Can An Overclocked GTX 690 be bottlenecked by PCIe 3.0 X8 or PCIe 2.0 X16 ?
We tested using Core i7-3770K at 4.6GHz and overclocked the GTX 690 that was used for this week’s complete platform upgrade. We cannot give you an exact answer to this question although we do have some interesting data. The GTX 690 is the fastest video card in the world and it is ideal for exploring potential bandwidth limitations using our ECS Z77H2-A2X Golden Series Motherboard.
We used even more demanding settings than in our platform upgrade evaluation as we overclocked our GTX 690 as far as it could go, to +132%, +150MHz core, and +550MHz memory over reference clocks. Our Core i7-3770K is also overclocked to 4.6GHz, and our 4GB of Kingston HyperX DDR3 is at 1866MHz.
One of Ivy Bridge’s advantages over older CPU architecture is PCIe 3.0, and there are 16 total lanes of PCIe 2.0 bandwidth available for graphics cards. PCIe 3.0 is an improvement over PCIe 2.0, including more efficient signaling. According to the Wikipedia, “PCIe 2.0 delivers 5 GT/s, but uses an 8b/10b encoding scheme that results in a 20 percent ((10-8)/10) overhead on the raw bit rate. PCIe 3.0 removes the requirement for 8b/10b encoding.”
It is considered that PCIe 2.0 16x is approximately equal to PCIe 3.0 8x bandwidth. Well the GTX 690 is the single fastest video card in the world and this experiment is to see if it can use more than the bandwidth that 8x PCIe 3.0 can provide it. Our benchmarks might also relate to how an overclocked GTX 690 might perform perform in the x16 slot in an older PCIe 2.0 motherboard, but don’t read too much into them without further testing.
In the case of the ECS motherboard, the primary slot is X16 – it uses nearly the full bandwidth available to it unless there is a second video card installed in the second slot. In this case, both cards share the bandwith and each one has the bandwidth of 8x PCIe 3.0 available. However, the secondary slot is X16_S, which means it is X8 PCIe 3.0 bandwidth whether there is one video card in the other primary slot or not. So it was an easy test to simply move our overclocked GTX 690 from the primary (X16) slot to the secondary (X8) slot and record the performance in the benchmarks.
Well, we were surprised!
It appears that in every case, using the top PCIe 3.0 X16 slot in the ECS Z77H2-A2X motherboard, is faster than using the second X16_S slot. This is about as far as we can take our testing with this motherboard as there is no other video card other than the GTX 690 which can do this. 
Fortunately we received an EVGA Z77 FTW motherboard for evaluation and we will continue to explore PCIe scaling in a follow-up. In the meantime, feel free to discuss this on our forum or leave a comment below. New this week, ABT will bring you the best of the tech news on a regular basis and you can expect a Thermaltake Overseer RX-I full tower evaluation this weekend.
Happy gaming!
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