290x PCIe 8x bottleneck?

[CustardInc]

I just finished my build in a prodigy m and I had to install the GPU in the second compatible PCIe slot (due to PSU and GPU length isues) which turns out to be an 8x slot. I've been noticing considerably lower framerates, I'm wondering if this is a limitation of 8x or if there's something wrong with my GPU?

[MrJim]

I just finished my build in a prodigy m and I had to install the GPU in the second compatible PCIe slot (due to PSU and GPU length isues) which turns out to be an 8x slot. I've been noticing considerably lower framerates, I'm wondering if this is a limitation of 8x or if there's something wrong with my GPU?

I could be wrong, but I thought that on most motherboards, the second 16x PCIe slot would only revert to 8x when a 16x card is installed in the first slot? Either way, if your motherboard is running PCIe 3, 8x shouldn't impact too greatly on performance.

[CustardInc]

Well the slot does physically have fewer pins in it (looked roughly half), but I was also under the impression that it wouldn't make much of a difference, I just can't think of another reason for the performance drop.

[herulach]

All all the other components the same? Assuming you're overclocking have you made sure your settings have followed over?

[CustardInc]

The only different thing is the case, no thermal issues there.

[Brian224]

Are you using an add-on card in one of the x1 slots? The second x16 slot may share bandwidth with these in which case it might slow down to x1 mode.

[CustardInc]

https://uk.msi.com/product/mb/Z87M_GAMING.html#overview

That's the mobo, I've got a wifi card in the top 1x slot, but like I said, it physically has fewer pins.

[Brian224]

Z87 should have more than enough PCIe capacity, so I doubt that the slot is going down to 1x (although I might be tempted to test with the wifi card removed anyway).

Looking at the manual, I noticed that the PCI express support for the slots can be changed individually in the PCI Subsystem Settings section of the BIOS. It wouldn't have mattered what the setting for this slot was until you rebuilt the machine, but might be worth checking now?

[CustardInc]

I did take a look in the BIOS, I can change PCI generations (I think) but not 16/8x. The hardware monitor in the BIOS says it's a 16x slot running in 8x mode, their website says it has 2 16x slots, this picture shows the two slots and that one with the GPU in has fewer pins... http://i.imgur.com/96zXD0C.png this can't really be a manufacturing mistake, can it?

edit: but the one with missing pins seems to have a lone pin on the empty end... O.o

[MrJim]

Although it's physically a 16x, it may be electrically only 8x. Here's a diagram showing the Z97 Chipset features:



According to the diagram, even if you're running the second slot in 8x PCIe 3 mode, you should still have plenty of lanes for other cards...

[AGTDenton]

With a single card and a single monitor at 1080p res you will notice no difference, there's a number of reviews where they recorded something like 1fps difference. Its more important on multi monitors or scarily high resolutions.

I run my 770 card at 8x and its fine, this is due to the way I have positioned my CPU HSF which overlaps the top x16 slot and I'm too lazy to reposition all the other cards just to get 16x.

[CAT-THE-FIFTH]

You should be fine IMHO. I ran my massively overclocked HD5850 1GB in a PCI-E 1.0 8X slot for years,and that was at a time when PCI-E 2.0 16X was common. Benchmark scores were in the same ballpark for systems using PCI-E 2.0 16X too.

[CustardInc]

Back on 16x - back to original framerate

[LtMatt]

You should be fine IMHO. I ran my massively overclocked HD5850 1GB in a PCI-E 1.0 8X slot for years,and that was at a time when PCI-E 2.0 16X was common. Benchmark scores were in the same ballpark for systems using PCI-E 2.0 16X too.

Hi Cat.

I agree, x8 is unlikely to be the source of your problems and it is plenty fast enough for all but the highest resolutions.

[scaryjim]

Back on 16x - back to original framerate

Strongly suggests that the second slot wasn't actually running at x8. x8 PCIe 3 is the same bandwidth as X16 PCIe 2.0, and should in no way affect your framerates, certainly not to the extent you suggest has happened. It sounds to me like something unusual is happening - perhaps that second slot is running x8 PCIe 2.0 from the chipset (rather than from the CPU), or it's negotiating down to a lower set of lanes for some reason. I can't find anything concrete from MSI or from any reviews to confirm, sadly.