[Tutorial] GPU Tools and GPU Memory Clock: Real and Effective Speeds Demystified

GPU Tools and GPU Memory Clock: Real and Effective Speeds Demystified

I recently received this strange image of a GTX 480 tortured by FurMark.

FurMark and an overclocked EVGA GTX 480 - Temperature spikes
FurMark and an overclocked EVGA GTX 480 – Temperature spikes

Hummm tortured is not really appropriate if we look at the current intensity and GPU power: 27A and 29W… And the FPS: 34 FPS is too low for a GTX 480 even with MSAA enabled. I think the system has a problem (look at the nice temperature spikes – may come from a bug in GPU-Z, I don’t know) but don’t ask me what’s kid of prob ;)

But that’s not the point!

The guy who sent the image asked me why the memory clock of his GTX 480 is 950MHz instead of 1900Mhz. If we look at the image, we see that GPU-Z is used to monitoring the clocks and temperature.

GPU-Z displays the real memory clock frequency or memory speed which is 950MHz (this is an overclocked memory since the stock speed of a GTX 480 is 924MHz). The real memory speed is the most important information. All other values you can read in other GPU tools or graphics cards reviews are the effective speed which depends on the type of memory (GDDR3, GDDR5) and people who have witten the specs, the reviews or the GPU tools ;)

Roughly said, there are 2 kinds of graphics memory currently in use: DDR (Double Data Rate) and QDR (Quad Data Rate or Quad-pumped). For example, the GDDR3 memory (on GTX 200 for example) is a DDR memory while the GDDR5 (on HD 5000 or GTX 400) is a QDR memory.

The effective speed of a DDR memory is:

DDR_effective_speed = real_speed x 2  

And the effective speed of a QDR memory is:

QDR_effective_speed = real_speed x 4  

Then for a GTX 480 (GDDR5) with stock memory speed we have:
- Real memory speed: 924MHz
- Effective DDR memory speed: 1848MHz
- Effective QDR memory speed: 3696MHz

For a HD 5870 (also GDDR5) we have:
- Real memory speed: 1200MHz
- Effective DDR memory speed: 2400MHz
- Effective QDR memory speed: 4800MHz

And for a GTX 275 (GDDR3) we have:
- Real memory speed: 567MHz
- Effective DDR memory speed: 1134MHz

IN practice, very often, the DDR speed is used in the specs of GDDR5 memory. For exemple, NVIDIA displays the effective memory speed of the GTX 480 as a DDR speed (1848MHz) instead of a QDR speed (3696MHz) (see specicifications HERE).

And what kind of speed is displayed in our GPU tools ?

As we said, GPU-Z displays the real clock speed. EVGA Precision, MSI Afterburner
or the new GPU Shark display the effective DDR speed:

GPU-Z shows real memory speed

EVGA Precision
EVGA Precision shows DDR memory speed

GPU Shark
GPU Shark shows DDR memory speed

I hope the memory speed you see in graphics cards reviews or in your favorite GPU tool is clear now.


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