AMD Considered GDDR5 For Kaveri, Might Release Eight-Core Variant

MojoKid writes "Of all the rumors that swirled around Kaveri before the APU debuted last week, one of the more interesting bits was that AMD might debut GDDR5 as a desktop option. GDDR5 isn't bonded in sticks for easy motherboard socketing, and motherboard OEMs were unlikely to be interested in paying to solder 4-8GB of RAM directly. Such a move would shift the RMA responsibilities for RAM failures back to the board manufacturer. It seemed unlikely that Sunnyvale would consider such an option but a deep dive into Kaveri's technical documentation shows that AMD did indeed consider a quad-channel GDDR5 interface. Future versions of the Kaveri APU could potentially also implement 2x 64-bit DDR3 channels alongside 2x 32-bit GDDR5 channels, with the latter serving as a framebuffer for graphics operations. The other document making the rounds is AMD's software optimization guide for Family 15h processors. This guide specifically shows an eight-core Kaveri-based variant attached to a multi-socket system. In fact, the guide goes so far as to say that these chips in particular contain five links for connection to I/O and other processors, whereas the older Family 15h chips (Bulldozer and Piledriver) only offer four Hypertransport links."

Comments

Latency vs bandwidth

[evilsofa]

DDR3 is low latency, low bandwidth. GDDR5 is high latency, high bandwidth. Low latency is critical for CPU performance while bandwidth doesn't matter as much. On video cards, GPUs need high bandwidth but the latency doesn't matter as much. This is why gaming PCs use DDR3 for system RAM and GDDR5 on their video cards. Video cards that cut costs by using DDR3 instead of GDDR5 take a massive hit in performance. The XBox One and PS4 use GDDR5 shared between the CPU and GPU, and as a result have the rough equivalent of a very low-end CPU paired with a mid-range GPU.

Re:Latency vs bandwidth

[Anonymous Coward]

False, XBox one uses pure DDR3.

It is also one of the key reasons why many games on XBox one cannot do 1080p (that, and the lack of ROPs - PS4 having twice as many ROPs for rasterization)

XBox One tries to "fix" the RAM speed by using embedded sRAM on-chip as a cache for the DDR3 for graphics. Remains to be seen how well the limitations of DDR3 can be mitigated. Early games are definitely suffering from "developer cannot be assed to do a separate implementation for Xbox One".

Kaveri, while related to the chips inside the consoles, is decisively lower performing part. Kaveri includes 8 CUs. XBox one has 14 CUs on die, but two of those are disabled (to improve yields), so 12. PS4 has 20 CUs on-die, with again two CUs disabled to improve yields, so 18.

On the other hand, Kaveri has far better CPU cores (console chips feature fairly gimpy Jaguar cores, tho both consoles feature 8 of those cores vs 4 on Kaveri)

Any integrated graphics setup that uses DDR3 is bound to be unusable for real gaming. Kaveri has a good integrated graphics setup compared to the competition, but it is far behind what the new consoles feature - boosting it with GDDR5 without also at least doubling the CU count wouldn't do much. Either way, it really isn't usable for real gaming. It beats the current top offering from Intel, but that's bit like winning in Special Olympics when compared to real graphics cards (even ~$200-250 midrange ones)

Re:Latency vs bandwidth

[Sockatume]

Latency in cycles is higher for GDDR5, but the clock speed's a lot faster, isn't it? As the real-time latency is the product of the number of cycles and the length of a cycle, I think it's pretty much a wash.

Re:Latency vs bandwidth

[Bengie]

DDR3 and GDDR5 have nearly the same latency when measured in nanoseconds. When measured in clock cycles GDDR5 is higher latency, but it has more cycles. This has to do with the external interface, which is more serial in nature than the internal. The data path for the internal is quite wide, but the external datapath is not because traces on a motherboard are expensive. To crank up the bandwidth, you increase the frequency. If the internal frequency remains fixed and the external frequency goes up, the external "latency" seeming goes up.

Of course, that would miss the point

[guacamole]

The whole point of AMD APUs is low cost gaming. That is, lower cost than buying a dedicated GPU plus a processor. Many already argue that you don't save much by buying an APU. A cheap Pentium G3220 with a AMD Radeon 7730 costs the same as the A10 Kaveri APU, and will give better frame rate. Even if the Kaveri APU prices come down, the savings will be small. If you have to buy the GDDR5 memory, there won't be any savings. It's understandable that AMD didn't take that route.

A Better Explaination At Anandtech

[Anonymous Coward]

Anandtech's writeup (which Hothardware seems to be ripping off) has a much better explanation of what's going on and why it matters.

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.

It's also worth noting that the Anandtech article implies that AMD is still on the fence on Kaveri APUs with more memory bandwidth, and that it may be something they do if there's enough interest/feedback about it.

Re:AMD could do a 24 core desktop chip right now

[TheLink]

They don't care because a desktop with a 24 core AMD CPU is likely to be slower than a 4 core Intel CPU for most popular _desktop_ tasks which are mostly single threaded. They're already having problems competing with 8 core CPUs, adding more cores would either make their chips too expensive (too big) or too slow (dumber small cores).

Sad truth is for those who don't need the speed a cheap AMD is enough - they don't need the expensive ones. Those who want the speed pay more for Intel's faster stuff. The 8350 is about AMD's fastest desktop CPU for people who'd rather not use 220W TDP CPUs, and it already struggles to be ahead of Intel's mid range for desktop tasks: http://www.anandtech.com/bench/product/697?vs=702

A few of us might regularly compress/encode video or use 7zip to compress lots of stuff. But video compression can often be accelerated by GPUs (and Intel has QuickSync but quality might be an issue depending on implementation). The rest of the desktop stuff that people care about spending $$$ to make faster would be faster on an Intel CPU.

A server with 24 cores will be a better investment than a desktop with 24 cores.

Re:news for nerds

[SuricouRaven]

Because it's electrically so delicate that you can't keep bit sync when shoving such high frequencies through a slot connector. The price of higher bandwidth, in both the analog and digital senses.

Re:AMD could do a 24 core desktop chip right now

[symbolset]

Fine. You do it your way. I want this technological achievement so I can do it mine.