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If we're discussing future GPUs, does anyone know what exactly AMD means by "next-gen memory" when it talks about Navi? At first I thought it means that HBM2 w

by mtgx 11y ago

If we're discussing future GPUs, does anyone know what exactly AMD means by "next-gen memory" when it talks about Navi? At first I thought it means that HBM2 would be available across their whole lineup, but I think if that was the case they would've just said so. I've also read a bit about how Nvidia wanted to go with HMC (Hyper Memory Cube) instead of HBM, but it seems it's still at least twice as expensive/GB, and they needed to go with high-end GPUs that have at least 16GB. HMC is not even at 8GB yet. Intel also seems to have adopted HMC for some servers. So is the "next-gen" memory HMC, or something else? AMD is supposed to come out with it in 2018, hopefully at 10nm.

redtuesday 11y ago
Was the difference between HMC and HBM not basically HMC had good latency comparable to normal main RAM like DDR3 plus good bandwith (up to 400 GB/s), while HBM had worse latency than DDR3 (like GDDR5) with a large bus and really good bandwith (1 TB/s)? Will indeed be interesting to see what AMD means with next gen memory.
- Tuna-Fish 11y ago
  No. HMC is fully buffered and provides a packet-based interface to the CPU it's connected into. This means that the CPU doesn't need to know anything about the memory it uses, and saves die space on the CPU side, but it adds latency. HBM has lower latency than HMC. Also, GDDR5 does not have higher latency than DDR3. The memory arrays themselves are the same and exactly as fast, and the bus doesn't add any extra latency. GDDR5 doesn't trade latency for bandwidth, it trades density for bandwidth. Things built to use GDDDR5 often have much higher latencies than things built to use DDR3, but this has nothing to do with the memory and instead about how the memory controllers in GPUs delay accesses to merge ones close to each other to optimize for bandwidth.
  - redtuesday 11y ago
    Thank you for the detailed explanation. Always thought the added latency came from GDDR5. Good to know.

by mtgx 11y ago

redtuesday 11y ago

Was the difference between HMC and HBM not basically HMC had good latency comparable to normal main RAM like DDR3 plus good bandwith (up to 400 GB/s), while HBM had worse latency than DDR3 (like GDDR5) with a large bus and really good bandwith (1 TB/s)? Will indeed be interesting to see what AMD means with next gen memory.

Tuna-Fish 11y ago
No. HMC is fully buffered and provides a packet-based interface to the CPU it's connected into. This means that the CPU doesn't need to know anything about the memory it uses, and saves die space on the CPU side, but it adds latency. HBM has lower latency than HMC. Also, GDDR5 does not have higher latency than DDR3. The memory arrays themselves are the same and exactly as fast, and the bus doesn't add any extra latency. GDDR5 doesn't trade latency for bandwidth, it trades density for bandwidth. Things built to use GDDDR5 often have much higher latencies than things built to use DDR3, but this has nothing to do with the memory and instead about how the memory controllers in GPUs delay accesses to merge ones close to each other to optimize for bandwidth.
- redtuesday 11y ago
  Thank you for the detailed explanation. Always thought the added latency came from GDDR5. Good to know.