WEBVTT 00:00:00.240 --> 00:00:06.560 bifurcation it's a fancy word that means to split something into two and today 00:00:04.960 --> 00:00:10.480 we're going to talk about bifurcating something that isn't a sandwich a 00:00:08.400 --> 00:00:14.960 sporting event or a marriage this episode is all about pci express 00:00:12.800 --> 00:00:19.039 bifurcation this is exactly what it sounds like taking a pci express slot 00:00:17.199 --> 00:00:23.359 and chopping it up so that you can use multiple devices inside of it 00:00:21.119 --> 00:00:27.680 but how does that work i mean you can't exactly shove two graphics cards in one 00:00:25.600 --> 00:00:31.039 slot to answer we reached out to philip meyer from tian and we'd like to thank 00:00:29.359 --> 00:00:36.079 him for his contributions so the thing to keep in mind here is how pci express 00:00:33.440 --> 00:00:40.079 lanes are configured on modern platforms your CPU has a certain number of 00:00:38.239 --> 00:00:46.079 controllers and each one can only support one device for example even 00:00:43.360 --> 00:00:51.039 though a current Intel CPU supports 16 lanes it's split up with four 00:00:48.160 --> 00:00:55.280 controllers controlling four lanes each meaning you can connect a maximum of 00:00:52.879 --> 00:01:00.000 four pci express devices of course you can connect one graphics card and use 00:00:57.280 --> 00:01:04.479 all 16 lanes and just call it a day but let's say that you want to use multiple 00:01:01.760 --> 00:01:08.799 pci express storage devices like NVMe solid state drives this is where 00:01:06.400 --> 00:01:13.680 bifurcation comes in you see one of the most common uses of bifurcation is when 00:01:11.119 --> 00:01:18.000 you want to connect several m.2 drives to a single pci express slot this can be 00:01:16.320 --> 00:01:21.280 done using a fancy card that looks something like this but here's the 00:01:19.600 --> 00:01:25.920 kicker if you're going to chop up just one slot you need to tell your 00:01:23.040 --> 00:01:30.320 motherboard inside the BIOS exactly how to divvy up those lanes otherwise it's 00:01:28.400 --> 00:01:33.600 just gonna see one of those four devices and you're gonna regret spending the 00:01:31.520 --> 00:01:36.880 money how exactly you split up these lanes depends on what platform you're 00:01:35.119 --> 00:01:41.360 using as mentioned earlier the most you can do on a mainstream Intel platform is 00:01:38.799 --> 00:01:45.439 four devices with four lanes each but on a server platform like AMD epic you 00:01:43.439 --> 00:01:50.159 might get something like eight devices albeit with just two lanes each if you 00:01:47.920 --> 00:01:53.680 use that many bifurcation is a lot more common in enterprise settings as data 00:01:52.000 --> 00:01:58.159 centers tend to use pci express lanes for things that you just wouldn't find 00:01:55.280 --> 00:02:01.759 at home such as fpgas and asics chips and devices that can be highly 00:01:59.680 --> 00:02:05.520 customized to work on very specific kinds of operations but that doesn't 00:02:03.600 --> 00:02:10.239 mean that there aren't additional uses for pci express bifurcation at home like 00:02:08.080 --> 00:02:14.800 running two graphics cards in one slot although it's common for gamers to run 00:02:11.840 --> 00:02:18.560 just one graphics card at x16 speed running a card at x8 will make virtually 00:02:17.040 --> 00:02:22.879 no difference to your performance as even high-end modern cards don't move 00:02:20.800 --> 00:02:27.840 enough data to really need those extra lanes now obviously if you have a board 00:02:25.200 --> 00:02:32.080 with two free full-length pci express slots the lanes are automatically 00:02:29.200 --> 00:02:36.400 divvied up by a switch called a mux chip so that each card runs at x8 speed but 00:02:34.560 --> 00:02:42.239 there's actually nothing stopping you from sticking a two slot riser card into 00:02:39.040 --> 00:02:44.239 your pci express x16 slot plugging in 00:02:42.239 --> 00:02:48.560 two graphics cards with the help of riser cables and telling your BIOS to 00:02:46.319 --> 00:02:53.920 bifurcate the slot into two x8 connections just think you could run 00:02:50.560 --> 00:02:55.440 nvlink on a one slot mini itx board 00:02:53.920 --> 00:02:59.519 assuming you could find a way to mount the cards in your case so hopefully you 00:02:57.840 --> 00:03:03.360 don't mind using duct tape but if you're interested in using pci express 00:03:01.040 --> 00:03:07.519 bifurcation regardless of what exactly you'll be plugging in you need to verify 00:03:05.680 --> 00:03:11.519 that your motherboard and BIOS support it not all motherboards allow you to 00:03:09.599 --> 00:03:15.760 enable it in the BIOS and if that's the case it's pretty much a show stopper so 00:03:13.680 --> 00:03:20.400 do your research before you decide to turn your home rig into some sort of 00:03:17.599 --> 00:03:24.400 crazy science experiment big thanks to pia for sponsoring 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