cool, i knew i was an idiot, but I just wanted a clearer def on why.
Your question was definitely not stupid. I don't like to see perfectly viable ideas get shot down for no reason
the thing that the paper suggest is that we put the cpu to work, while the gpu works.
It's more complicated than that. The slides are a bit scatter brained, but I'll just reference the "Profit!" slide, because I think that is what the OP is referring to. What it suggests is moving the two components of the algorithm which the GPUs perform poorly at, but which the CPU can quickly process while the GPU is doing more intensive and suitable work. It's like hiring someone at your restaurant to peel potatoes and take out the trash, so your cooks can do what they do best.
In some sense, this has already been applied to mining. bitcoind actually does half the work for you, and "pre-hashes" the results it returns (this is where midstate comes from). Hence, some of the work has been offloaded to the CPU-based bitcoind.
And a lot of the recent improvements to the GPU mining algorithms have come from pre-computing, on the CPU, various values.
And we also only generate Difficulty-1 shares from the GPU, because anything else would slow the GPU down. The CPU is responsible for determining if that share is actually useful in generating a block.
However, those are only once per unit of work optimizations. Unlike the password recovery slides, there is no repeated computation that the CPU could help with in mining, because there are no parts of the mining algorithm that are better suited for the CPU. No complicated generation or verification. Since the CPU isn't better
at any part of it, it's best to leave the whole algorithm on the GPU.
but there are nothing to do for the cpu while the gpu works.
That's not the case at all. The CPU can be hashing as well, and on most modern CPUs that would give you an extra 3 or 4 MH/s ... roughly the same increment you see in each new revision of the GPU code.
The real reason why we don't utilize the CPU in this manner is not because you can't. You most certainly can run a CPU miner alongside your GPU miners (with a bit of effort). People don't usually do it, though, because the CPUs have terrible MH per Watt performance. i.e. you're wasting electricity. It's far more suitable to buy a low power, cheap CPU and have it just twiddling its thumbs so it can snap into action when it's time to give the GPU new work. Like the squeegee boys on porn shoots.