007/09/25 12:13 PM
                          Thank you (NT)Michael S2007/09/25 12:53 PM
                Serialization delayJonathan Kang2007/09/25 07:26 AM
                  Serialization delayMichael S2007/09/25 01:57 PM
                    Serialization delayJonathan Kang2007/09/26 05:24 AM
                      Serialization delayDavid W. Hess2007/09/26 06:39 AM
                        Serialization delayJonathan Kang2007/09/26 09:56 AM
                          Serialization delayDavid W. Hess2007/09/27 02:21 AM
                            Serialization delayJonathan Kang2007/09/27 04:36 AM
                              Serialization delayDavid W. Hess2007/09/27 05:31 PM
                      Serialization delayrwessel2007/09/26 01:26 PM
                        Serialization delayJonathan Kang2007/09/27 07:16 AM
                          Serialization delayrwessel2007/09/27 12:20 PM
                            Serialization delayJonathan Kang2007/09/28 04:38 AM
                              Serialization delayrwessel2007/09/28 01:00 PM
                                Serialization delayJonathan Kang2007/10/01 07:07 AM
                                  Cache coherent latencyDavid Kanter2007/10/01 07:20 AM
                                    Cache coherent latencyblaine2007/10/01 10:36 AM
                                      Critical word first on coherent interconnectsDavid Kanter2007/10/01 11:10 AM
                                        Does ccHT do critical word first?blaine2007/10/02 07:10 AM
                                    Cache coherent latencyJonathan Kang2007/10/01 12:34 PM
                                      Cache coherent latencyDavid Kanter2007/10/01 01:13 PM
                      Serialization delayMichael S2007/09/28 04:32 AM
                        Serialization delayanonymous2007/09/28 10:25 AM
                          Serialization delayMichael S2007/09/29 09:06 AM
        New article - The Common System Interface: Intel's Future InterconnectJonathan Kang2007/09/25 07:05 AM
      New article - The Common System Interface: Intel's Future Interconnectjigal2007/09/23 12:37 PM
        CSI, PCI and HTDavid Kanter2007/09/23 12:46 PM
        New article - The Common System Interface: Intel's Future InterconnectJonathan Kang2007/09/25 07:39 AM
          New article - The Common System Interface: Intel's Future Interconnectjigal2007/09/25 02:16 PM
            New article - The Common System Interface: Intel's Future InterconnectMichael S2007/09/26 03:14 AM
              New article - The Common System Interface: Intel's Future InterconnectAnonymous2007/09/26 09:41 AM
                New article - The Common System Interface: Intel's Future InterconnectJonathan Kang2007/09/26 09:59 AM
            New article - The Common System Interface: Intel's Future InterconnectJonathan Kang2007/09/26 05:48 AM
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