How to tell the graphic card memory usage?

My application seems to be slow, but in terms of CPU and RAM, it seems that it is OK. So, I want to know how much memory of the graphic card I am using. I've seen some questions about this on SO, but they talk about Linux or NVidia. I would like to have this information for ATI 开发者_StackOverflowcards on Windows.

Thanks.

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How about the OpenGL debugger?

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if you use OpenSceneGraph in order to render scene, there is a stats monitor that shows the usage of GPU.