AMD Kaveri Docs Reference Quad-Channel Memory Interface, GDDR5 Option
by Anand Lal Shimpi on January 16, 2014 10:51 PM EST
Our own Ryan Smith pointed me at an excellent thread on Beyond3D where forum member yuri ran across a reference to additional memory controllers in AMD's recently released Kaveri APU. AMD's latest BIOS and Kernel Developer's Guide (BKDG 3.00) for Kaveri includes a reference to four DRAM controllers (DCT0 - 3) with only two in use (DCT0 and DCT3). The same guide also references a Gddr5Mode option for each DRAM controller.
Let me be very clear here: there's no chance that the recently launched Kaveri will be capable of GDDR5 or 4 x 64-bit memory operation (Socket-FM2+ pin-out alone would be an obvious limitation), but it's very possible that there were plans for one (or both) of those things in an AMD APU. Memory bandwidth can be a huge limit to scaling processor graphics performance, especially since the GPU has to share its limited bandwidth to main memory with a handful of CPU cores. Intel's workaround with Haswell was to pair it with 128MB of on-package eDRAM. AMD has typically shied away from more exotic solutions, leaving the launched Kaveri looking pretty normal on the memory bandwidth front.
In our Kaveri review, we asked the question whether or not any of you would be interested in a big Kaveri option with 12 - 20 CUs (768 - 1280 SPs) enabled, basically a high-end variant of the Xbox One or PS4 SoC. AMD would need a substantial increase in memory bandwidth to make such a thing feasible, but based on AMD's own docs it looks like that may not be too difficult to get.
There were rumors a while back of Kaveri using GDDR5 on a stick but it looks like nothing ever came of that. The options for a higher end Kaveri APU would have to be:
1) 256-bit wide DDR3 interface with standard DIMM slots, or
2) 256-bit wide GDDR5 interface with memory soldered down on the motherboard
I do wonder if AMD would consider the first option and tossing some high-speed memory on-die (similar to the Xbox One SoC).
All of this is an interesting academic exercise though, which brings me back to our original question from the Kaveri review. If you had the building blocks AMD has (Steamroller cores and GCN CUs) and the potential for a wider memory interface, would you try building a high-end APU for the desktop? If so, what would you build and why?
I know I'd be interested in a 2-module Steamroller core + 20 CUs with a 256-bit wide DDR3 interface, assuming AMD could stick some high-bandwidth memory on-die as well. More or less a high-end version of the Xbox One SoC. Such a thing would interest me but I'm not sure if anyone would buy it. Leave your thoughts in the comments below, I'm sure some important folks will get to read them :)
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Unspoken Thought - Friday, January 17, 2014 - link
Excellent ideas. This was along the lines of my thoughts as well. I would like to see graphics modules up for purchase so I could potentially choose which bandwidth to utilize. That way it is still left up to the consumer and would allow for multiple price points.Video cards would have to be redesigned to all proper cooling of the module. Aftermarket GPU cooling would have to be retooled as well. Memory manufactures could push this through to see higher revenue, just remember we will likely see price premiums on all components go up.
An alternative would be to have a dedicated area of the motherboard allotted for graphics memory modules. Again, we have to look at location, cooling, and direct access to the GPU whether through the chipset or some other silicon.
Mr Perfect - Friday, January 17, 2014 - link
It sounds good, but the generally accepted problem with that is the massive latency the GPU would have to deal with when accessing the system's RAM over the PCIe bus. That's the entire reason why GPUs have massive frame buffers on the card, they can't afford to go out to main memory.lmcd - Friday, January 17, 2014 - link
What about putting GDDR5 on a mini-card? Would be interesting given all the direct memory access available to Kaveri and Hawaii GPUs.tiquio - Thursday, January 16, 2014 - link
I'd me most interested in a large L4 cache like intel did because this seems to be the most affordable and reasonable option. Intel bills the eDRAM at $50. If AMD were do something similar they could charge a small premium for those who value/need more bandwidth and for those who don't need it, they can save some money and forego the L4.mczak - Thursday, January 16, 2014 - link
You need to have a L3 before you can have a L4 :-).testbug00 - Friday, January 17, 2014 - link
Um.Kind of.
People are to used to using Lx to describe something.
AMD could implement something with eDRAM like Intel did without an L3.
Colin1497 - Thursday, January 16, 2014 - link
Is it possible that Kaveri simply shares its memory controller with ps4? Random thought...jjj - Friday, January 17, 2014 - link
AMD most likely recycled blocks from the PS4 SoC.I've been saying for a while that they needed 20nm parts with at least 18CU this year (early better than late) but i highly doubt we will see much from AMD anytime soon. They are not eager to jump on new processes or to offer significant perf gains at decent prices if they aren't forced to.
As for GDDR or SRAM it's rather costly , Mavbe go with what Nvidia is doing with Volta and put some RAM on an interposer.
The problem with integrated GPU is that they've been chasing 1080p and failed to get quite there. Now we are going 4k and APUs are years behind so the timing stinks for fast APUs. Ofc matching consoles in perf is not a bad thing so it wouldn't be a waste of time if it can be done at acceptable prices.
It's sad thought that AMD can't do fast CPUs anymore. With Intel wasting die are on GPUs we don't use and it's grotesque prices for more than 4 cores ,there was a market to exploit there.The DIY market lacks energy and enthusiasm because we aren't getting what we want and nobody gives a damn.
Hell, i would go ARM and Linux if we had faster chips there.Guess ARM desktops is something nobody is trying to do yet and AMD might not have the resources for such a gamble.
jimjamjamie - Saturday, January 18, 2014 - link
I wonder how Nvidia's new Tegra chip fares against the lower-end APUs? May be an interesting match-up and perhaps even serious competition in the coming year.xTRICKYxx - Friday, January 17, 2014 - link
It would be cool to have a beefy APU in a NUC form factor. Although, it would probably be a little louder.