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So will it close the gap with NVLink?
by anonymousDan 3y ago
So will it close the gap with NVLink?
- p1esk 3y agoWhat makes you think nvlink will stay the same?
- anonymousDan 3y agoI don't know either way, hence my question! Is there some fundamental design issue that means NVLink (or ultra Ethernet in the future) are likely to maintain their bandwidth advantage? Or is PCIe likely to close the gap?
- transpute 3y agoRecent thread: https://news.ycombinator.com/item?id=39729509 https://news.ycombinator.com/item?id=39729509
- touisteur 3y agoRecent announcement during GTC was Blackwell would do x2 on nvlink, 900GB/s => 1800GB/s. And iirc nvlink can do multi-node (gpus in different servers) too now... so... no?
- z4y5f3 3y agoNVLink advertises combined bandwidth in both direction, so the 1800 GBps NVLink on Blackwell is actually 900 GBps for everyone else. PCIe can also do multi-node direct transfer via PCIe switches and has been already widely adopted. NVLink still has the power and chip arena advantage even if the bandwidth is similar.
- touisteur 3y agoI was just saying they've announced x2 already, around the same pace as PCIe. And it's G-Byte-ps on nvlink, while G-bit-ps on PCIe, right? I'm probably missing something... Anyway, good read here https://community.fs.com/article/an-overview-of-nvidia-nvlink.html https://community.fs.com/article/an-overview-of-nvidia-nvlin...
- nsteel 3y agoBoth the PCIe and NVLink numbers here are full-duplex bandwidth. So no, you're not missing anything, you are correct. NVLink 5.0 is far ahead (but presumably it's not actually shipping yet either). Next-gen PCIe 7.0 512GB/sec full-duplex bandwidth actually sits between NVLink 2.0 (300GB/sec) and NVLink 3.0 (600GB/sec). NVLink 5.0's 1800GB/sec comes from 18 NVLinks, with each NVLink comprised of 2 lanes of 200Gbps (single-duplex). So in normal single-duplex numbers, each NVLink is 400Gbps, and 18 links together provide 7200Gbps. When advertised as full-duplex bandwidth that's 14400Gbps (or 1800GB/sec). In comparison, PCIe 7.0 uses only 128Gbps lanes: 128Gbps x 16 lanes x 2 directions = 4096Gbps (or 512GB/sec). PCIe has always been way behind cutting-edge serdes speeds. NVidia just made it more obvious to those working outside of specialist HPC and Networking. But we must cut PCIe some major slack, it has to (eventually) work on relatively cheapo hardware from hundreds of different vendors, in a restricted power and thermal environment i.e. consumer devices.
- touisteur 3y agoAh thanks for the detailed breakdown. You're right that PCIe has more problems to deal with.
- z4y5f3 3y agoDepends. NVLink advertises bidirectional bandwidth, whereas PCIe and standard networking calculate bandwidth in a single direction. So a 1800 GBps NVLink is actually 900 GBps in PCIe and standarding networking terms. Therefore, a 512 GBps PCIe would sit between the current H100 NVLink (450 GBps) and next generation B200 NVLink (900 GBps). With that being said, NVLink still has lower power draw and smaller chip area, so it would still have a competitive advantage even if the bandwidth is similar.
- anonymousDan 3y agoWut, I did not know that regarding NVLink marketing gimmicks! Thanks for the info.
- nsteel 3y agoI think you've mixed your Bytes and bits here. Not sure where the power and area numbers come from but I'd expect NVLink to be worse on both counts for similar bandwidths since they shipped much, much earlier. PCIe 7.0 is 512GB/s and not 512Gbps. 512GB/s is the full-duplex bandwidth. It sits between NVLink2 and NVLink3. I gave a fuller breakdown in a sibling thread.
- z4y5f3 3y agoChecked your numbers in another thread - excellent breakdown. Thanks for the clarification. I did not read the original anandtech post so I did not realize the 512GBps already refers to the full-duplex bandwidth. You are right that PCIe 7.0 x16 sits between V100 and A100 NVLink.
- aseipp 3y agoNVLink doesn't have as strict latency requirements as PCIe does. Stricter latency requirements mean you need more die area per GB/s of bandwidth. In practice this means that if you think of it from an mm^2 perspective, you can get much higher bandwidth -- higher GB-per-second-per-mm^2 -- with NVLink than you can PCIe. For many compute/bandwidth heavy workloads where that latency can be masked, this makes NVLink a superior choice, because die space is limited.
- pezezin 3y agoNVidia advertises their latest GPUs as having 18 NVLinks, but each link is 4 lanes. Make a 72x PCIe link, and you will see that the numbers are very similar.