Max Bandwidth Available From PCIE for GPUs

What is really Bottlenecking your Graphics Card?

Steve Smith explains why the amount of bandwidth available from the PCIE Serial bus, is not the source of the infamous bottlenecking in games we all loathe.

Episode #5-30 released on April 16, 2015

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This may be a question people do not ask often, but it is an important one. The available amount of PCIE Lanes, their generation, and other factors do determine how a GPU Card will perform in your computer.

Let's start with how the speed of PCI Express is calculated. The PCI Express lane has a measurement of transfer calculated in Gigatranfers per second, or GT/s. 1 GT/s is equal to 100 MB/s.

PCIE V1.0 has a transfer rate of 2.5GT/s, or 2.5 Gbit/s per lane. For a 16 lane graphics card, this means the total bandwidth available as 32Gbit/s, or 4 GB/s.

PCIE V2.0 has a transfer rate of 5GT/s, or 4 Gbit/s per lane. For your 16 lane graphics card, this means a total bandwidth available as 64 Gbit/s, or 8GB/s.

PCIE V3.0, the current generation, has a transfer rate of 8 GT/s, or 7.877 Gbit/s per lane. For your 16 lane graphics card, this means we currently have 126.032 Gbit/s, or 15.754 GB/s available.

And in the future, PCIE V4.0 will have a transfer rate of 16 GT/s, or 15.754 Gbit/s per lane. For your 16 lane graphics card, this means you will have an available bandwidth of 252.064 Gbit/s, or 31.508 GB/s available.

Now, it has been theorized that, to this day, we are yet unable to bottleneck a modern graphics card, on the PCI Express V2.0 bus, even with a Generation 3.0 PCI Express Graphics card. This being said, the usual suspect for any bottlenecking, is usually the CPU and API, and how they supports draw calls. This does render some games unplayable on some systems, and is the notable reason why Assassin's Creed: Unity had some many issues as launch.

Which brings us to how to create a modern system that should, at least in theory, be able to play virtually all video games, without too much issues. I am able to guarantee quality beyond low or medium, depending on the system choice, and graphics card choice.

Most games will now expect you to have a processor with, at least, 4 logical cores and a frequency of 3.0 GHZ or higher, 8 GB of system ram, hopefully none of that dedicated to an onboard graphics adapter, access to PCI Express V2.0 or better. A graphics card that is not too low end. For reference, you'd get a better experience with a GTX 750ti card, that a GTX 730 card. Even if you think you are saving money, if at any point you risk upgrading, you're spending more money than you perceive. You'll, also, need a hard drive with more than enough space. If your mainboard does not support UEFI, please stay at 2TB or less. If it does support UEFI, you can opt for greater than 2TB, because you can then boot from a GPT partition under Windows 8 or better. For better speeds, consider using a SSD for your operating system. If you have the cash, buy a bigger SSD and install your games on it, and reduce the load times, and transfer rates to your graphics card. Using SATA cables which are just long enough will, also, reduce overall lag by reducing resistance caused latency.

Now, if you get the best of everything on a given platform, you will always get the best performance, but with one caveat, your computer monitor's refresh rate will never get any faster. Even if your computer could produce more than 60FPS, if your monitor's refresh rate is 60HZ, you should turn on vertical sync, also known as, v-sync.

Just as a note, one of the many bottlenecks in gaming is related to the number of draw calls, and in relationship to that, Direct X 12 is supposed to fix the issue of CPU bottlenecking related to draw calls from the graphics card. This, however, will only fix games that support Direct X 12, that, also, have Direct X 12 capable graphics cards, in Windows 10, which will require a mainboard that is, also, UEFI enabled. Needless to say. The technique to make games playable on any computer will work, as long as you don't intend to use a game that supports Direct X 12. If you want to play Direct X 12 games, you need to upgrade your mainboard, and processor to those that are UEFI enabled. Users with Windows 7 Service Pack 1 and Windows 8.1, provided you have an UEFI mainboard, will be able to upgrade to Windows 10. Those with Direct X 12 Graphics cards, will, also, be able to enjoy the benefits Microsoft claims to date.

By the way, for those wondering, each game has a different requirement for bandwidth, also, based on the quality of the image. For that reason, we can only determine the maximum available bandwidth for the graphics card, each game will determine how much it may need. When the VRAM and GPU is not enough, the game's FPS will drop, sometimes dramatically. When this occurs, please reduce the image quality to restore the framerate to 60 FPS or better.

Host : Steve Smith | Music : Jonny Lee Hart | Editor : Steve Smith | Producer : Zed Axis Productions

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