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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  • agaelebe - Friday, March 02, 2007 - link

    Wow! A lot of dicussion in here.
    And, by the way, very interesting article.

    I'm a software engineer from Brazil and I'm planning to change my PC this year.
    I've bem using AMD processors since the K6.
    Today I've a XP Mobile 2500+(@2.2ghz), 1gb ram, 200gb and an AGP 6600GT
    My PC is not very slow, but I'm thinking in going dual core to speed things up(office applications, web development and some games).
    I can run some of the newest games, but not in high graphics.
    I expect that my PC can run C&C 3 (Already run the demo in 1024 medium, but have some craches although it's not running it slow)

    So, today I'm thinking in 3 options:
    1) Stay with this computer and wait until AMD launchs it's new architecture (I pretend to go with an average price Kuma)

    2) Go with Intel Core 2 Duo (e6300 or e6400). They're not expensive and for games I can easily make an overclock and gain more performance.

    3) Buy a good AM2 board and a cheap Atlhon X2 (3600) and wait new AMD processors and then change only the processor.

    Here in Brazil the taxes are to high, so I'm planning in buying a PC with these specs:

    - CORE 2 Duo e6300/6400 or X2 3600/3800
    - mid-tier motherboard (
    - 2 x 1gb DDR 800 4-4-4-12
    - 2 x 250 gb
    - X1950pro 256 or 512
    - 500watts power

    So the prices are below:

    e6300 box US$ 300 (same price for a X2 4200+ box)

    x23800 box US$ 220

    motherboard: US$ 220

    ram: US$ 400

    video: US$ 450

    DVD: US$ 70

    case: US$ 150

    HDs : US$ 250

    Power: us$ 180

    So I plan to spent about 2000 dollars (Sadly, I can buy this same PC in US for the half of the price).

    My new PC should spent not to much power so I can leave it turned onall day long(max 150watts on iddle without monitor), otherwise I'll keep my old computer turned on just for downloding stuff)

    So, If someone has an opinion, I'd like to "hear" it. You can give another options to, or make some comments about the specs I'm choosing now.

    I had Pentium 75 and after that only AMD CPUs... Should know I surrender to the Core 2 Duo or believe that AMD can really beat it until the end of 2008?

    And thanks for the cooperation and patience.
    Reply
  • Zebo - Saturday, March 03, 2007 - link

    Athlon 64 AM2's arnt exactly slow so if you're an AMD fan just get one..like a 3800+ or 3600+ and overclock it. It will be at least 4x faster than what you have now and accept K8L Agena core later. It will be cheaper than C2D by about $50 USD and You'll also pay cheap for a GeForce 6100 Motherboard which is only $50 USD. Overall expect the the AM2 system to be about $100 USD cheaper.

    Keep in mind that C2D is 20% faster clock for clock in most apps so it's not exactly a quantum leap here getting a C2D.. Gap gets a lot larger when overclocking since C2D's overclcok higher like 3.2Ghz is common on air vs. only 2.8Ghz for AM2, so, at the end of the day a C2D setup is able to be about 40% faster over most benchmarks. That is getting significant and why enthusiasts are buying C2D's.
    Reply
  • agaelebe - Friday, March 02, 2007 - link

    And,as always, sorry with the errors and not so good writing... Reply
  • Kiijibari - Thursday, March 01, 2007 - link

    Hi,

    never heard of of that before, does anybody know what it is ?
    So far I see 2 pad areas at the DIE photo, therefore I assume that it would be also 2 interfaces, e.g. x8 PCIe like Sun uses ?

    bb

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

    It should be some management/coodrination stuff (can-t remember the name of that bus).
    Every northbridge and CPU has that.
    Reply
  • davecason - Thursday, March 01, 2007 - link

    Anand,

    Great article! I know it took a lot of time and I wanted you to know I really appreciate your effort. It is the kind of article that keeps me coming back to your site.

    -Dave
    Reply
  • yyrkoon - Thursday, March 01, 2007 - link

    quote:

    On average, about 1/3 of all instructions in a program end up being loads, thus if you can improve load performance you can generally impact overall application performance pretty significantly.


    Page 5, paragraph 4 'pretty significantly'. Well is it, or is it not it ?

    http://www.wikihow.com/Avoid-Colloquial-%28Informa...">http://www.wikihow.com/Avoid-Colloquial-%28Informa...

    Aside from my gripe concerning writing style, good article :)
    Reply
  • trisweb2 - Friday, March 16, 2007 - link

    Usually we criticize writing style based on a whole experience... obviously Anand is one of the best technical review writers on the Internet; if you bother to read his articles more fully perhaps you'd realize that. The colloquial writing sometimes brings it to a more personal level that a reader can better relate to and understand -- it works especially well in this case, where it's a future design, we really don't know how it's going to perform. That he can guess and say "pretty significantly" tells me he understands the uncertainty of the situation, and the language communicates that fact perfectly well. It would be more confusing if he said it would impact performance "significantly" as you want him to, as that would imply that he was more certain than he might actually have been.

    Masters are allowed to bend the rules, and Anand is one, so lay off.
    Reply
  • yyrkoon - Thursday, March 01, 2007 - link

    *Is it, or is it not*

    /me hangs head in shame
    Reply
  • baronzemo78 - Thursday, March 01, 2007 - link

    Any rough guess as to how Barcelona will compete with Core2 in gaming? Many articles have shown how Core2 gets you a slight FPS boost in games that aren't graphics card limited. I'm curious how Barcelona will fit in with the overall picture of DX10 cards like G80 and R600. Reply

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