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Truth about x16 and x8 speeds

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youra6

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3) Now, this is something that many people around here are not aware of, but PCI-Ex bandwidth is primarily important when your graphics card(s) do not have enough onboard memory as is required for the particular test at hand. If you run a test (say you run Crysis @1920x1200, 2xAA, with Very High texture quality) that uses 750MB of vram, and your cards have only 512MB of vram, this will cause your cards to 'farm out' the difference in memory usage to the system memory. To do this, large amounts of data must pass very quickly over the PCI-Ex bus. This is the scenario where the PCI-Ex bandwidth comes into play heavily. Otherwise, it really isn't terribly important.
Click to expand...

This is written by a guy on another overclocking forum. Now, I have actually never heard of this before. I always thought faster cards took more advantage of faster bandwidth. It's just something interesting I found.

He also says:
Many people think this question come down entirely to 'better card' = 'higher bandwidth requirement', but it's actually not that simple. In fact, it's actually kinda 'wrong', some of the time.
Click to expand...

Any comments on this guy's explanation?

Thanks
 
It's undeniable that a faster card requires more bandwidth. However, when you consider that a 5870 (fastest single GPU on the market at the moment) loses a mere 1-2% going from 16x to 8x, it's clearly not as important as some people think.

Like he said if the GPU doesn't have enough memory then PCIe width is tremendously important, though you don't have any business trying to do a >512mb setting on 512mb of ram in the first place IMO.
 
Like 99% of people, even here, know what at >512MB setting is though. Most just crank it up blindly.
 
EarthDog said:
A link to the information you provided would be nice... ;)
Click to expand...

He never wrote down a source of where he got his info from. I do have the link of the the original post

Am i allowed to post links of other forums? I recently got yelled at by a overclock.net mod for linking to other sites... stupid if you ask me
 
ROFLMAO! Yup! Good call! :)
__________


No big on the links man.. thought it wasnt a forum user but someone/thing more reliable. But as you can see, that is at least supported by some other forum user. :)
 
so when it comes down to it, do you guys think that "its wrong some of the time?"

Agree or disagree?

Many people think this question come down entirely to 'better card' = 'higher bandwidth requirement', but it's actually not that simple. In fact, it's actually kinda 'wrong', some of the time.
Click to expand...
 
I do indeed disagree.
A faster card will always require more bandwidth then a slower card, assuming you are comparing apples to apples.
If you're comparing a low end card that requires a lot of CPU time (say it can't draw triangles) to a high end card that requires only a little bit (it can draw triangles) the bandwidth requirement for the high end card may be lower due to the triangles not haven't to be drawn elsewhere and sent intact.

That said, there isn't much that falls into the can't-do-it-but-the-cpu-can category these days.
 
youra6 said:
so when it comes down to it, do you guys think that "its wrong some of the time?"

Agree or disagree?
Click to expand...

i would have to agree... in really does depend just because the card is faster doesnt always mean needs more bandwidth. to keep this post short since i rewrote it like 8 times now. the one main big area i want to point out where it doesnt matter. is that say you playing at 1920x1080x32Bit using 8xAA,8xAF. now this is where the gpu does all the work and the cpu doesnt have much to do. the role would be different if we were talking something CPU bound but for this instance we are not. now then with such a high res you do need more bandwidth as the texures are higher res,etc. now add into the fact you have the gpu doing AA and AF to the image. if you give the gpu a x16 pipeline to work with at those settings it wont need that much bandwidth. because the gpu has to apply the AA and AF effects to the image, that takes time. A faster card would be able to apply AA/AF to the image faster but wouldnt be a big performance increase using x16 over x8.


when we talk about CF or SLI it is a different matter.
 
Evilsizer said:
i would have to agree... in really does depend just because the card is faster doesnt always mean needs more bandwidth. to keep this post short since i rewrote it like 8 times now. the one main big area i want to point out where it doesnt matter. is that say you playing at 1920x1080x32Bit using 8xAA,8xAF. now this is where the gpu does all the work and the cpu doesnt have much to do. the role would be different if we were talking something CPU bound but for this instance we are not. now then with such a high res you do need more bandwidth as the texures are higher res,etc. now add into the fact you have the gpu doing AA and AF to the image. if you give the gpu a x16 pipeline to work with at those settings it wont need that much bandwidth. because the gpu has to apply the AA and AF effects to the image, that takes time. A faster card would be able to apply AA/AF to the image faster but wouldnt be a big performance increase using x16 over x8.


when we talk about CF or SLI it is a different matter.
Click to expand...

If you compare the same settings (5839xAA at whatever res) on different cards, the faster one will still need more bandwidth then the slower one.
 
I don't know why this is a discussion, he clearly states it all right there in common sense.

slower cards make the PCIe bus interact with the system RAM, which means more bandwidth going across the bus. a PCIe x16 bus can transfer 8GB/s. most DDR2-DDR3 RAM transfer around that or more, up to 12GB/s and beyond. So it's safe to say that RAM can fully saturate the PCIe bus. Although, with management protocols you could have the card use it's full bandwidth and leave the left over to the RAM transfers, effectively limiting the bandwidth that system RAM is allowed to use.

But then you have another problem, if the RAM isn't transferring data fast enough, you'll still see horrible performance. It's a lose lose scenario either way. It's like trying to fit a gallon of water into 1 cup slowly by waiting for it to evaporate before pouring more in.

Vram is very fast, transferring at 50-100's of GB/s give or take, so it's MUCH faster than even the fastest system RAM, which means even if the PCIe bus had loads more bandwidth the system RAM would still be a bottleneck compared to the Vram.
 
Its not a slower card. Its any card that goes beyond its ram usage and has to write to system ram.
 
720x770 said:
I don't know why this is a discussion, he clearly states it all right there in common sense.

slower cards make the PCIe bus interact with the system RAM, which means
Vram is very fast, transferring at 50-100's of GB/s give or take, so it's MUCH faster than even the fastest system RAM, which means even if the PCIe bus had loads more bandwidth the system RAM would still be a bottleneck compared to the Vram.
Click to expand...

And I dont understand why a person cannot get a second opinion? I wont automatically take someone's word for it. Always better to sure than to be sure with false knowledge. What may be obvious to one may not be obvious to another. I appreciate the help, but please keep that tone to yourself if you will.

I think you are getting confused with PCI-E bandwidth and GPU VRAM bandwidth.

They dont operate the same way. the GPU and onboard RAM bandwidth is completely independent from the GPU to system bandwidth. I am fairly certain the "50-100" gb/s refers to the card's internal memory bandwidth, not the external PCI Express bandwidth IIRC
 
Last edited:
Bobnova said:
If you compare the same settings (5839xAA at whatever res) on different cards, the faster one will still need more bandwidth then the slower one.
Click to expand...

in a cpu bound situation i can see that... when you place all the load on the GPU with aa/af the PCIE width wont matter much for the faster card. the card is reprocessing the same data over and over, there is nothing new to send to it.
 
Evilsizer said:
in a cpu bound situation i can see that... when you place all the load on the GPU with aa/af the PCIE width wont matter much for the faster card. the card is reprocessing the same data over and over, there is nothing new to send to it.
Click to expand...

cpu bound as in lets say for example.... 3dmark06?
 
youra6 said:
cpu bound as in lets say for example.... 3dmark06?
Click to expand...

no cpu bound is when the resolution is low, ie 1280x1024 and lower. gpu bound is around 1600x1200 and higher. Does also include things that improve IQ like AA and AF.
 
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