Power Gating

With 1.45 billion transistors on die, Llano relies on extensive power gating in order to keep things in order. The APU is split into two independent power islands: the CPU and the GPU. The memory controller and North Bridge both live on the GPU's power island. Each island has its own independent voltage source.

Everything from an individual CPU core to the entire GPU or virtually the entire APU package can be power gated. AMD provided photon recombination images to show the impact power gating the GPU can have on leakage current:

Although not depicted above, Llano can also fully power gate the x86 CPU cores or both the CPU and GPU if the entire APU is in a deep sleep state. Being able to completely power gate CPU cores or the GPU is an important part of enabling the next major feature of Llano: Turbo Core.

Turbo Core

All processors whether CPUs, GPUs or APUs have to be designed to strict thermal and power limits. OEMs need to know exactly what sort of chassis they'll be able to build around these chips and as a result the chip vendors provide guidance in the form of specifications, including the chip's thermal design point (TDP).

In the old days of microprocessors things were simple. You had a single core that ran all the time and it consumed all of the available thermal budget allocated for that core. AMD and Intel eventually enabled dynamic clock frequencies which let your single core underclock itself when it wasn't being used, which helped reduce power and extend battery life. Then came the multi-core era.

CPUs couldn't just start putting out twice as much heat now that they had two cores; instead, each core had to consume less power. The chip guys achieved this by running the cores at lower frequencies and voltages than they did in the single-core days. Two cores paved the way to four cores, which meant another reduction in clock speed per core. Sure we got much better multi-threaded performance, but for single-threaded applications performance wasn't as great as it could be. Users had to make a tradeoff: good multi-threaded performance or good single-threaded performance; you couldn't have both. Until power gating came along that is.

Without power gating you can never really shut off power to an idle core. The transistors aren't switching but power is still dissipated thanks to leakage current. Remember that transistors don't simply stop conducting electricity when they're off. The smaller they get, the more leaky our beloved transistors become. Power gating lets you physically block the flow of current to the transistors that are being gated, so when they're off, they're actually off. With an idle core shut off, now you have the extra TDP headroom to run any active cores at higher frequencies.

Intel does this with a technology it calls Turbo Boost. Intel looks at current draw and thermal sensors spread out all over the chip and determines when it has the available thermal headroom to turbo up any active cores. AMD implements a similar technology in Llano (and previously in their hex-core desktop parts) called Turbo Core.

I say similar but not identical because AMD's approach differs in a very important way. While Intel looks at current draw and temperature data, AMD looks at workload. Each activity within the Llano APU is assigned a certain power weight (e.g. an integer multiply is known to require a certain amount of power). Llano is aware of the operations it's currently working on and based on the weights associated with these operations it comes up with a general estimate of its power consumption on a per core basis. I mention this is an estimate because it correlates digital activity to power consumption; it doesn't actually measure power consumption.

Based on the number of events and their individual weights, AMD estimates the power consumption of each core and determines how much TDP headroom exists in the system. If the OS is requesting the highest p-state from the CPU and there's available TDP headroom, Llano will turbo up any active cores up to a maximum frequency. Like Sandy Bridge, Llano is able to temporarily exceed the APU's maximum TDP if it determines that the recent history of power consumption has been low enough that it'll take a while for the APU to ramp up to any thermal limits.

One major limit of Llano's Turbo Core is that the GPU can't turbo up in the event of the CPU cores being idle. Only the CPU cores can turbo up if they have available headroom. I suspect future versions of Llano will probably enable GPU Turbo Core as well:

It's unclear to me at this point what shortcomings or advantages exist for AMD's Turbo Core method vs. Intel's Turbo Boost. At the bare minimum the two are finally comparable although they use different approaches to attain a similar end result. AMD doesn't yet have a method of actually displaying Turbo Core frequencies, unfortunately, so we're operating a bit blind at this point. Over time I hope to have a better idea of how AMD's solution stacks up.

The GPU Introducing Mobile Llano
POST A COMMENT

177 Comments

View All Comments

  • Dustin Sklavos - Tuesday, June 14, 2011 - link

    I disagree emphatically. Having used Atom-based and E-350-based netbooks (and let's not kid ourselves, the E-350 is a netbook chip), the E-350 machines just feel snappier. I'd never buy an Atom-based computer, but I love the E-350. Reply
  • ppeterka - Tuesday, June 14, 2011 - link

    Absolutely agree with Dustin... Recently bought an E-350 to replace my wife's painfully struggling Atom n455 netbook. The user experience went through the roof. My wife is happier than ever (and that's a very good thing, and a very good benchmark! :))) )

    She is a "not-professional-but-quite-demanding" user. (20-30 browser tabs, Office, and video streaming at once, Twitter with multiple accounts, and so on). She used to have a C2D T6600 + GeForce 310 (or 210 , don't exactly remember) Toshiba notebook too, but since we acquired the Brazos one, she didn't turn the old one on. Have to sell it now...

    I know, Brazos is weak if compared to anything other than Atom. But magically it manages to absolutely fill its role. Hats off to AMD on that one!
    Reply
  • Broheim - Tuesday, June 14, 2011 - link

    I wouldn't describe your wife as a "quite demanding" user.
    Browser tabs don't take up CPU cycles once loaded (unless the page uses AJAX, and even then a simple http request isn't a daunting task even for a 20 year old processor) and only uses a little bit of memory.
    I'd call MS Office pretty basic computer usage .
    today video decoding is almost always hardware accelerated, so most of the work is offloaded onto the GPU (where E-350 has the upper hand).
    but I digress.

    just look at the benchmarks, there isn't much of a difference between the E-350 and a D525 (or similar) in CPU intensive tasks...
    any percieved "snappiness" on your part is down to other aspects of the system (such as HDD for instance).
    Reply
  • Iketh - Tuesday, June 14, 2011 - link

    are you forgetting single-threaded results? Reply
  • Broheim - Wednesday, June 15, 2011 - link

    1 synthetic benchmark... big whoop.

    they are equal in everything else
    Reply
  • duploxxx - Friday, June 17, 2011 - link

    perhaps actually read some reviews, they all agree on one thing, the system feels much smoother for daily tasks. I am sure you don't have both or used both so have no idea what you are talking about. just launching OS or any application is enough to notice the difference. I owned a n570 so i do know. Reply
  • Broheim - Monday, June 27, 2011 - link

    that is subjective opinion and the only subjective opinion that matters to me is my own. You fail to realize that the perceived user experience is a product of the system as a whole and not just a single component.... Reply
  • sinigami - Wednesday, June 15, 2011 - link

    i've only gotten to run two single threaded benchmarks on the E-350, and out of those two, the bench that showed the biggest improvement over the Atom was CPUMark99, by 6% percent, over an N455 in an MSI netbook.

    Granted, i didn't get to run all four of my single-threaded benchmarks on the E350 against the N455, but i will, and soon.
    Reply
  • maroon1 - Tuesday, June 14, 2011 - link

    Atom N455 is single core.

    There are new and better Atoms like N550
    Reply
  • sinigami - Wednesday, June 15, 2011 - link

    and, BTW, the N455 even plays 720p MKV. Who would expect any more than that out of a $250 netbook?

    But at the price of the E-350, i do expect more. And by more, i mean more than just ION level graphics that might let you send out 1080p to an external display. I want some significant CPU horsepower.
    Reply

Log in

Don't have an account? Sign up now