Managing Idle Power: Introducing ZeroCore Power

AMD has been on an idle power crusade for years now. Their willingness to be early adopters of new memory standards has allowed them to offer competitive products on narrower (and thereby cheaper) memory buses, but the tradeoff is that they get to experience the problems that come with the first revision of any new technology.

The most notable case where this has occurred would be the Radeon HD 4870 and 4890, the first cards to use GDDR5. The memory performance was fantastic; the idle power consumption was not. At the time AMD could not significantly downclock their GDDR5 products, resulting in idle power usage that approached 50W. Since then Cypress introduced a proper idle mode, allowing AMD to cut their idle power usage to 27W, while AMD has continued to further refine their idle power consumption.

With the arrival of Southern Islands comes AMD’s latest iteration of their idle power saving technologies. For 7970 AMD has gotten regular idle power usage down to 15W, roughly 5W lower than it was on the 6900 series. This is accomplished through a few extra tricks such as framebuffer compression, which reduce the amount of traffic that needs to move over the relatively power hungry GDDR5 memory bus.

However the big story with Southern Islands for idle power consumption isn’t regular idle, rather it’s “long idle.” Long idle is AMD’s term for any scenarios where the GPU can go completely idle, that is where it doesn’t need to do any work at all. For desktop computers this would primarily be for when the display is put to sleep, as the GPU does not need to do at work when the display itself can’t show anything.

Currently video cards based on AMD’s GPUs can cut their long idle power consumption by a couple of watts by turning off any display transmitters and their clock sources, but the rest of the GPU needs to be minimally powered up. This is what AMD seeks to change.

With Southern Islands AMD is introducing ZeroCore Power, their long idle power saving technology. By implementing power islands on their GPUs AMD can now outright shut off most of the functional units of a GPU when the GPU is going unused, leaving only the PCIe bus interface and a couple other components active. By doing this AMD is able to reduce their power consumption from 15W at idle to under 3W in long idle, a power level low enough that in a desktop the power consumption of the video card becomes trivial. So trivial in fact that with under 3W of heat generation AMD doesn’t even need to run the fan – ZeroCore Power shuts off the fan as it’s rendered an unnecessary device that’s consuming power.

Ultimately ZeroCore Power isn’t a brand new concept, but this is the first time we’ve seen something quite like this on the desktop. Even AMD will tell you the idea is borrowed from their mobile graphics technology, where they need to be able to power down the GPU completely for power savings when using graphics switching capabilities. But unlike mobile graphics switching AMD isn’t fully cutting off the GPU, rather they’re using power islands to leave the GPU turned on in a minimal power state. As a result the implementation details are very different even if the outcomes are similar. At the same time a technology like this isn’t solely developed for desktops so it remains to be seen how AMD can leverage it to further reduce power consumption on the eventual mobile Southern Islands GPUs.

Of course as impressive as sub-3W long idle power consumption is on a device with 4.3B transistors, at the end of the day ZeroCore Power is only as cool as the ways it can be used. For gaming cards such as the 7970 AMD will be leveraging it not only as a way to reduce power consumption when driving a blanked display, but more importantly will be leveraging it to improve the power consumption of CrossFire. Currently AMD’s Ultra Low Power State (ULPS) can reduce the idle power usage of slave cards to a lower state than the master card, but the GPUs must still remain powered up. Just as with long idle, ZeroCore Power will change this.

Fundamentally there isn’t a significant difference between driving a blank display and being a slave card card in CrossFire, in both situations the video card is doing nothing. So AMD will be taking ZeroCore Power to its logical conclusion by coupling it with CrossFire; ZeroCore Power will put CrossFire slave cards in ZCP power state whenever they’re not in use. This not only means reducing the power consumption of the slave cards, but just as with long idle turning off the fan too. As AMD correctly notes, this virtually eliminates the idle power penalty for CrossFire and completely eliminates the idle noise penalty. With ZCP CrossFire is now no noisier and only ever so slightly more power hungry than a single card at idle.

Furthermore the benefits of ZCP in CrossFire not only apply to multiple cards, but multiple-GPU cards too. When AMD launches their eventual multi-GPU Tahiti card the slave GPU can be put in a ZCP state, leaving only the master GPU and the PCIe bridge active. Coupled with ZCP on the master GPU when in long idle and even a beastly multi-GPU card should be able to reduce its long idle power consumption to under 10W after accounting for the PCIe bridge.

Meanwhile as for load power consumption, not a great deal has changed from Cayman. AMD’s PowerTune throttling technology will be coming to the entire Southern Islands lineup, and it will be implemented just as it was in Cayman. This means it remains operationally the same by calculating the power draw of the card based on load, and then altering clockspeeds in order to keep the card below its PowerTune limit. For the 7970 the limit is the same as it was for the 6970: 250W, with the ability to raise or lower it by 20% in the Catalyst Control Center.

On that note, at this time the only way to read the core clockspeed of the 7970 is through AMD’s drivers, which don’t reflect the current status of PowerTune. As a result we cannot currently tell when PowerTune has started throttling. If you recall our 6970 results we did find a single game that managed to hit PowerTune’s limit: Metro 2033. So we have a great deal of interest in seeing if this holds true for the 7970 or not. Looking at frame rates this may be the case, as we picked up 1.5fps on Metro after raising the PowerTune limit by 20%. But at 2.7% this is on the edge of being typical benchmark variability so we’d need to be able to see the core clockspeed to confirm it.

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  • CeriseCogburn - Thursday, March 08, 2012 - link

    Interesting, amd finally copied nvidia...
    " This problem forms the basis of this benchmark, and the NQueen test proves once more that AMD's Radeon HD 7970 tremendously benefits from leaving behind the VLIW architecture in complex workloads. Both the HD 7970 and the GTX 580 are nearly twice as fast as the older Radeons. "

    When we show diversity we should also show that amd radeon has been massively crippled for a long time except when "simpleton" was the key to speed. "Superior architecture" actually means "simple and stupid" - hence "fast" at repeating simpleton nothings, but unable to handle "complex tasks".
    LOL - the dumb gpu by amd has finally "evolved".
    Reply
  • chizow - Thursday, December 22, 2011 - link

    ....unfortunately its going to be pitted against Kepler for the long haul.

    There's a lot to like about Southern Islands but I think its going to end up a very similar situation as Evergreen vs. Fermi, where Evergreen released sooner and took the early lead, but Fermi ultimately won the generation. I expect similar with Tahiti holding the lead for the next 3-6 months until Kepler arrives, but Kepler and its refresh parts winning this 28nm generation once they hit the streets.

    Overall the performance and changes AMD made with Tahiti look great compared to Northern Islands, but compared to Fermi parts, its just far less impressive. If you already owned an AMD NI or Evergreen part, there'd be a lot of reason to upgrade, but if you own a Fermi generation Nvidia card there's just far less reason to, especially at the asking price.

    I do like how AMD opened up the graphics pipeline with Tahiti though, 384-bit bus, 3GB framebuffer, although I wonder if holding steady with ROPs hurts them compared to Kepler. It would've also been interesting to see how the 3GB GTX 580 compared at 2560 since the 1.5GB model tended to struggle even against 2GB NI parts at that resolution.
    Reply
  • ravisurdhar - Thursday, December 22, 2011 - link

    My thoughts exactly. Can't wait to see what Kepler can do.

    Also...4+B transistors? mind=blown. I remember when we were ogling over 1B. Moore's law is crazy.... :D
    Reply
  • johnpombrio - Wednesday, December 28, 2011 - link

    Exactly. If you look at all the changes that AMD did on the card, I would have expected better results: the power consumption decrease with the Radeon 7970 is mainly due to the die shrink to 28nm. NVidia is planning on a die shrink of their existing Fermi architecture before Kepler is released:

    http://news.softpedia.com/news/Nvidia-Kepler-Is-On...

    Another effect of the die shrink is that clock speed usually increases as there is less heat created at the lower voltage needed with a smaller transistor.

    The third change that is not revolutionary is the bump of AMD's 7970's memory bus from 384 bits (matching the 580) from the 6970's 256 bits along with 3GB DDR5 memory vs the GTX580's 1.5GB and the 6970's 2GB.

    The final non revolutionary change is bumping the number of stream processors by 33% from 1,536 to 2,048.

    Again, breaking out my calculator, the 35% bump in the number of stream processors ALONE causes the increase in the change in the benchmark differences between the 7970 and the 6970.

    The higher benchmark, however, does not show ANY OTHER large speed bumps that SHOULD HAVE OCCURED due to the increase in the memory bus size, the higher amount of memory, compute performance, texture fill rate, or finally the NEW ARCHITECTURE.

    If I add up all the increases in the technology, I would have expected benchmarks in excess of 50-60% over the previous generation. Perhaps I am naive in how much to expect but, hell, a doubling of transistor count should have produced a lot more than a 35% increase. Add the new architecture, smaller die size, and more memory and I am underwhelmed.
    Reply
  • CeriseCogburn - Thursday, March 08, 2012 - link

    Well, we can wait for their 50%+ driver increase package+ hotfixes - because after reading that it appears they are missing the boat in drivers by a wide margin.
    Hopefully a few months after Kepler blows them away, and the amd fans finally allow themselves to complain to the proper authorities and not blame it on Nvida, they will finally come through with a "fix" like they did when the amd (lead site review mastas) fans FINALLY complained about crossfire scaling....
    Reply
  • KaarlisK - Thursday, December 22, 2011 - link

    What is the power consumption with multiple monitors? Previously, you could not downclock GDDR5, so the resulting consumption was horrible. Reply
  • Ryan Smith - Thursday, December 22, 2011 - link

    "On that note, for anyone who is curious about idle clockspeeds and power consumption with multiple monitors, it has not changed relative to the 6970. When using a TMDS-type monitor along with any other monitor, AMD has to raise their idle clockspeeds from 350MHz core and 600Mhz memory to 350MHz core and the full 5.5GHz speed for memory, with the power penalty for that being around 30W. Matched timing monitors used exclusively over DisplayPort will continue to be the only way to be able to use multiple monitors without incurring an idle penalty." Reply
  • KaarlisK - Thursday, December 22, 2011 - link

    Thank you for actually replying :)
    I am so sorry for having missed this.
    Reply
  • ltcommanderdata - Thursday, December 22, 2011 - link

    Great review.

    Here's hoping that AMD will implement 64-bit FP support across the whole GCN family and not just the top-end model. Seeing AMD's mobile GPUs don't use the highest-end chip, settling for the 2nd highest and lower, there hasn't been 64-bit FP support in AMD mobile GPUs since the Mobility HD4800 series. I'm interested in this because I can then dabble in some 64-bit GPGPU programming on the go. It also has implications for Apple since their iMacs stick to mobile GPUs, so would otherwise be stuck without 64-bit FP support which presumably could be useful for some of their professional apps.

    In regards to hardware accelerated Megatexture, is it directly applicable to id Tech 5's OpenGL 3.2 solution? ie. Will id Tech 5 games see an immediate speed-up with no recoding needed? Or does Partially Resident Texture support require a custom AMD specific OpenGL extension? If it's the later, I can't see it going anywhere unless nVidia agrees to make it a multivendor EXT extension.
    Reply
  • Ryan Smith - Thursday, December 22, 2011 - link

    Games will need to be specifically coded for PRT; it won't benefit any current games. And you are correct in that it will require and AMD OpenGL extension to use (it won't be accessible from D3D at this time). Reply

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