What does PCIe 2.0 x16 (x4 mode) mean?

[threadysparrow]

I know what PCIe 2.0 is, but what is the (x4 mode)?

[rmonty]

x4 mode is when the socket is running in a physical 16x (full length slot), but is electrically wired to be a x4 slot.

In other words it will run at a hypothetical 1/4 the speed of a true 16x slot.

[DaftBehemoth]

It means the PCI-E lane uses a quarter of the full bandwidth of a PCI-E lane. The full length of bandwidth for the slot would be x16. There's also an x8 spec that runs half the bandwidth. 

[threadysparrow]

x4 mode is when the socket is running in a physical 16x (full length slot), but is electrically wired to be a x4 slot.

In other words it will run at a hypothetical 1/4 the speed of a true 16x slot.

so no crossfire? shit...

[target39]

I know what PCIe 2.0 is, but what is the (x4 mode)?

It means it will only run at 1/4 bandwidth of x16. If it's a PCIe 2.0 x4 link, that means the expansion card can only communicate to the board with 2GB/s bandwidth.

[target39]

so no crossfire? shit...

CrossfireX can run at x4, it just won't perform as good as x16/x16 crossfire.

[DaftBehemoth]

so no crossfire? shit...

You can still run crossfire, it just won't be optimal. The difference between 8x and 16x are negligible, and I would imagine that 4x isn't horrible. I never recommend running a card in less than an 8x PCI-E 2.0 slot though

[AMICLG]

CrossfireX can run at x4, it just won't perform as good as x16/x16 crossfire.

You don't need x16/x16 most modern cards don't even use 2.0 x8's full bandwidth.

[target39]

You don't need x16/x16 most modern cards don't even use 2.0 x8's full bandwidth.

If you use a graphics card that won't take a hit with PCIe 2.0 x4 (2GB/S), it may be time for a GPU upgrade. Even SATA II is 50% faster. Even with PCIe 2.0 x8, you only get 4GB/S bandwidth.

[AMICLG]

If you use a graphics card that won't take a hit with PCIe 2.0 x4 (2GB/S), it may be time for a GPU upgrade. Even SATA II is 50% faster. Even with PCIe 2.0 x8, you only get 4GB/S bandwidth.

A lot of mid level cards are only doing 3.85 gb/s so if your just getting into pc gaming this isn't a problem unless you want to upgrade .

[Emperor_Piehead]

You can still run crossfire, it just won't be optimal. The difference between 8x and 16x are negligible, and I would imagine that 4x isn't horrible. I never recommend running a card in less than an 8x PCI-E 2.0 slot though

The difference between x16 and x4 if you have a good gpu is around 10fps

[AMICLG]

The difference between x16 and x4 if you have a good gpu is around 10fps

I wasn't talking about x4 but x8.

[Glenwing]

If you use a graphics card that won't take a hit with PCIe 2.0 x4 (2GB/S), it may be time for a GPU upgrade. Even SATA II is 50% faster. Even with PCIe 2.0 x8, you only get 4GB/S bandwidth.

 

2GBps is over 3x the speed of SATA III.  SATA II gets 300MBps (0.3GBps).

 

A Titan might be bottlenecked by PCIe 2.0 x4.

[Emperor_Piehead]

I wasn't talking about x4 but x8.

you said you wouldn't recommend below a 2.0x8 slot and 10fps would be the performance difference in going below that.

[threadysparrow]

 

2GBps is over 3x the speed of SATA III.  SATA II gets 300MBps (0.3GBps).

 

A Titan might be bottlenecked by PCIe 2.0 x4.

I just have a 7870 and I wanted to get another one so I can use 3 monitors.

[AMICLG]

you said you wouldn't recommend below a 2.0x8 slot and 10fps would be the performance difference in going below that.

Well if you run below 2.0 x8 the gpu will start to bottleneck things that use a lot of bandwidth like shaders etc..

[SkilledRebuilds]

I have personally experienced Generation 1 PCIE 1.0&2.0 in 1x and BF3 Maxed is 50-80fps but cripples down to 28fps for long periods on a 7970.

PCIE 1.1 and 2.0 1x  - Hawken is the killer, 45-70fps average down to 3-12 fps. Limited for sure.

 

Since going through that experience I had a deeper look into the limitations of what type of slot was needed to keep about 90% of the cards power.

Just a theoretical number but 90% or higher most people wouldnt notice in a crossfire situation.

 

If you had crossfire 16x / 16x of two cards and had 100fps (being the baseline 100% for comparison)

 

Crossfire 16x / 4x would see you lose about 10fps from the second card, however as stated in the above post, shaders take a small hit on the second gpu as well.

But realistically you will not notice it with your eyes, only on benchmarks comparing against the baseline.

Look up PCIE Scaling using 3770k and a 7970/680
PCIE 2.0 4x 7970 still at 96%,