CPU-boundedness of the video system Part I - Analysis
Processor dependence
While reading various reviews of video cards you must have come across the phrase - "the results are CPU-bounded". The meaning of this phrase is easiest to demonstrate with a diagram like that:
 Diagram 1On this diagram, we can see the results of testing GeForce 7800GT and 7800GTX video cards in Half-Life2 game. As you can see, the effect gained from merging two video cards into the SLI mode does not add to the performance results (measured in FPS) at all, despite our expectations. At the same time, the boundary of results is absolutely the same for both the single GeForce 7800GTX and for a pair of GeForce 7800GTX video cards in the SLI mode. Evidently, the rise of results is limited to something else, and the CPU is just the "something else". We took the diagram from here. Let me remind you that we used AMD Athlon64 4000+ running at 2.4 GHz as the CPU. As you understand, the above diagram is not at all a special case for Half-Life2 and the resolution 1024ő768. You can find examples of diagrams like that over here - http://www.3dnews.ru/video/ati_x1900xtx_crossfire/index04.htm. It's simply meant to show that to see the CPU-boundedness in tests, you've got to have a really powerful video subsystem.
…Or a "weak" CPU. Actually, if instead of the CPU AMD Athlon64 4000+ used in our test setup we take a CPU of smaller performance, then the FPS value which limits the performance of video cards will be lower. Similarly, if we take a CPU of even smaller capacity, then the boundary of performance will be even lower. To make it clearer what we mean, let's illustrate it with a diagram similar to Diagram 1 which we brought in above but slightly change its positioning. We position the columns of results vertically, with the conditional performance of the CPU positioned horizontally. Therefore, we get Diagram 2.
 Diagram 2Evidently, Diagram 2 is quite conditional. We intentionally decided not to mark the labels of the CPUs for the left-hand and middle group of results but restricted to the descriptive terms like "a CPU weaker than Athlon 64 4000+". The red touches stand for the limit the results come up against. The left-hand and middle groups of results show only the presumptive results on the way to weaker CPUs. We'll know later what the real results look like.
Here, our desire to see what the "CPU-boundedness" looked like in reality brought us over to thinking about the methodology of tests run with a «weak CPU». We thought that couldn't be easier. Just take a few processors of various clock speeds and verify that. Unfortunately, the comparability of results in such tests would hardly be authentic. What should we use as a CPU performance measure? All these ratings and measures are all a nice kettle of fish - ratings for Athlon and Sempron are different. It is also not quite clear how to compare the AMD and Intel platforms. That is why we followed a different way.
Methodology for CPU-boundedness tests
We decided to use the real CPU clock speed in MHz as the measure of CPU performance, because that can be adjusted within some range and thus we can expect a proportional change in the CPU performance without conversions to whatever ratings. As regards the other parameters of modern CPUs like cache memory size, number of channels in the memory controller, etc., for now we'd better leave the effect of these outside consideration, although they can be taken into account properly. The most important for us now is the linearity of clock speed as a measure of CPU performance.
We took AMD Athlon64 4000+ as the basis. AMD processors prove as very good performers indeed just at gaming applications, and although AMD Athlon64 4000+ is no longer a flagship model for now, it is still one of the most powerful CPUs in the line to date. The indubitable convenience of AMD Athlon64 4000+ for this test is in that its multiplier is not fixed and can be adjusted towards reduction. So we decided to make use of just this feature of the CPU. Varying merely the CPU multiplier, we produced a "line" of processors running at various clock speeds but offering absolutely identical performance in all the other respects – the cache size, the system bus speed, etc. It was just that which allowed us to deduce certain regularities related to the joint operation of CPU and the video subsystem in graphic applications.
Since we prepared the material for quite a long time, we used various video cards as the video subsystem, but that will not affect the quality of the final result. Moreover, while working at the material we used that as indirect endorsement of correctly selected methodology according to which different video subsystems under certain conditions should behave in absolutely the same way, and we made sure of that more than once.
All started then when GeForce 7800GTX was the most powerful video card ever. The drivers used for tests were of version 81.85. As we already stated, the material was prepared for a long time, so the driver version is so old. Later on, we did not upgrade the version of the video drivers to provide comparability and consistence of the test results. However, as we'll see it later, the driver version is not decisive for the deduction of produced regularities. For the purpose of verification we once again tested the same with drivers of version 84.21 and produced similar results. I'd like to note for ATI fanciers that we conducted absolutely the same experiments with Radeon X1900XTX and produced absolutely the same results (within the measurement error).
OK, then, off we go.
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