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CPU PCIe Lanes

1dmf

1dmf

1dmf

Associate
Joined
30 Nov 2006
Posts
468
Hi,

What does the PCI lanes on a CPU mean?

Haswell-E Comparison

Does this have anything to do with SLI and x16 Dual vs x8 Dual?

16 x 2 = 32 which would imply you need the 40 lanes to get true dual x16 SLI.

Is this controlled by the Mother Board or the CPU or both.

A little confused and would appreciate your guidance.

Thanks,
1DMF
 
PCIE devices require lanes to work. Think about it as traffic on a highway, the more lanes you have the more easily the traffic flows through. Restrict those lanes and you restrict the traffic (or data) through those lanes. It's the same with dual/triple/quad channel memory.

Thankfully most of it is "Gamer's BS (tm)" and those lanes are far harder to fully congest than people claim.

However.

On certain Intel CPUs the lanes are quite heavily restricted. A modern GPU likes to run at X16, well the 5820k for example only has 24 lanes. So if you ran two graphics cards you could only run them X16 X8 because there are only 24 lanes in total available to the GPUs.

This is fine of course. X16 X8 has hardly any deficit over X16 X16 but most modern boards carry a mini PCIE slot which can take, for example, an SSD and that SSD will want at least four lanes.

So then you are left with X16 X4 which is far from ideal, and your performance will start to suffer.

It's OK if you know what system you want and have it all planned and don't plan on adding to it later, but things get more murky when it comes to future upgrades. Your graphics subsystem will suffer if you add too much hardware.
 
ALXAndy said:
So then you are left with X16 X4 which is far from ideal, and your performance will start to suffer.
Click to expand...

I imagine it would move to 8x/8x rather than 16x/4x also pci-e slots have been shown time and again to rarely if at all change performance untill you get to silly low slot numbers especially as it's version 3.
 
Thanks for clarifying, I thought I'd got a handle on it, but wasn't sure.

So true dual x16 requires the 40 lanes CPU.

As you say the 5820k only has 28, which seems a bit of kick in the teeth for Haswell-E when the 40 lane CPU's are so expensive in comparison.

Your comment on SSD has me intrigued, so even though the form factor for SSD is SATA III 6GB, it still required some of the PCI lanes?

Or am I confusing connectors?

I am considering this SSD : Samsung 512GB

I take it this doesn't require any PCIe lanes to function?
 
Tythus said:
I imagine it would move to 8x/8x rather than 16x/4x also pci-e slots have been shown time and again to rarely if at all change performance untill you get to silly low slot numbers especially as it's version 3.
Click to expand...

It will run X16 X4. I know, I've tried it. My old rig had a pretty badly spaced board and to run my sound card I had to put the second GPU in the very bottom slot and hack SLI to run, and it ran at X16 X4. It will basically allocate all of the bandwidth it can.
 
1dmf said:
Thanks for clarifying, I thought I'd got a handle on it, but wasn't sure.

So true dual x16 requires the 40 lanes CPU.

As you say the 5820k only has 28, which seems a bit of kick in the teeth for Haswell-E when the 40 lane CPU's are so expensive in comparison.

Your comment on SSD has me intrigued, so even though the form factor for SSD is SATA III 6GB, it still required some of the PCI lanes?

Or am I confusing connectors?

I am considering this SSD : Samsung 512GB

I take it this doesn't require any PCIe lanes to function?
Click to expand...

The SATA controller should not eat lanes, no.

I'm talking about the mini PCIE slot that many boards now have. They support things like WIFI adapters, SSDs and so on.

Another example are the OCZ Revodrive PCIE SSD cards. They have more than one SSD on them and a raid controller. Some cards have four SSDs on and two RAID controllers and so on.

They require an X4 slot, and will not operate on less. This is because they bypass the limits of SATA controllers and thus need a lot of bandwidth.

So if you had a rig with SLI and a Revodrive like I do you would struggle.

5820k is fine if you want to live with the basics. Not that it's worth the silicon it's made of due to disgusting DDR4 prices.

Intel love locking cheap things. It's something they stopped doing for a while but losing a factory obviously made them blame their audience rather than themselves so they're fresh with the padlock again :rolleyes:
 
Yes, just been doing some homework and apparently it is SATA Express ( M.2 form factor).

Toms Hardware

So now I'm thinking If I want to break 6GBS I need this form factor not that Samsung drive and look at the 5930K so I can run true dual SLI x16.

Got any advice on this?
 
1dmf said:
Yes, just been doing some homework and apparently it is SATA Express ( M.2 form factor).

Toms Hardware

So now I'm thinking If I want to break 6GBS I need this form factor not that Samsung drive and look at the 5930K so I can run true dual SLI x16.

Got any advice on this?
Click to expand...

If you're buidling a serious rig then you need a serious CPU.

The 5820k is a cut down CPU aimed at gamers building a regular sort of rig with just a single GPU and that's about it.

For video editing and so on yes, you either want a 3930k, 4930k or 5930k. I mention all three because there's absolutely nothing in it. Yes, Haswell is faster, but it goes like this..

Sandy. The slowest of the lot, but, she clocks well. I've had 4.9ghz out of my 3970x which I paid just over £300 for.

Ivy. Clocks slightly lower, but IPC make it about even.

Haswell. Clocks even lower, but again, IPC makes it about even. Anand concluded that if you have a 3930k @ around 4.8ghz there is no point buying anything else as you'll gain nothing.

So, if you want to save a few beans then keep an eye out for a second hand hex on Sandy or later, as DDR3 is literally less than half the price of DDR4. I paid £160 for 16gb of Dom Plats @ 1866 and I'm sure they have some clocking room in them.

X79 boards are going for a song. I've seen very good ones fetching £60.
 
Yes, I have been chewing the fat in the overclocking forum regarding my current Q6600 and replacing the CPU with QX9650, where I quickly got shot down over!

Of course they all recommended i5 2500k or an i7 on 1150 socket.

But I've decided like I did when I built my Q6600 rig 8 years ago, to go for it.

So I am prepared to pay the premium as I probably won't upgrade again for another 8 years!

The missus offered to buy me the 5960X , but reading a lot of reviews this actually performs worse in a gaming environment than the other two Haswell-E, but hey games will eventually utilise the 8 cores and then I'm sure it'll be a different story.

But as you say DDR4 is expensive , So I think I'd settle for the 5930K so I can spend the £300 on 16GB 2800mhz DDR4 ram. Heck I spent £250.00 on 4GB of DDR2 ram 8 years ago!

I can always upgrade the CPU later when prices come down.

Only now having spoken with you, the build price has just gone up as I didn't know about this PCIe Express / x4 / M.2 SSD's stuff, which I am now going to have to get as they seem so much faster than standard 6GBS SSD.

So thanks for that ;)
 
1dmf said:
Hi,

What does the PCI lanes on a CPU mean?

Haswell-E Comparison

Does this have anything to do with SLI and x16 Dual vs x8 Dual?

16 x 2 = 32 which would imply you need the 40 lanes to get true dual x16 SLI.

Is this controlled by the Mother Board or the CPU or both.

A little confused and would appreciate your guidance.

Thanks,
1DMF
Click to expand...
yes for 2x 16 (full sli) u need haswell-e
However theres almost none performance drop if u use 2x8...~1%
 
Most SATA/USB functions run off the southbridge which has its own separate DMI link from the CPU, though you'd have to look at the specifications of each the specific motherboard to see if they have anything running off the CPU lanes.

The 5820K is absolutely fine unless you want to run with quad crossfire/SLI, ALXAndy has had an agenda against Haswell-E and specifically the 'value' 5820K since well before it was released so I would take his opinion about it with a grain of salt, he just seems to be upset that people can now get a £300 CPU that's better than his £800 one and resents anyone doing so.
 
Last edited:
mmj_uk said:
Most SATA/USB functions run off the southbridge which has its own separate DMI link from the CPU, though you'd have to look at the specifications of each the specific motherboard to see if they have anything running off the CPU lanes.

The 5820K is absolutely fine unless you want to run with quad crossfire/SLI, ALXAndy has had an agenda against Haswell-E and specifically the 'value' 5820K since well before it was released so I would take his opinion about it with a grain of salt, he just seems to be upset that people can now get a £300 CPU that's better than his £800 one and resents anyone doing so.
Click to expand...

I have an agenda against ram that costs over double its predecessor yes.

I guess you missed all of the teething issues people were getting too? I was right. Totally right.

As for a £300 cpu being better than my £300 CPU?

Don't think so. Not seen any 5820ks going near 4.9ghz and thus they won't be any faster, and for lanes and so on they're completely inferior.

You keep your RS Turbo and I will keep my Ferrari thanks.
 
Well I have decided to get the 5930K for the 40 lanes so I can run dual x16 and PCIe SSD.

I just am unsure which one of these I should get?

G.SKILL 480GB SSD Phoenix Blade - which I believe is a PCIe 2.0 x8
Intel P3600 400GB SSD - PCIe 3.0 x4
Samsung 512GB XP941 SSD - M.2 (PCIe 3.0 x4)

I have a heat concerns over the XP941 being flat against the motherboard and am unsure if the Intel out performs the G.Skill

Would the PCIe 3.0 x4 be better than PCIe 2.0 x8 because that would be all 40 lanes used up wouldn't it?

Do you have any advice on these 3 SSD's ?

Thanks,
1DMF
 
Yes I have seen that there are PCIe adaptors with heatsink for M.2 SSD drives and it is definitely a consideration.

I did email OCUK sales support and they said they don't see any heat issues with the M.2 in any builds they have used them in.

The dilemma is now choosing which one to go for, I have the G.Skills in my shopping cart, but still can't quite decide so have yet to place order!

The XP941 is a lot cheaper and with the adaptor it should eliminate heat concerns, and assume it doesn't impede performance in any way?

I can't seem to find any XP941 vs Phoenix Blade reviews / benchmarks so only have the specs to go by which does put the G.Skills ahead, but would be nice to see some real world comparisons to help me decide if the £200 extra is worth it?
 
1dmf said:
Well I have decided to get the 5930K for the 40 lanes so I can run dual x16 and PCIe SSD.

I just am unsure which one of these I should get?

G.SKILL 480GB SSD Phoenix Blade - which I believe is a PCIe 2.0 x8
Intel P3600 400GB SSD - PCIe 3.0 x4
Samsung 512GB XP941 SSD - M.2 (PCIe 3.0 x4)

I have a heat concerns over the XP941 being flat against the motherboard and am unsure if the Intel out performs the G.Skill

Would the PCIe 3.0 x4 be better than PCIe 2.0 x8 because that would be all 40 lanes used up wouldn't it?

Do you have any advice on these 3 SSD's ?

Thanks,
1DMF
Click to expand...

PCIE 3.0 offers more bandwidth for the lanes. Again, think of it as traffic.

Actually saturating a PCIE lane is very difficult, but I've had a go at it with my Titan Black SLI rig and it was definitely possible.

However, please bear in mind I was running five overlaid lots of data at once. So think of running five games at once, all on top of one another.

Great choice on the SSDs. Personally I would go with the G.Skill because it looks the best :D But PCIE SSDs really do break the barriers put up by SATA ports.
 
Thanks Andy, appreciate your input.

I have gone for the G Skill, having done in-depth research it seems all these M.2 SSD's run ridiculously hot to the point they throttle themselves and performance drops considerably.

XP941 vs M6e

In the above tests between the Samsung and the Plextor, they both run really hot and that was with the PCIe x4 adapter & heatsink, and although the Samsung performs better it also runs much hotter.

Plus trying to find a M2P4A - Bplus adaptor is near impossible unless I want to pay over £50.00 and get it from USA or Australia.

When I way it all up, the G Skill although expensive looks to be the better option.

I think I'll be glad I didn't go down the XP941 route in the M.2 slot on the motherboard, it would have melted something I'm sure!
 
I've also been pondering getting a 5930K or 5960X but the current DDR4 prices are sickening and there isn't much competition on the motherboard front. So I've decided to upgrade my 2600K to a 4790K and keep my current RAM. I've bought a GByte board that I can upgrade FOC by the time DDR4 is more competitively priced and Broadwell is released.
 
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