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How many PCIe lanes do I need?

Quartz

Quartz

Quartz

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Or, Rocket Lake vs Ryzen vs Ice Lake Xeon vs Threadripper.

Consumer Ryzen and Rocket Lake motherboards come with 24 PCIe lanes: 16 for the GPU, 4 for the NVME, and 4 for everything else, multiplexed out to 16 or so. Threadripper and Xeon (Ice lake) boards have 64 or more natively.

But if you add in anything extra either it cuts into the 16 GPU lanes, thus reducing the GPU's performance (marfginally right now, but what of the future?), or uses some of the multiplexed 4 lanes, meaning it doesn't operate at full speed. Consider a USB video capture to a multiplexed NVME drive. So how many PCIe lanes should a consumer CPU supply? I think an extra 8 for one or two add-in cards and an extra 4 for a second NVME drive seems about right, for 36 total. How about you?
 
Don't forget higher power consumption.
If AM5, or LGA 1700 were to come with more lanes that would be an extra expense added to everything. Eventually driving down the monetary cost of doing so, but maybe not the power usage.
PCIe 5.0 makes far more sense as at the cost of some latency it can be split.
That's always assuming that signal quality for PCIe 5.0 doesn't require twice as expensive track layouts.
 
Quartz said:
I don't particularly care about that as it's going to be modest and I'm not sure it's of particular concern to most, either.
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Yes, but my point was that if it carries into every place the new socket gets sold to, then there will be places where it does matter.
Sure laptop gets different dies (Renoir only had PCIe 3.0, Cezanne is rumoured to be the same), but the total power budget which the unicore already takes up in some mainstream desktop processors is significant.
 
Quartz said:
Exactly how much extra power is providing those extra PCIe lanes going to require? 1W? 2? I don't know but I'll bet it's trivial.
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I guess if it is properly power-gated, very little.
And signal integrity shouldn't be too expensive (for PCIe 4.0 at least) now that the motherboard manufacturers have had the experience over the last year+.
Entry class boards (A520 etc.) would just drop support for the extra stuff.
So maybe my concern is unfounded.
 
I think Ryzen should have had 32 lanes for desktop from the start. That would have been enough for 99.9% of users. 16->GPU, 8->Chipset and (8)-> 2 * m.2 ports. 24 means compromising, I have to run my second m.2 in gen 2 (off the chipset) or buy new hardware, a lot of mobo's cannot run more than 1 m.2 without dropping SATA ports or other things.
 
Quartz said:
Or, Rocket Lake vs Ryzen vs Ice Lake Xeon vs Threadripper.

Consumer Ryzen and Rocket Lake motherboards come with 24 PCIe lanes: 16 for the GPU, 4 for the NVME, and 4 for everything else, multiplexed out to 16 or so. Threadripper and Xeon (Ice lake) boards have 64 or more natively.
Click to expand...

Rocket lake CPUs have 28 lanes. 8 being for the DMI, used for communication with the chipset. Albeit limited to gen 3 speeds.
 
KompuKare said:
Don't forget higher power consumption.
If AM5, or LGA 1700 were to come with more lanes that would be an extra expense added to everything. Eventually driving down the monetary cost of doing so, but maybe not the power usage.
PCIe 5.0 makes far more sense as at the cost of some latency it can be split.
That's always assuming that signal quality for PCIe 5.0 doesn't require twice as expensive track layouts.
Click to expand...

a gen 5 slot would need a gen 5 device plugged into it to make use of all the bandwidth. I don't think we will see those for a few years given that gen 4 capable GPUs have just come on to the market.
splitting it down doesn't help either because of that requirement for device compatibility.
 
Annoys me - another reason I've not hurried on from my X79 platform with 40 PCI-e lanes from the CPU and additional 8 from the chipset.
 
I will love some more pci express connectivity
I dislike having to check the manual to see what shares connectivity
I was researching PLX switches and they are so expensive
 
Rroff said:
Annoys me - another reason I've not hurried on from my X79 platform with 40 PCI-e lanes from the CPU and additional 8 from the chipset.
Click to expand...

Budget and core requirements depending, a Threadripper Pro 3955WX (16c/32t) would give 128 pcie lanes. Cost would be around £1,800 for CPU and mobo (the WRX80 boards aren't cheap but are excellent). Stepping down (a bit) to Threadripper non-pro 3960X, which I have, gives 'just' 64 lanes. The CPU is a smidge more than the Pro ironically, due to its higher core count, but it's offset by being able to buy a TRX40 mobo for half the price of a WRX80 one (total £1,700).

With only £100 in it I'd want the Pro personally, as 16 cores is enough for me. If it'd been out when I ordered that's where my money would have gone, plus the 'WEPYC' (Workstation Epyc) is a nicer platform imo. With the next Threadripper CPUs due out 'soon' (Q4 iirc) it's obviously likely worthwhile to wait and either get the next gen based on Milan/Ryzen 5000 or save a chunk of cash on the existing SKUs when they inevitably fall in price.
 
I'm looking at the Intel page for the Ice Lake Xeons and I'm not seeing any single-CPU chips. Nor is Google turning up any workstation motherboards. I hope they'll release workstation Xeons sometime soon.
 
20+4 is more than enough for a consumer CPU, especially as running even an RTX 3090 at 8x 4.0 PCI-E has no impact at all for people playing games, and by the time a card come around that does the system will likely be due for replacement as the CPU will end up being the bottleneck.
So you've got 4x 4.0 lanes (dedicated) for one NVMe drive, 8x 4.0 lanes (flexible) for 'other' cards/drives and then finally the 4x 4.0 lanes (flexible) from the chipset (x570) left to work with.

This is why for more workstation-y AM4 builds my default go to non-commercial board is the ASUS Pro WS X570-ACE, as it offers wonderful flexibility with the lane layout.
 
Journey said:
especially as running even an RTX 3090 at 8x 4.0 PCI-E has no impact at all for people playing games,
Click to expand...

The problem comes when you have direct GPU-NVME access as in the PS5. Which is coming soon. Suddenly those 8 lanes get reduced to 4. 16 lanes down to 12 isn't an issue; 8 lanes down to 4? I think that that's going to be an issue, even for PCIe v4. Roll on PCIe v5!
 
Quartz said:
The problem comes when you have direct GPU-NVME access as in the PS5. Which is coming soon. Suddenly those 8 lanes get reduced to 4. 16 lanes down to 12 isn't an issue; 8 lanes down to 4? I think that that's going to be an issue, even for PCIe v4. Roll on PCIe v5!
Click to expand...

That isn't how it works, it just skips the CPU, the NVMe, and GPU still call to main system RAM, you lose no lanes.
 
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