Question PCIEx16 drops to x 8 when M2 SSD is installed???

[Fakum]

I am looking to build a new PC with a 4090 GPU and an M2 ssd (and also considering it to support M2 Gen 5 for future upgrade). I am looking at the specs on a couple of Gigabyte boards and there are notes in there that I dont understand:

GIGABYTE Z790 AORUS Xtreme & GIGABYTE Z790 AORUS Master :

"1 x PCI Express x16 slot, supporting PCIe 5.0 and running at x16 (PCIEX16)
* The PCIEX16 slot shares bandwidth with the M2C_CPU connector. The PCIEX16 slot operates at up to x8 mode when a device is installed in the M2C_CPU connector.
* For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16 slot. "

Does this mean that if I install a 4090 into the PCIEx16 slot it drops down to x8 if I install an M2 SSD? Or is this typical for all boards anyway? How do I avoid any performance drops (If any) using these parts?

Thanks

 

[In2Photos]

Current motherboards have multiple M.2 slots. Some of those slots share PCI lanes with the PCIe slots. In the one you mentioned it appears that the 3rd slot (M2C, likely means there is an M2A and M2B) shares lanes with the PCIex16 slot. So you will only see x8 on that PCI slot if you use the M2C connector. Note that PCIe5.0x8 is the same speed as PCIe4x16 so you aren't really losing any performance with the 4090 as it is a PCIe4 card. Future cards may see a performance hit.

 

[blckgrffn]

Current motherboards have multiple M.2 slots. Some of those slots share PCI lanes with the PCIe slots. In the one you mentioned it appears that the 3rd slot (M2C, likely means there is an M2A and M2B) shares lanes with the PCIex16 slot. So you will only see x8 on that PCI slot if you use the M2C connector. Note that PCIe5.0x8 is the same speed as PCIe4x16 so you aren't really losing any performance with the 4090 as it is a PCIe4 card. Future cards may see a performance hit.

Hmmm… I think it is possible it will only run as an eight lane slot with that drive installed, and only at the maximum device supported speed.

So it will be a functional PCIE 5 x8 slot, then negotiate down to PCIE 4 as that’s the highest standard the 4090 supports, as you pointed out.

To get the functional PCIe 4 x16 speed out of the 8 lanes, both the slot and the device will need to be PCIe 5.

 

[In2Photos]

Hmmm… I think it is possible it will only run as an eight lane slot with that drive installed, and only at the maximum device supported speed.

So it will be a functional PCIE 5 x8 slot, then negotiate down to PCIE 4 as that’s the highest standard the 4090 supports, as you pointed out.

To get the functional PCIe 4 x16 speed out of the 8 lanes, both the slot and the device will need to be PCIe 5.

Now that you mention I think you're right. It will run at x8 won't it?

 

[Hail The Brain Slug]

So the reason why some Z790 boards do this is so one of the M.2 slots is PCI-e 5.0 for the newly releasing and upcoming PCI-e 5.0 SSDs. If you are using PCI-e 3.0 or PCI-e 4.0 SSDs, just use the other M.2 slots and you will see full speed on those SSD's and your GPU's link speed.

Alder and Raptor lake only have 16x PCI-e 5.0 lanes, 4x PCI-e 4.0 lanes, and the chipset's PCI-e 4.0 lanes. Because of that limitation they have to rob lanes from the PCI-e 5.0 slots in order to use some of those lanes for a PCI-e 5.0 M.2 SSD.

 

[Fakum]

Thanks for the responses, I absorbed most of it. What I got out of it was:

The 4090 wont be impacted by this, and it is advisable that I install the M2 in one of the other M2 slots that is not shared with the GPU PCIEx16 slot. However, in the future, I will need that M2 slot that is shared with the GPU PCIEx16 slot if I install a Gen 5 M2. And if I do that, the M2 Gen 5 SSD will run at its Gen 5 speed, but the GPU Slot will drop down to a PCIEx8 but it wont impact the performance of the GPU in any way? Correct?

Also, Im not clear if there are any other high end gaming motherboards that eliminate this sharing (at least yet?).

 

[blckgrffn]

Thanks for the responses, I absorbed most of it. What I got out of it was:

The 4090 wont be impacted by this, and it is advisable that I install the M2 in one of the other M2 slots that is not shared with the GPU PCIEx16 slot. However, in the future, I will need that M2 slot that is shared with the GPU PCIEx16 slot if I install a Gen 5 M2. And if I do that, the M2 Gen 5 SSD will run at its Gen 5 speed, but the GPU Slot will drop down to a PCIEx8 but it wont impact the performance of the GPU in any way? Correct?

Also, Im not clear if there are any other high end gaming motherboards that eliminate this sharing (at least yet?).

It probably won't impact the performance of your card, and if it does it might be detectable by benchmarks or something but unlikely by you.

Unless the game is heavily dependent on streaming in textures or some corner case where your 24GB card is having its framebuffer overwhelmed. Decidedly corner case (looks at MS flight sim) for now, might be different in the future but of low risk.

PCIe5 NVME drives look to be hot garbage imo for the next year at least until they can bring the controller power usage under control and significantly push past PCIe 4 speed limits.

And by that point, will it matter or will you have moved on? Who knows.

 

[Fakum]

Thanks, I primarily use the PC for Digital Combat Simulator World (Combat Flight Sim), they just introduced MultiThreading, so that is still WIP. My current rig doesnt handle it well at all. But thanks!

Current PC Specs: Windows 10 Pro - 64 Bit / GIGABYTE 3866MHz GA-Z170X-Gaming 7 MOBO / Intel i7-6700K 4.2GHz Processor (OC to 4.Eight) / G.SKILL TridentZ Series 32GB DDR4 3200 Ram / Samsung 870 SSD 2TB / EVGA 2080 Ti FTW3 Hybrid/ Onboard Audio card / SteelSeries Arctis 7 /LG-Ultragear 38" IPS LED Ultrawide HD Monitor (3840 x 1600)/ Track IR4 / Thrustmaster TPR Pendular Rudder Pedals / Virpil HOTAS VPC Constellation ALPHA-R & VPC MongoosT-50CM3 Throttle

 

[blckgrffn]

Nice! You got a great run out of that PC!

If I may, the 8 core, 3D cache CPU from AMD drops in just about 2 weeks. I think that this might make a great choice for you, and you can have all of your PCIe5 and eat it too with the right AMD board, which is likely to be in that same stratum where you are currently shopping.

The 3D cache parts particularly effective at simulation type applications and will likely deliver the same performance or better than a 13700k/13900K while also saving upwards of 100 peak watts.

I also mention this as you are moving from maybe 400W total of power consumption to a point where your card might take that alone. This will likely change heating and cooling aspects of your PC room so it is worth considering IMHO.

I am not trying to denigrate the performance of those high end 13th gen parts but point out that AMD has extremely healthy competitor worth your consideration in wholistic sense.

 

[Fakum]

Yes, just over 6 years on this build! I appreciate your suggestion, I was going with Intel frankly because that is what I am acquainted with. Seems most of the Flight simmers are with Intel also. I have read a tad about AMD being quite the contender for gaming, but i havnt read much and im not sure at all how it handles Multi Threading for DCS? Now your making me more curious LOL. Im not in a hurry, so I have time to research. Im not sure at all what the disadvantages would be going with AMD (If any?) And your right about the 4090 power consumption, especially the one I am targeting, which it is water cooled (RTX 4090 SUPRIM Liquid). I will be getting out of the 860 Watt family into the 1000 watt family going the Intel i9-13900K route (Will be using another Corsair Hydro Series H115i Liquid cooler for this build also). I am a bit over budget so I am entertaining all suggestions. Right now Im at about $3900.00 with current parts list. I still have to add the CPU cooler, and I am robbing the Silent fans out of this rig.

-Intel Core i9-13900K
-ASUS ROG Maximus Z790 Hero
-MSI Gaming GeForce RTX 4090 SUPRIM Liquid X 24G
-G.Skill Trident Z5 RGB Series (Intel XMP) 32GB (2 x 16GB) 288-Pin SDRAM DDR5 6000 CL36-36-36-96 1.35V
-SAMSUNG 990 PRO SSD 2TB PCIe 4.0 M.2 Internal Solid State Drive
-MSI MPG A1000G PCIE 5 & ATX 3.0 Gaming Power Supply
-Tower: Lian Li PC-O11DW 011

 

[blckgrffn]

Yes, just over 6 years on this build! I appreciate your suggestion, I was going with Intel frankly because that is what I am acquainted with. Seems most of the Flight simmers are with Intel also. I have read a tad about AMD being quite the contender for gaming, but i havnt read much and im not sure at all how it handles Multi Threading for DCS? Now your making me more curious LOL. Im not in a hurry, so I have time to research. Im not sure at all what the disadvantages would be going with AMD (If any?) And your right about the 4090 power consumption, especially the one I am targeting, which it is water cooled (RTX 4090 SUPRIM Liquid). I will be getting out of the 860 Watt family into the 1000 watt family going the Intel i9-13900K route (Will be using another Corsair Hydro Series H115i Liquid cooler for this build also). I am a bit over budget so I am entertaining all suggestions. Right now Im at about $3900.00 with current parts list. I still have to add the CPU cooler, and I am robbing the Silent fans out of this rig.

-Intel Core i9-13900K
-ASUS ROG Maximus Z790 Hero
-MSI Gaming GeForce RTX 4090 SUPRIM Liquid X 24G
-G.Skill Trident Z5 RGB Series (Intel XMP) 32GB (2 x 16GB) 288-Pin SDRAM DDR5 6000 CL36-36-36-96 1.35V
-SAMSUNG 990 PRO SSD 2TB PCIe 4.0 M.2 Internal Solid State Drive
-MSI MPG A1000G PCIE 5 & ATX 3.0 Gaming Power Supply
-Tower: Lian Li PC-O11DW 011

That's going to be quite the monster!

I would look into the AMD parts - the 3D cache parts are the CPUs to have for MS Flight Sim, bar none. I don't know how they will operate for you specifically, but they are incredibly solid gaming CPUs, especially if that is the specific use for your rig. Also, with the big cache you can more easily go with lesser ram and maybe consider getting more of it instead of paying for the speed.

It would also really assist you staying in budget

 

[Fakum]

Thanks, Off to a new research quest,, LOL,,, thanks

 

[BonzaiDuck]

Those are the specifics of motherboards you want to know as you go forward to pick your other parts. Someone mentioned that PCIE 5.0 x8 had a similar bandwidth to version 4.0 x16. That tells me that there's not so much a loss for x8 versus x16 even in more recent generations of hardware.

I myself am at a crossroads, contemplating a build with newer parts. The attempt is always to build a system that is good for gaming and everything else. I'm not sure my heart is in it at this stage of my life.

 

[igor_kavinski]

I myself am at a crossroads, contemplating a build with newer parts. The attempt is always to build a system that is good for gaming and everything else. I'm not sure my heart is in it at this stage of my life.

Don't mind but do you still game? Your other posts make it seem like you spend most of your computing time doing backups and managing your personal finances electronically.

 

[VirtualLarry]

Don't mind but do you still game? Your other posts make it seem like you spend most of your computing time doing backups and managing your personal finances electronically.

BestBuy on ebay has an HP Victus 15L tower in white black, with an i3-12100 and a GTX 1650 4GB, brand new, for $529.

HP - Victus Gaming Desktop - Intel Core i3-12100F - 8GB Memory - NVIDIA GeFor... 196548449187 | eBay

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Sorry, that was a note for BonzaiDuck. I'll PM him too.

 

[Fakum]

Im failing to see the relevance of this to my inquiry?

 

[BoomerD]

I just pulled up the manual for my MSI Z690 Tomahawk DDR4 board. It has 4 PCI-e slots:

PCI_E1: PCIe 5.0 x16 (From CPU)
PCI_E2: PCIe 3.0 x1 (From Z690 chipset)
PCI_E3: PCIe 3.0 x4 (From Z690 chipset)
PCI_E4: PCIe 3.0 x1 (From Z690 chipset)

It also has 4 M.2 slots:

• M2_1 slot (From CPU)
  • Supports up to PCIe 4.0 x4
  • Supports 2242/ 2260/ 2280/ 22110 storage devices
• M2_2 slot (From Z690 chipset)
  • Supports up to PCIe 4.0 x4
  • Supports 2242/ 2260/ 2280 storage devices
• M2_3 slot (From Z690 chipset)
  • Supports up to PCIe 3.0 x4
  • Supports up to SATA 6Gb/s
  • Supports 2242/ 2260/ 2280 storage devices
• M2_4 slot (From Z690 chipset)
  • Supports up to PCIe 4.0 x4
  • Supports up to SATA 6Gb/s
  • Supports 2242/ 2260/ 2280 storage devices

There is nothing about a reduction in speed from any of the PCI-e slots, and nothing about:

If M2_1 is occupied by a SATA-type M.2 device, SATA3_5 will be disabled.
If M2_2 is occupied by a SATA-type M.2 device, SATA3_1 will be disabled.
If M2_2 is occupied by a PCIe-type M.2 device, SATA3_0 will be disabled.

(that's from my old ASRock Z390 Phantom Gaming 4 board)

MAYBE it's because in your case, both the PCI-E slot and the M.2 slot are Gen 5.

I did some looking at the MSI site for Z790 boards. The Z790 Meg Ace board has similar language to yours:

*PCI_E1 slot will run at x8 speed when installing device in the PCI_E2 slot or M2_4 slot.
**M2_4 slot will be unavailable when installing device in the PCI_E2 slot.

 

[igor_kavinski]

How do I avoid any performance drops (If any) using these parts?

Simple.

Avoid Gigabyte since they are too cheap to give you full bandwidth.

Try looking at ASROCK or MSI mobos.

 

[BoomerD]

Simple.

Avoid Gigabyte since they are too cheap to give you full bandwidth.

Try looking at ASROCK or MSI mobos.

See my post above. It looks like even MSI boards do the shared bandwidth thing when both PCI-E and M.2 slots are Gen.5.

ASRock says the same thing:

If M2_5 is occupied, PCIE1 will downgrade to x8 mode.

 

[igor_kavinski]

See my post above. It looks like even MSI boards do the shared bandwidth thing when both PCI-E and M.2 slots are Gen.5.

ASRock says the same thing:

There's a difference though. On ASROCK mobos (mine included and the one you also mention), the sharing speed downgrade happens when you use a 4th or 5th M.2 slot. If you just use the primary slot like many will do, there is no slowdown. So if the Gigabyte mobo provides slots that don't cause the slowdown, that's well and good. But if they provide only one PCIe 5.0 slot and you have to choose between that or having x8 PCIe graphics, that's kinda lame.

 

[BoomerD]

There's a difference though. On ASROCK mobos (mine included and the one you also mention), the sharing speed downgrade happens when you use a 4th or 5th M.2 slot. If you just use the primary slot like many will do, there is no slowdown. So if the Gigabyte mobo provides slots that don't cause the slowdown, that's well and good. But if they provide only one PCIe 5.0 slot and you have to choose between that or having x8 PCIe graphics, that's kinda lame.

I think most boards only offer one PCI-E 5.0 M.2 slot...the rest are 4.0 or even 3.0. It just depends on which one the manufacturer has set as 5.0 (from the CPU)

Even MSI's most powerful Z790 board, the "Godlike" only has one M.2 5.0 slot...and shares bandwidth.

From the spec sheet:

7x M.2
M.2_1 Source (From CPU) supports up to PCIe 4.0 x4 , supports 22110/2280/2260 devices
M.2_2 Source (From Chipset) supports up to PCIe 4.0 x4 , supports 2280/2260 devices
M.2_3 Source (From Chipset) supports up to PCIe 4.0 x4 / SATA mode, supports 2280/2260 devices
M.2_4 Source (From CPU) supports up to PCIe 5.0 x4 , supports 22110/2280 devices
M.2_5 Source (From Chipset) supports up to PCIe 3.0 x4 / SATA mode, supports 2280/2260 devices
M.2_6 Source (From Chipset) supports up to PCIe 4.0 x4 , supports 2280/2260 devices
M.2_7 Source (From Chipset) supports up to PCIe 4.0 x4 , supports 2280/2260 devices
6x SATA 6G

*PCI_E2 slot will be unavailable when installing M.2 SSD in the M2_4 slot.
**SATA5~8 will be unavailable when installing M.2 PCIe SSD in the M2_5 slot.
***SATA5~8 will be unavailable when installing M.2 SATA SSD in the M2_5 slot.

And from the user manual:

PCI_E1 slot will run at x8 speed with when installing device in the PCI_E2 slot or M2_4 slot. ∙ PCI_E2 slot will be unavailable when installing M.2 SSD in the M2_4 slot.

The Z690 Godlike is also limited with bandwidth sharing:

• 3x PCIe x16 slots
  • Supports x16/x0/x4, x8/x8/x4
  • PCI_E1 supports PCIe 5.0 x16 (From CPU) 1
  • PCI_E2 supports PCIe 5.0 x8 (From CPU)2
  • PCI_E3 supports PCIe 4.0 x4 (From Z690 chipset)2

1. PCI_E1 will run at x8 speed when install M.2 PCIe SSD in the M2_3 slot
2. M2_3 will be unavailable when installing device in the PCI_E2 slot
3. PCI_E3 will be unavailable when installing M.2 SATA SSD in the M2_4 slot. PCI_E3 will run at x1 speed and M2_4 will run at x2 speed when installing PCIe device in PCI_E3 slot and M.2 PCIe SSD in M2_4 slot simultaneously

 

[igor_kavinski]

Even MSI's most powerful Z790 board, the "Godlike" only has one M.2 5.0 slot...and shares bandwidth.

I rechecked the specs for my mobo. You are right. PCIe 5.0 for both NVMe SSD and x16 PEG doesn't seem to be available simultaneously. PEG downgrades to x8 with PCIe 5.0 NVMe installed.

 

[mv2devnull]

If you want simultaneous PCIe 5.0 x16 and PCIe 5.0 x4, then you do have to get PCIe 5.0 x20 from somewhere.
If the CPU has only PCIe 5.0 x16 to offer, then where would the remaining PCIe 5.0 x4 come from?
From chipset that will share the CPU-chipset connection with other traffic too?

For many PCIe generations CPU's have had x16 to offer and motherboards typically did connect two
x16 slots to that, so you could either have one x16 card or two x8 cards. The new trend is to
connect one x16 slot and one(?) M.2 slot to those 16 lanes. (And AMD has more than x16 on the CPU?)

 

[Hail The Brain Slug]

If you want simultaneous PCIe 5.0 x16 and PCIe 5.0 x4, then you do have to get PCIe 5.0 x20 from somewhere.
If the CPU has only PCIe 5.0 x16 to offer, then where would the remaining PCIe 5.0 x4 come from?
From chipset that will share the CPU-chipset connection with other traffic too?

For many PCIe generations CPU's have had x16 to offer and motherboards typically did connect two
x16 slots to that, so you could either have one x16 card or two x8 cards. The new trend is to
connect one x16 slot and one(?) M.2 slot to those 16 lanes. (And AMD has more than x16 on the CPU?)

The new standard has been for the CPU to offer 20 lanes, so you have your 16x pcie slot lanes and 4x for one direct CPU attached M.2.

The problem here is that the CPU only offers 16x 5.0 lanes, and the other 4 are pcie 4.0 for the direct attached M.2. The board manufacturers decided to wire it up so that an M.2 slot can rob some lanes from the slots to offer a pcie 5.0 m.2.

And AMD's AM5 platform offers 24 pcie 5.0 lanes, so most boards have 16x for the slots and two direct cpu attached 5.0 m.2 slots.

 

[blckgrffn]

OK @Fakum I am going off script here, but hear me out.

Your 2080ti is a very powerful card, even though your monitor resolution is punishing. I think your lowest frame rates are likely CPU related, however and it would be interesting to see if paired with something massively more modern if the gaming experience would be much more pleasing.

Buy a 5800X 3D, a B550 or X570 board that is solid and you are out maybe $500 after tax. Then proceed to reuse everything. Ram, SSD, don't even have to re-install windows likely, just need to re-activate with your key. Or you could buy an NVME, clone your SATA drive to it and keep that to the side, many options there. Buy a $35 Thermalright cooler of your choice for the CPU or get the right bracket (maybe you already have it) for your AIO.

The tuning on the 5800X 3D is extremely minimal, turn on XMP for the ram and walk away.

But the 5800X 3D is a very solid CPU, especially for gaming and especially for simulation games. I think you might be pleasantly surprised to realize so much gains at a fraction of a price of the full rebuild.

And if you wanted to, you could justifiably pair a 4090 with the 5800X3D if you discovered the 2080ti is quite doing it for you. That CPU is really good at maintaining high minimum framerates even though you will likely lose some of the max frame rate capability. The B550 and X570 platforms are also fully PCIe 4 ready so you can get the best storage currently available and there would be zero bottleneck on a card like the 4090 which is also PCIe 4.