2.8M views · Aug 31, 2017 · 9:41 · 1 HN recommender · 7 mentions · discussed 2017–2026
Linus Tech Tips tests whether chaining many PCIe riser extensions causes loss of GPU performance. They keep adding extensions to see how far the signal will go.
Why Hacker News points people here: HN cites it as empirical evidence that PCIe cables can be chained to roughly three meters before failing.
“The limit is around 3 meters.”HN comment, 2022
“Linus Tech Tips has a bunch of good ones”
Fun challenge of chaining PCI extenders with a graphics card.
I love a good “PCI Challenge”, where someone tries to see how much functionality they can wring out of PCI devices in a weird situation. Linus Tech Tips has a bunch of good ones: - How crazy of a device can you use with a server motherboard’s PCI hot-swap BIOS feature: link - How many PCI extenders can you use with a high end graphics card: this video
Cited as source for PCIe cables over 3m.
It's a shame you can't get a cable to link computers via PCIe (supports cables over 3m[0]). Multiple x16 connections that would lead to some decent bandwidth while being low-latency, without the network overhead or shelling out for high-end switches. You can skip halfway. [0] this video
“The limit is around 3 meters.”
Shows PCIe extensions can chain to about 3 meters.
Here is an LTT video about how many PCI-E extensions one can daisychain without problems. The limit is around 3 meters. this video
“They got to several meters.”
Riser cables strung together to several meters before failure.
LTT has a video where they tried to see how many PCIe riser cables they could string together before it stopped working.[1] They got to several meters. Maybe you could argue that it's worse inside a PC case since there's more EMI, but it seems like your PCIe riser cable would have to be very out of spec before you'd notice anything. [1] this video
“I don't care much for Linus Tech, but”
Linked for riser cable length, despite disliking the channel.
Someone needs to get this man a speed square, also a longer riser cable [1]. [1]I don't care much for Linus Tech, but this video
“PCIe is more like a packet-switched network than a bus”
Demonstration of PCIe behaving like a packet network.
PCIe is more like a packet-switched network than a bus, which is incidentally why things like Thunderbolt (effectively external PCIe) and sillier demonstrations like this video work. ...and with things like link , you can get even higher latencies.
“I wonder if that benchmark actually loaded the PCIe bus to any significant degree”
I wonder if that benchmark actually loaded the PCIe bus to any significant degree after the initial benchmark startup, or just updated a couple small parameters on a single scene and thus mainly just tested the local computation on the GPU? You'd want to somehow monitor the PCIe bus error rate - with a marginal signal and lots of errors -> retries, something that loads the bus harder (loading new textures etc) could suffer way more. They do briefly show a different PCIe riser made out of generic ribbon cable [1, 3:27], and say that one failed after chaining only two ~200mm lengths. The quality of the riser cable certainly matters. [1] this video
