Overclocking the GPU

I read through the intro to overclocking and found it very useful, and I can understand why overclocking/cooling the CPU and RAM take precedence over the other components - except the GPU. It seems to me that the new generations, not so new any more, of video cards are basically becoming miniature motherboards with processors and memory of their own. If one wanted to overclock for gaming purposes (why else?), why shouldn't overclocking the GPU be the primary concern and overclocking the CPU be only secondary?

 

Because a game doesn't just use the GPU. The only thing the video card ram is used for is textures and pixel shader work, most every thing else is done in system ram via the CPU. Video games push in every direction in a system. A 9800GTX will be useless in a system with only 1GB of ram. The same 9800GTX will be just as useless in a single CPU 2Ghz system. On the other hand a quad core CPU at 3Ghz would be useless for gaming if the only videocard in the system was a 7300gt. The trick is to find a nice balance all around . But I see what your getting at. Most systems tend to have weaker video cards, at least compared to there CPU ram size/speeds.

 

So when overclocking, how do the GPU and GDDR clock speeds relate to the CPU and DDR? More generally, how and why does the "system" as a whole speed up with the clock rate of the CPU?

 

It depends on the system and how your overclocking. In older systems the FSB was the primary bus and all other buses were divided or multiplied by it. For example if 133 x 10 = 1.33Ghz is stock on a P3 system the rates and dividers would be 133 / 0.5 = 66Mhz (AGP bus speed) x 8 (assuming 8x AGP), 133 / 0.25 = 33Mhz your PCI bus rate. Your ram is also on a divider running either 133 100 or 66 (133 divided by 1, 0.75, or 0.50). So if you upped your FSB to say 140 every thing will change. Your core speed would be 1.4Ghz your PCI buss speed will be 35Mhz, your AGP bus speed will be 70MHz and your ram speed will be 140Mhz, 105Mhz or 70Mhz. The maths being: 140 x 10 = 1400 for CPU, 140 / 0.5 = 70 for AGP and 140 / 0.25 = 35 for PCI. To make things even more complected older systems some times use the FSB as a reference clock for USB, PS/2 and onboard video as well. As for video cards you cannot overclock most video cards by simply overclocking the AGP or PCIe buss as most video cards have there own clock crystals to use as a reference clock. The two exceptions I know of being the 9600GT and the 9800GTX read more about Nvidia using the PCIe clock as the GPU reference clock here. Hope I didn't confuse you to much.

 

Just a few more questions... did I understand correctly from your guide that the expansion slots, or pci at least, can't be overclocked because their cards are designed for specific, unchanging speeds? Also, you say that on "older" systems the CPU bus acted as the master bus determining via ratios the speeds of all other buses, but how is it done in "newer" systems? Finally, you've mentioned a couple times things called "crystals" which sound very intriguing, what are they?

 

In older systems you don't have a choice. if you overclock the FSB the PCI bus goes up with it. On newer motherboards there is a PCI lock that uses a reference crystal to get the stock 33Mhz for the PCI bus instead of the FSB so now matter what the overclocked setting the PCI bus remains at 33Mhz. AGP does the same thing and uses the PCI as its reference and just times it by 2 to get 66Mhz. Normally this is enforced at all times, but some times there is an option to disable/enable the PCI lock in teh BIOS. In general overclocking the PCI bus is a bad idea because a lot PCI cards use the 33Mhz PCI clock as a reference for there own internal clocks. Most sensitive to this effect are hard drive controllers causing data corruption and the loss of whole partitions. Most newer motherboards use reference clocks independent of the FSB normally crystals or "cans" are used as reference clocks leaving the FSB alone to be adjusted with out driving other buss out of spec.

 

Excellent explanation ACE very well covered and from:

Howto: Overclock C2q (quads) And C2d (duals) - A Guide V1.7

"PCI Express Frequency – Set this to 100 MHz. If you don’t, I believe the PCIe bus speed will increase proportionally with your FSB which is something you DON’T want to do to your expensive video board.

PCI Clock Synchronization - Use 33.33 MHz here. Again, if you leave the setting on auto, the PCI clock will creep up proportionally with your FSB which can damage cards you may have there aren't designed to run at higher frequencies."

ACE bought up a very important point if you are about to OC go into BIOS and make sure these settings are set to above. Hands up who has left them on "Auto", OC and then had RAID0 corrupt, <<<<<<<pick me pick me!

So in theory if you dont lock those settings you can end up damaging the Video Card and really messing with DATA transcription.

If you really want to OC your Video Card you most often do it within OS using something like RivaTuner for monitoring your Video Card and ATI Tool for that range (not sure what for nVidia):

http://www.majorgeeks.com/download737.html

As with any OC you must do it in small stages and monitor those TEMPS. OC on the GPU will significantly increase overall system temps, CPU, MOBO etc etc if you do not have adequate GPU cooling, be very cautious.