AMD Virtualization Improvements

The performance-related improvement to Barcelona comes in the way of speeding up virtualized address translation. In a virtualized software stack where you have multiple guest OSes running on a hypervisor there's a new form of memory address translation that must be dealt with: guest OS to hypervisor address translation, as each guest OS has its own independent memory management. According to AMD, currently this new layer of address translation is handled in software through a technique called shadow paging. What Barcelona offers is a hardware accelerated alternative to shadow paging, which AMD is calling Nested Paging.

Supposedly up to 75% of the hypervisor's time can be spent dealing with shadow pages, which AMD eliminates by teaching the hardware about both guest and host page tables. The translated addresses are cached in Barcelona's new larger TLBs to further improve performance. AMD indicates that Barcelona's support for Nested Paging requires very little to implement; simply setting a mode bit should suffice, making the change easy for software vendors to implement.

Power Management

The most recent aspect of Barcelona's design that AMD revealed is how it handles power management. Although all four cores still operate on the same power plane (same voltage), Barcelona's Northbridge now runs on a separate power plane. Barcelona's core and Northbridge voltages can vary between 0.8V - 1.4V independently of one another.

In a conventional platform architecture, the Northbridge and the CPU are already on separate power planes given that the Northbridge is external to the CPU. The benefit of this arrangement is that the two chips can power down independently of one another, so when the memory controller has little to do, it can power down until needed. With AMD's K8, this wasn't true as the Northbridge and CPU core(s) were on the same power plane. In Barcelona, they are separated to improve power efficiency.

The individual cores still share the same reference voltage, but each core has its own PLL so that they can run at different clock speeds depending on load. While voltages of all four cores have to be equal, clock speed and thus current draw can be reduced depending on load - which will amount to power savings under normal usage conditions. The implications on the desktop are particularly interesting since it's rare that most desktop workloads will keep all cores pegged at 100% utilization.

Barcelona supports up to 5 independent p-states for each core, varying only in clock speed. The p-states are completely hardware controlled, so you will not need a driver to enable support for the power management features. AMD also increased the amount of clock gating done on Barcelona compared to K8 at both the block level and logic level. AMD wouldn't give us any more detail than this, but given how long it's been since the K8's introduction we'd expect that there's a lot that can be done.

The performance efficiency enhancements to Barcelona, coupled with updated power management, further clock gating and 65nm process allow AMD's first quad core part to operate within the same thermal envelope as current Opterons.

Getting Spendy with Transistors - L3 cache Final Words
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  • chucky2 - Friday, March 2, 2007 - link

    Can you post the link that originates at AMD's own website then that says specifically that AM2+ CPU's are guaranteed to work - understandably maybe not supporting every new feature - in current AM2 boards?

    Not a news post from DailyTech, The Inquirer, Toms, whatever...one that's on AMD's site itself.

    And No, AMD could make AM2+ completely incompatible with current AM2 boards and they probably wouldn't see much drop if at all from the large OEM's. The large OEM's would just ensure that when the AM2+ CPU's came in, AM2+ motherboards would likewise come in.

    Believe me, I want to see the link...because I'm desperately awaiting 690G or MCP68, whichever comes first (which is probably MCP68 at the pace AMD is moving on 690G).

    Chuck
    Reply
  • yacoub - Thursday, March 1, 2007 - link

    quote:

    In order to keep die sizes manageable, AMD constructed its quad-core Barcelona out of four cores each with a 128KB L1 and 512KB L2,


    You say 128kb L1 per core but the diagram image just beneath that text shows a 64bit L1 cache. Please confirm which it is.

    Thanks.

    Awesome article, btw. Seems like quite a significant group of changes to the CPU. Looking forward to seeing how it stacks up against the best Quad Core2 Intel can offer. =)
    Reply
  • yacoub - Thursday, March 1, 2007 - link

    also, please forgive my hasty typing - I wrote "128kb" and "64bit" - I meant "128KB" and "64KB" Reply
  • JarredWalton - Thursday, March 1, 2007 - link

    L1 is 128K total - 64K data and 64K instruction. Reply
  • Beenthere - Thursday, March 1, 2007 - link

    AMD doesn't do knee-jerk reactions like Intel because AMD has superior products. AMD continues to take market share from Intel in every segment and Barcelona will continue that trend. Barcelona looks to be every bit as superior to Intel's hacked/patched/glued together chips as Opteron was when introduced. Intel's chips depend on huge cache size for their performance and that crutch won't work after the intro of Barcelona.

    For those without a clue, AMD didn't start design of Barcelona last week or last year. It's been in the development pipeline for many years and thr performance will demonstrate exactly why AMD's long term platform stability is the right choice for most enterprise buyers. Intel is gonna feel the pain again.
    Reply
  • Roy2001 - Thursday, March 1, 2007 - link

    Facts please, no BS. Reply
  • zsdersw - Thursday, March 1, 2007 - link

    Idiocy incarnate. Reply
  • Regs - Thursday, March 1, 2007 - link

    AMD, like Intel, start numerious projects. Just not all of them get to this finish line. Actually a lot of them don't even reach the end of the planning phase before being scratched.

    As for Intel and their large caches...well I'd say it's amazing how half their die (if not more) is used for cache and still had enough space for all the core logic that's kicking the crap out of the K8 now.

    Common sense!
    Reply
  • erwos - Thursday, March 1, 2007 - link

    Looks like some good improvements coming down the pipe. The cache size issue makes me nervous, though - 512kb per core is starting to look a little antiquated, and there's no information about the bandwidth to the L3 cache (which, presumably, is slower than L2). Reply
  • SmokeRngs - Thursday, March 1, 2007 - link

    In the past, AMD did not need the large cache sizes that Intel did for their processors. This was very obvious in regards to the Netburst architecture. However, while Core2 is much better than Netburst there are still disadvantages for Intel.

    I'll explain a little background as far as I understand it. In the K7 and Netburst days, Intel had to have the cache to make up for their long pipeline. Branch mispredictions are going to happen and the penalty on the long pipeline of the Netburst processors hurt their IPC badly. The shorter pipeline on the K7 did not have the same performance penalty due to the shorter pipeline. With K8, the on die memory controller also negated the need for large L2 caches due to the reduced latency when accessing main memory. This has been one of the major performance aspects for the K8 architecture.

    The Core2 architecture obviously does not have the on die memory controller so the need for larger caches is still present and Intel sees improvement due to the larger caches. Barcelona still has the on die memory controller and the previous efficiency is still there and still negates the need for large caches. This is just the difference between architectures. While having a larger cache on the K8 did improve performance some in some usage scenarios, it wasn't on the same scale as the improvements Intel received with a larger cache.

    AMD can't compete with Intel in regards to cache size. However, other architecture differences make up for the lack of large amounts of cache. Barcelona having a smaller cache does not seem to be a big problem. If it was a big problem, AMD probably would have gone with a larger cache to get the extra performance. Bigger does not always mean better or at least enough better to warrant the extra.

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

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