Folding Forum

Holdolin wrote:I agree ProDigit. Up front cost is the thing here. If you have the up-front capital, the i'd highly recommend 2080ti's all day long. They are the most efficient cards out there in terms of PPD/Watt according to https://docs.google.com/spreadsheets/d/ ... edit#gid=0 . Problem most of us have is we just don't have that kind of capital to build systems out of those monsters.

foldy wrote: The PPD should be an average value and PPD depends on work unit and is different on Windows or Linux. But again you can use it as relative value to compare the GPUs.

foldy wrote:...Knowing that RTX 2080 has 25% more shaders than RTX 2070 it doesn't make sense that it only has 10% more PPD....

kbeeveer46 wrote: I ended up buying the RTX 2070 because it had the best PPD/price. Even looking at it now a week later, some of the prices I found have already changed.....

bruce wrote:

foldy wrote:My theory is CPU loads data through pcie to GPU and then GPU processes data without CPU until it has a result. Then CPU gets result from GPU through pcie and sends next data to GPU dependent on the result. So the CPU to GPU communication occurs in peaks and then CPU spin waiting time.
If the pcie bus speed is too slow or has bad driver usage like nvidia opencl on Windows then it takes too long for the CPU to GPU communication which makes GPU get idle for short time which slows down FAH. If CPU speed is too slow then the same occurs. Then we have a ratio of CPU/pcie to GPU time and if it is CPU/pcie 1 to GPU 99 that is great. If it is CPU/pcie 10 to GPU 90 we have a light bottleneck. And if it is CPU/pcie 50 to GPU 50 then we have bad performance.

That's my theory, too. Certainly the PCIe data transfers occur in peaks and the CPU spin-waits which, for the sake of increased game frame rates, the CPU is always ready to transfer data without the added delay of interrupt processing and task switching overhead. (That's probably insignificant for FAHBench).

When the GPU finishes the processing that has been assigned to it, it will pause until a new block of data is transferred so it's ready to be processed. It doesn't matter whether the PCIe bus is slow or the CPU is slow or the CPU is busy with some other task -- the result is the same -- nothing for the GPU to process yielding lower average performance. It's sort of like your browser opening a document over a slow internet connection. As soon as the first page of data is displayed, you start reading and it doesn't bother you if pages 2, 3, ... have been loaded into memory or not; they'll be ready to read before you're ready to look at them.

All of the atoms are divided into multiple pages. As soon as the positions of all nearby atoms are available for the current time, the GPU can begin summing the forces on each atom. As this process advances, I suspect that this data must be returned to main RAM. Once the last page is available to the CPU, updated positions of all atoms for the next time step can be computed and sending them to the GPU begins again.

What I do not know is how the software figures out how to allocate VRAM. (What constitutes a "page"?) Clearly, once the first page is transferred, the GPU can start running and the second page can start transferring. If multiple pages fit in VRAM, other shaders can processing them in parallel so the GPU doesn't need to wait again until the last page has been computed.