AM4 PCIE Lanes

Troezar

Troezar

Troezar

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Joined
6 Aug 2009
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Quick question. How many devices can you connect before it affects performance i.e. reducing bandwidth to the GPU or disabling ports\slots?

24 lanes. 16 for a GPU, 4 for an nvme drive. Doesn't seem to leave much to add SSDs etc.

I would be expecting to run one or two nvme drives a GPU at x16 and 2-4 SSDs all at full speed as a bare minimum.

Edit: I now see Ryzen has 4 lanes for the Nvme and 16 for the GPU off the CPU with the intention of avoiding a potential bottleneck of the DMI connection.

When referring to the 24 lanes are they just talking about the CPU lanes rather than the chipset lanes?
 
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Yes just the CPU lanes they are talking about, x16 for the GPU, splits to x8-x8 if using 2 graphics cards or anything is insalled in the 2nd PCI-e x16 slot, x4 for the first nvme drive connected direct to the CPU, and the other x4 are to top up the south bridge lanes.

The first nvme drive runs at PCI-e 3.0 x 4 and if your board is equipped with a 2nd M.2 slot, this runs via the south bridge at PCI-e 2.0 x4 so basically half speed, however if you use the 2nd M.2 port, you loose the use of the 3rd PCI-e x16 slot which runs at x4 electrically, I guess these lanes are diverted to the 2nd M.2 slot, its only slowed down to PCI-e 2.0 x4 because of the DMI transfer, all 8 sata ports are still available unless I installed an M.2 SSD instead of an nvme then I loose only SATA3 port, according to my MSI Manual.

I was hoping with RyZen 2 they would just add the missing 4 PCI-e lanes to allow both nvme drives to run at full speed direct to the CPU, id have paid the extra, blistering fast RAID0.

However, they have somehow managed to allow raiding of the 2 nvme drives even with 1 running direct to the CPU and the other running via the SB at a slower speed, ive not really studied into how they managed it, but I get 3200mb read and 3200mb write on 2 Samsung Evo 960's 256gb each RAID0
 
Yes they make it add clear add mud. A simple diagram would make it much simpler to work out what you can and can't do. It seems the mainstream options from Intel and AMD both make comprises.

It is swaying me towards Threadripper as I don't need absolute top end gaming performance. However I want a platform that will last a ling time and be a good all rounder that will have decent upgrade potential.
 
Troezar said:
Yes they make it add clear add mud. A simple diagram would make it much simpler to work out what you can and can't do. It seems the mainstream options from Intel and AMD both make comprises.

It is swaying me towards Threadripper as I don't need absolute top end gaming performance. However I want a platform that will last a ling time and be a good all rounder that will have decent upgrade potential.
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Yes, with threadripper you certainly don't need to worry about PCI-e lanes that's for sure lol, pretty sure TR2 is just around the corner though, so you may want to hold off for a couple of months if you're thinking of going that way.

I basically run an MSI x470 Gaming Pro Carbon AC, R7 1700 @ 3.9ghz, 32gb (4x8gb) G.Skill Trident Z RGB 3200mhz at 2933mhz, 1 x ASUS GeForce 1070 8gb OC GFX card, 1 x Creative sound blaster AE-5, 2 x 960 nvme drives, 1 x Samsung evo 850 500gb SATA, and 1 x Western Digital 3tb green, all at full speed except the RAM, but that's pretty much because im using all 4 slots.

Anyway, heres a diagram, this is for X370, doesn't show the 2nd M.2 slot, just remember it shares the lanes from the 3rd PCI-e x16 (x4) slot out of the south bridge:

symu4y.png
 
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That's a well utilised system you've got. Impressed it all runs at full speed. Not sure how they're getting both those Nvme drives working so quickly? Aren't you in danger of saturating the DMI 3.0 link with the chipset Nvme?

I don't plan to upgrade until 7nm AM4 or TR4 so plenty of time to weigh it up and save some cash!
 
Troezar said:
That's a well utilised system you've got. Impressed it all runs at full speed. Not sure how they're getting both those Nvme drives working so quickly? Aren't you in danger of saturating the DMI 3.0 link with the chipset Nvme?

I don't plan to upgrade until 7nm AM4 or TR4 so plenty of time to weigh it up and save some cash!
Click to expand...

That's what I meant in my earlier post, I really don't know how they managed to get RAID running between the 2 nvme drives when 1 runs direct to the CPU and the other at half speed via the chipset ??? ok my drives out of the box are 3200 read, and still 3200 read in RAID0, but they are 1500 write out of the box, but raided together they are 3200 write too or there abouts.

its easy with threadripper, just run all the PCI-e lanes to the CPU, you've got 64 of them to use to the CPU alone, without the chipset ones too, that's potentially 4 m.2 slots at full speed, 2 x PCI-e x16 slots and 2 PCI-e x8 (x16) slots :D:D:D
 
In terms of saturation, guess its no difference to Intel and z370 having a trio of M.2 drives running at PCIe Gen 3.0x4 via the DMI 3.0 seems to cause no issue with the later so I expect the information works okay and no real noticeable impact unless the devices are all tugging on bandwidth at the exact same time.
 
This is why I went TR - just didn't want to be limited at all when it comes to total system bandwidth.
Have got 2 more 960 Evos on there way to go into my RAID array, things like this just wouldn't even be an option if not on a HEDT platform.
 
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