A Faster Memory Controller

When AMD integrated a memory controller on-die, we knew that every time we saw a new AMD processor, we'd get a slightly enhanced memory controller. In Barcelona, the tweaks are significant and should provide for a tangible improvement in memory performance.

One strength of Intel's FB-DIMM architecture used in Xeon servers is that you can execute read and write requests to the AMB simultaneously. With standard DDR2 memory, you can do one or the other, and there's a penalty for switching between the two types of operations. If you have a fairly random mixture of reads and writes you can waste a lot of time switching between the two rather than performing all of your reads sequentially then switching over to writes. The K8's memory controller made some allowances for preferring reads over writes since they take less time, but in Barcelona the memory controller is far more intelligent.

Now, instead of executing writes as soon as they show up, writes are stored in a buffer and once the buffer reaches a preset threshold the controller bursts the writes sequentially. What this avoids is the costly read/write switch penalty, helping improve bandwidth efficiency and reduce latency.

The K8 core (Socket-940/939/AM2) featured a single memory controller that was 128-bits wide, but in Barcelona AMD has split up the DRAM controller into two separate 64-bit controllers. Each controller can be operated independently and thus you get some improvements in efficiency, especially when dealing with quad core implementations where the individual cores working on independent threads all have their own memory access patterns.

Barcelona's Northbridge is also set up to handle higher bandwidth than before. Deeper buffers are present, allowing for higher bandwidth utilization, and the Northbridge itself is ready for use with future memory technologies (e.g. DDR3). We'd expect one or two revisions past Barcelona will be when AMD switches memory technologies, but the new core will initially debut with DDR2 support.

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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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