New Prefetcher

Prefetching is done in many areas of the system and by many different components. When NVIDIA introduced its nForce2 chipset, it stressed the ability of its intelligent prefetcher to make use of a very wide, at the time, 128-bit memory bus. More recently, when Intel introduced its Core 2 processor family it stressed the importance of its three prefetchers per core in drastically reducing perceived memory latency.

AMD's K8 core had two prefetchers per core - one instruction and one data. The Barcelona core still retains the same number of prefetchers, but improves on them. The biggest change is that the data prefetcher now brings data directly into the L1 data cache, as opposed to the L2 cache in the K8. AMD looked at the accuracy of its core prefetchers and realized that they were doing quite well, so it only made sense to prefetch into a low latency L1 and avoid polluting the L2 cache. AMD has also increased the flexibility of its L1 instruction cache prefetcher to handle two outstanding requests to any address.

At first glance it looks like Intel's prefetchers in Core 2 are greater, at least in quantity, than what AMD has planned even for Barcelona. Remember that Intel's Core 2 processor features two data and one instruction prefetcher per core, plus an additional two L2 cache prefetchers, all of which are well managed as to not eat into "demand" bandwidth. At the same time, we must keep in mind that Intel needs these prefetchers to help mask its longer trip to main memory. From a CPU perspective, the advantage here is for Intel, but as a platform the true winner is tough to determine.

Each Barcelona core gets its own set of data and instruction prefetchers, but the major improvement is that there's a new prefetcher in town - a DRAM prefetcher. Residing within the memory controller where AMD previously never had any such logic, the new DRAM prefetcher takes a look at overall memory requests and attempts to pull data it thinks will be used in the future. As this prefetcher has to contend with the needs of four separate cores, it really helps the entire chip improve performance and can do a good job of spotting trends that would positively impact all cores. The DRAM prefetcher doesn't pull data into the CPU's L2 or L3 caches either; instead it features its own buffer to avoid polluting the caches. The buffer is approximately 20 - 30 cache lines in size and happens to be the same buffer that is used for Barcelona's write bursting we mentioned on the previous page.

A Faster Memory Controller Getting Spendy with Transistors - L3 cache
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  • JarredWalton - Thursday, March 01, 2007 - link

    Games have quite a lot of LOAD instructions, like most programs, as well as plenty of branches (esp. in the AI routines). Most likely the boost that Core 2 gets is due in a large part to the better instruction reordering and branch prediction, although the cache and prefetchers probably help as well. Given AMD was better than NetBurst due to memory latency, through in better OOE (Out of Order Execution) logic and keep the improved latency and they should do pretty well.

    Naturally, everything at this point is purely speculation, but in the next few months we should start to get a better idea of what's in store and how it will perform. One problem that still remains is that even if AMD can be competitive clock-for-clock, Intel looks primed to be able to go up to at least 3.6 GHz dual core and 3.46 GHz quad core if necessary. AMD has traditionally not reached clock speeds nearly as high as Intel, possibly due in part to having more metal layers (speculation again - process tech and other features naturally play a role), so if they release 2.9GHz Barcelona at $1000 you can pretty much guarantee Intel will launch 3.2 and/or 3.46 GHz Kentsfield (and/or FSB1333 3.33 GHz).

    On the bright side, at least things should stay interesting in the CPU world. :D
    Reply
  • yyrkoon - Thursday, March 01, 2007 - link

    Yes, interresting indeed, but from experience, AMD has always been too vocal in what they plan on doing, especially during the times they are in a 'rut'.

    What this usually means to me, is that AMD is trying to blow smoke up our backsides, we'll see though.

    Keep in mind, my main desktop system, and my backup server for that matter, both are AMD systems. The phrase "cost effective" applies here.
    Reply
  • kilkennycat - Thursday, March 01, 2007 - link

    Yesterday, Intel announced that they were converting a fourth fab to 45nm. A great deal of confidence in that process. And a few days ago they announced desktop shipments of Penryn-based CPUs pulled forward into 2007. Looks as if AMDs 'window of opportunity' is likely to be very small. IBM has not yet announced a successful implementation of a RAM on their 45nm process. Intel had their RAM design on 45nm up and running late 2005. Reply
  • archcommus - Thursday, March 01, 2007 - link

    True but the move to 45 nm might not make a huge difference in real world performance, just like the move to 65 nm didn't for AMD. Their next full blown architecture will still be a ways off. Reply
  • Roy2001 - Thursday, March 01, 2007 - link

    Dislike AMD's move to 65nm process, move to 45nm has shown that Penryn would eats less power and runs faster thanks to its high K material and metal gate. Reply
  • smitty3268 - Thursday, March 01, 2007 - link

    Every process shows that in theory before chips are actually being made on it. We'll see what actually happens when Penryn is released, not before. Reply
  • chucky2 - Thursday, March 01, 2007 - link

    Has AMD given any indication of how probable dropping an Agena or Kuma CPU into an existing AM2 motherboard will go?

    Especially AMD's own newly released 690G or the upcoming nVidia MCP68?

    Chuck
    Reply
  • mamisano - Thursday, March 01, 2007 - link

    It has been stated in the past that AM2+ based products will run in AM2 based boards. The limitation, if I understand it correctly, will be the lack of support of the new power features.

    Someone correct me if I am wrong :)
    Reply
  • chucky2 - Thursday, March 01, 2007 - link

    Then it should be no problem for AMD to confirm through AnandTech that this is the case.

    Surely if Barcelona is this close to shipping (only a few months away), AMD must know if Agena and/or Kuma will work in current AM2 motherboards, especially their own 690 series their just about to release.

    All I'm asking for is a definite either way, it shouldn't be that hard for AMD to do at this point.

    Chuck
    Reply
  • mino - Friday, March 02, 2007 - link

    AMD stated PUBLICLY to anyone who listened that AM2+ stuff will plug into AM2, just BIOS update needed.

    Why should they react to any consumer who ask on some forum the same question every second week ?

    Most important is they said it WILL(not "may") work with AM2-spec boards to big Tier 1 OEM's.
    They can not make it incompatible therefore. They would be out of bussines in no time.
    Reply

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