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Steve132

OpenGL State Change Benchmark Data?

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I haven't been on this forum in a very long time...so I dont' know how much is kept up around here, but anyway... I had a relativeliy advanced question for you all.. It is well understood that OpenGL is a state machine, and to manage it, you run state changes on the machine to manipulate the current render state as geometry passes through... In addition, certain state changes are inherently more expensive than others...for example, it is more expensive to change the current bound texture than it is to change the current render color...the same goes for changing the clear color vs turning off the depth buffer, etc. ANYWAY. My point is this... How would I go about coming up with the actual state change approximate costs for each state change operation in opengl? Is there a spec with this data? For example, a change to the current color might be cost 1x, whereas a change to the texture might be 30x....is there some table somewhere giving costs for state change operations? This might be implementation dependant, if so, then is there a general table with approximations? What might be helpful even more is this...can anyone think of a way to collect this data imperically? perhaps an opengl state benchmark program? How would such a program be written? Write me back.

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I don't know if there really is a significant change in speed by changing different states.
( matrix changes might be slower than other changes )
But that's just my thoughts. I don't know.

But anyway, shouldn't gDEBugger or some other OpenGL debugger be able to do this?

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I've read something that suggested that the answer to your question is simply: don't worry about it, states are optimized already and what happens in the hardware is constantly improving.

At least that is how I took this rant on why scene graphs are a waste of time.

Of course that is only one source and I've yet to see a similar sentiment anywhere else.

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State changes can be put into 3 or 4 different groups

First group and by far the slowest group is where the driver has to do a lot of validation work, it has to check weither there actually is data there and is it ok and so on.
this group includes texture changes and simmilar.

Then theres the middle group, shure it takes some processing to work things out it's relatively fast, so fast they could be ignored, just don't go state trashing with them ok.
This group includes most other states.

Finally theres a group that really are not state changes, they just behave like them, glColor is a good example, in reallity it's just a variable that everytime you call glVertex it's color value gets sent to the GPU, and no real processing is required.

There is a fourth group to that eats a lot of processor time, though they are not technically state changes, but they can stall the CPU or the GPU and that is so very bad.
This group includes functions like glFlush, glReadPixels and so on.

Well this is at least how it looks today, but comming in openGL 3.0 we have a new object model, it makes textures be validated on texture load and not everytime you want to use them, thus speeding up most state changes.
So in the future, state changes are not so much of a bother as it used to be.

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lc_overlord...Thanks..those 4 state groups...is there a practical way to calculate what states go where? Or perhaps you could point me to a list or way of figuring it out for myself. That would be very helpful to me.

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no, generally only glBindTexture, glAccum, glClear, some of the FBO and stuff and most texture operations is in the first group.
glColor, glTexcoord, glNormal, glFogCoord and such are in the third group.
the rest is in the second one, some in the second group can be in the gray area between group one and two, though most of this varies between harware and software implementations and thus it's a bit hard to say for sure.
There is this new extention from nvidia that can mesure time in picoseconds that can preform these tests but it's only for the latest quadro cards.

Anyway the general idea is to keep glBindTexture calls to a minimum and let shaders do all the hard work, then it doesn't matter how fast the statechanges are.

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I originally began a research of this kind but I later realized the information was often unreliable and prone to change between drivers (not even hardware!).

A thing I noticed however is that the cost of something is in some way related to the workload: I guess this does not surprise you!

On some nVIDIA papers it is stated that the more you go down in the hardware, the less state change is expensive. I guess this has to do with the fact there's often more validation on the 'near end' than on the frame buffer output (consider BlendFunc against VertexAttribPointer: it does make sense).

I am also confident benchmarking this is of little pratical use.

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Quote:
Original post by Krohm
On some nVIDIA papers it is stated that the more you go down in the hardware, the less state change is expensive. I guess this has to do with the fact there's often more validation on the 'near end' than on the frame buffer output (consider BlendFunc against VertexAttribPointer: it does make sense).


Yea, that is why the new object model in opengl 3.0 will be faster since most of the validation work is already done.

Theoretically even with todays hardware one could write a API that use virtual texturing (that is, not having to bind textures, you just use them any way you like).

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I don't know any specs with actual opengl state change costs. I don't think that the relative costs of state changes are really important. Because usually there is no alternative state change(s) to a state change. It doesn't matter if it's in group1, group2 or gropu3. We just have to write optimized code and avoid as much unnecessary state changes as we can. It is - we can't replace glBindTexture with something cheaper. I mean I can't :p

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no, currently we can't directly replace glBindTexture, i think we will within a year or so, but today we can work around it by using things like multitexturing together with shaders and optimizing the usage of texture state changes.

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