[Fudzilla]AMD Fiji HBM limited to 4GB stacked memory? - Overclock.net - An Overclocking Community
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[Fudzilla]AMD Fiji HBM limited to 4GB stacked memory?

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post #1 of 127 (permalink) Old 02-19-2015, 02:00 PM - Thread Starter
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Cannot fit more on 2.5D interposer

In early January we heard a thing or two about an AMD card codenamed Fiji, the one that comes with High Bandwidth Memory (HBM).

The card is expected in the second part of 2015, and it will beat Nvidia Pascal to market. Nvidia is also using High Bandwidth Memory (HBM) for its 2016, 16nm GPU, but the company is using another technique, quite different compared to AMD's Fiji.

AMD is using what is calls a 2.5D-IC silicon interposer, which means that there will be two separate chips on the same silicon interposer and package substrate. Fiji in 28nm will be one of these chips, and the second batch of chips will be the High Bandwidth Memory (HBM) memory designs. However, there is a catch with AMD's approach.

From what we've learned, Fiji is limited to 4GB memory. With the current memory technology the GPU would simply be too big to put on an interposer and package. The interposer should be viewed as a stack of conductors that lets the GPU and HBM memory communicate at much higher speeds than ever before. The interposer then gets into the package that goes on PCB. You could say the interposer is the middle-man that makes things faster.

Hynix has HBM memory with 1024-bit wide interface and 1Gb/s per pin data-rate. This results with 128GB/s bandwidth per memory chip. In case of four 8 Gb chips (1GB) with a 1000MHz core clock you can end up with total bandwidth of 512GB/s. There are indications that HBM memory on Fiji might work at 1.25GHz, which would result in 640 GB/s. The Geforce GTX 980 has 224GB/s bandwidth, while Geforce Titan Back has 336 GB/s.

SK Hynix has listed that 1GB, 128GB/s chips with 1.0Gbps speed packaged in 5mKGSD are available now, January of 2015. These HBM chips feature 4Hi stack VDD/VDDQ 1.2V. The old GDDR5 needs 1.5V to work, meaning that HBM is not only faster, it is more power efficient as well. Fiji could end up having more than twice the bandwidth or Nvidia's current and upcoming Titan cards.

Back to the AMD Fiji approach with 2.5 stacking and interposer. Our sources claim that you cannot put more than four memory chips on the interposer, meaning that Fiji is limited to 4GB of memory. Eight chips next to a massive GPU would result in massively big chip (remember the GPU and memory chips are on the same board - interposer) and then put on the package. This is why we don’t think 8GB HBM Fiji will happen with this generation, but with time Hynix will come with more dense HBM memory chips making 8GB 2.5 D cards possible.


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The only thing that comes to mind is that AMD could be using 4GB HBM memory on the interposer and then put some additional GDDR5 chips on the PCB. Think of it as L2 and L3 cache with some older CPUs. Level two cache would be really faster, while Level tree cache would be slower, but would be able to get some important things done.

We don’t think that this would happen as GDDR5 memory would be significantly slower than the HBM part, making the similar case than with Geforce GTX 970 memory. Fiji has a good chance to beat whatever Nvidia comes up with in 2015, but it will probably have a hard time fighting against Pascal. Pascal is coming in 2016 and one can only hope that it will happen in early 2016.

We believe Fiji and Pascal will enable single GPU 4K gaming at acceptable frame rates and prices.

Source: http://www.fudzilla.com/news/graphics/36995-amd-fiji-hbm-limited-to-4gb-stacked-memory
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post #2 of 127 (permalink) Old 02-19-2015, 02:44 PM
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hmm, dunno if i read this right, but 8GB VRAM possible by doing two stacks, one on top of GPU die and the other to the side of it? Just wondering.

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post #3 of 127 (permalink) Old 02-19-2015, 02:51 PM
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Not news, we have known this.

Gen 1 HBM is limited to 4 GB, Gen 2 HBM goes higher.
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post #4 of 127 (permalink) Old 02-19-2015, 02:53 PM
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I dont think this is news actually. 1st gen HBM was confirmed to be limited to 4GB some time ago.

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post #5 of 127 (permalink) Old 02-19-2015, 02:54 PM
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Originally Posted by PostalTwinkie View Post

Not news, we have known this.

Gen 1 HBM is limited to 4 GB, Gen 2 HBM goes higher.

We know the stacks are limited to 1GiB.

We still don't know if the GPU will be limited to four stacks...I'm leaning against such a limitation, but time will tell.

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post #6 of 127 (permalink) Old 02-19-2015, 02:55 PM
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post #7 of 127 (permalink) Old 02-19-2015, 02:58 PM
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Originally Posted by Blameless View Post

We know the stacks are limited to 1GiB.

We still don't know if the GPU will be limited to four stacks...I'm leaning against such a limitation, but time will tell.

Yea, but even if the GPU is limited to address 4 whole stacks, do we need more than 4 stacks of Gen 2 when that hits? 32 GB of VRAM?!?

(If I did that right)
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So are they saying pascal will have 3d HBM stacks?

It will have 3DHBMULMBGASYNC Technology™
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post #8 of 127 (permalink) Old 02-19-2015, 03:01 PM
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So are they saying pascal will have 3d HBM stacks?

Been known since 2012 buddy. Some slide during a kepler presentation i think.
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post #9 of 127 (permalink) Old 02-19-2015, 03:04 PM
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Originally Posted by PostalTwinkie View Post

Yea, but even if the GPU is limited to address 4 whole stacks, do we need more than 4 stacks of Gen 2 when that hits? 32 GB of VRAM?!?

Probably not, but Gen 2 HBM isn't due till ~Q2 2016. Kind of a long wait for potential Fiji parts that may need more than 4GiB.

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post #10 of 127 (permalink) Old 02-19-2015, 03:10 PM
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Originally Posted by Blameless View Post

Probably not, but Gen 2 HBM isn't due till ~Q2 2016. Kind of a long wait for potential Fiji parts that may need more than 4GiB.

Which is one of the big debate topics around it.

At one point I was firmly OK with it, and said 4GB is more than enough for the large large majority. Then I have started looking at games like Shadows of Modor, and some of the up and coming, and I can see why 4K guys are going to want more. However, if the new APIs can truly combine VRAM into one pool, that solves that problem pretty solidly if done right.
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