bifurcation it's a fancy word that means to split something into two and today
we're going to talk about bifurcating something that isn't a sandwich a
sporting event or a marriage this episode is all about pci express
bifurcation this is exactly what it sounds like taking a pci express slot
and chopping it up so that you can use multiple devices inside of it
but how does that work i mean you can't exactly shove two graphics cards in one
slot to answer we reached out to philip meyer from tian and we'd like to thank
him for his contributions so the thing to keep in mind here is how pci express
lanes are configured on modern platforms your CPU has a certain number of
controllers and each one can only support one device for example even
though a current Intel CPU supports 16 lanes it's split up with four
controllers controlling four lanes each meaning you can connect a maximum of
four pci express devices of course you can connect one graphics card and use
all 16 lanes and just call it a day but let's say that you want to use multiple
pci express storage devices like NVMe solid state drives this is where
bifurcation comes in you see one of the most common uses of bifurcation is when
you want to connect several m.2 drives to a single pci express slot this can be
done using a fancy card that looks something like this but here's the
kicker if you're going to chop up just one slot you need to tell your
motherboard inside the BIOS exactly how to divvy up those lanes otherwise it's
just gonna see one of those four devices and you're gonna regret spending the
money how exactly you split up these lanes depends on what platform you're
using as mentioned earlier the most you can do on a mainstream Intel platform is
four devices with four lanes each but on a server platform like AMD epic you
might get something like eight devices albeit with just two lanes each if you
use that many bifurcation is a lot more common in enterprise settings as data
centers tend to use pci express lanes for things that you just wouldn't find
at home such as fpgas and asics chips and devices that can be highly
customized to work on very specific kinds of operations but that doesn't
mean that there aren't additional uses for pci express bifurcation at home like
running two graphics cards in one slot although it's common for gamers to run
just one graphics card at x16 speed running a card at x8 will make virtually
no difference to your performance as even high-end modern cards don't move
enough data to really need those extra lanes now obviously if you have a board
with two free full-length pci express slots the lanes are automatically
divvied up by a switch called a mux chip so that each card runs at x8 speed but
there's actually nothing stopping you from sticking a two slot riser card into
your pci express x16 slot plugging in
two graphics cards with the help of riser cables and telling your BIOS to
bifurcate the slot into two x8 connections just think you could run
nvlink on a one slot mini itx board
assuming you could find a way to mount the cards in your case so hopefully you
don't mind using duct tape but if you're interested in using pci express
bifurcation regardless of what exactly you'll be plugging in you need to verify
that your motherboard and BIOS support it not all motherboards allow you to
enable it in the BIOS and if that's the case it's pretty much a show stopper so
do your research before you decide to turn your home rig into some sort of
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