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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  • JustKidding - Friday, March 02, 2007 - link

    So what you are saying is that it's not the size of your cache that matters as much as how well you use it. Reply
  • VooDooAddict - Thursday, March 01, 2007 - link

    quote:

    Smaller cache will mean fewer transistors which should mean better yields, lower power consumption and cheaper to produce.


    With Cache size differences usually having small impact on performance for Athlon64s, the slight trade off for better yields and margins seems the better choice for AMD here.
    Reply
  • Regs - Thursday, March 01, 2007 - link

    Where was this article 8 months ago? ;)


    I agree with Anands closing article that AMD now needs it's own "snowball effect" for the next couple of years. 4-5 years with a sitting target against a giant like Intel prooved to be costly in terms of competivness.

    We all saw it coming when Intel developed the first Pentium M. It looks like AMD got the message as well and started the Barcelona project. Maybe AMD learned their lesson.
    Reply
  • iwodo - Thursday, March 01, 2007 - link

    So bascially all intel 's C2D improvement are made into Barcelona. And apart from Virtualization improvement there are nothing new from AMD that Intel doesn't have?

    On performance note Barcelona doesn't seem to offer better clock scaling. I.e even if it is 30% faster then its current K8 it will only have slight advantage against C2D clock per clock. Not to mention it is up against Penryn. Although Penryn is nothing much then a few minor tweaks and more cache. It does allow intel to scale higher in clock speed.

    And given AMD slow roll out rate, and AMD limited production capacity Barcelona never seem like much of a threat.

    The article does not mention anything about FP improvement. Are AMD keeping them secret for now or is that all we are going to see?
    Reply
  • Spoelie - Thursday, March 01, 2007 - link

    The FP improvement is the SSE improvement, and according to the theory it's more powerful than what core2 duo is offering.

    There are improvements mentioned that are not in core2 (+ other way around, like instruction fusing), and improvements that are inspired on the same principle but implemented differently. The architectures themselves differ widely (see earlier article that compares K8 with Core2 - reservation station etc.) so different implementations of principally the same optimizations on a different architecture will have vastly different effects. Even after these improvements, the capabilities (how much can you decode, etc) of each read nothing alike. And if it were all the same, AMD has the platform advantage, so it would still end up faster by virtue of nothing else but that. Some guesstimates made by varying sites would put Barcelona ahead in FP code and at the same level or slightly behind in INT code. But those are just guesstimates.

    What I'm trying to say here is that barcelona is still very different from core2, and that we just don't know yet in which direction the pendulum will swing ;)
    Reply
  • Shintai - Thursday, March 01, 2007 - link

    No....precisely in theory is where Barcelona lacks. Core 2 Duo could in theory do 6 64bit or 3 128bit SSE instructions per cycle. Barcelona can do 4 64bit or 2 128bit. AMD provided this information aswell. Reply
  • Griswold - Thursday, March 01, 2007 - link

    Wishful thinking. Reply
  • Spoelie - Thursday, March 01, 2007 - link

    Hmmm, in the earlier article, there was explicit emphasis on the fact that 2 of the 3 units are symmetric in core2, but I'm not too sure what it means. It does imply however that those 3 units of core2 can only be used fully in certain combinations, and are not 3 independent units. On 128-bit performance, what was said is this: "so the Core architecture has essentially at least 2 times the processing power here [compared to K8]". Not 3 times, but "at least" 2 times, so again the 3 times will probably only be in certain situations.

    The next paragraph said this:
    "With 64-bit FP, Core can do 4 Double Precision FP calculations per cycle, while the *Athlon64* can do 3."
    So K8 was not at such a big disadvantage when it came to 64-bit SSE, if Barcelona doubles everything SSE, it should come ahead in this area.

    So to me it looks like for 128-bit, core2 will be faster in some situations, on par in others, and for 64-bit, Barcelona would be ahead.

    If this is wrong, I do not know where some of the articles I read over time came from, implying Barcelona would be better overall in SSE.
    Reply
  • Shintai - Friday, March 02, 2007 - link

    Core 2 got 3 individual SSE ports:
    http://www.realworldtech.com/page.cfm?ArticleID=RW...">http://www.realworldtech.com/page.cfm?ArticleID=RW...

    AMD says 4 double:
    http://www.anandtech.com/showdoc.aspx?i=2768&p...">http://www.anandtech.com/showdoc.aspx?i=2768&p...

    And 64 or 128bit doesnt matter. I dont know how you think that way.
    Barcelona got 2 SSE ports. They are able to do 2 128bit or 4 64bit. Most 128bit actually contains 2 64bit or 4 32bit.
    Core 2 got 3 SSE ports. They are able to do 3 128bit or 6 64bit.
    Reply
  • flyck - Friday, March 02, 2007 - link

    core duo has 3 SSE units but they are not symmetric, meaning that not every unit can execute all commands. Core duo can do at best 4DP flops/ cycle. the same as barcelona. Reply

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