Pyrit - Decreasing Gains with more GPUs

[cramrod]

I recently moved over from a single Nvidia GPU (CUDA) setup to a multiple ATI (Cal++) setup. As I was installing the ATI cards, I noticed I was getting decreasing per card performance with each new card I added.

1 Radeon HD card yields ~ 63,000 PMK/s
2 Radeon HD cards yields ~ 50,000 PMK/s (per card)
3 Radeon HD cards yields ~ 45,000 PMK/s (per card)

I dug around and found a few instances of people reporting the same thing but I couldn't find a case where a solution was found. Furthermore, I couldn't find anyone who could explain the underlying reason the problem was occuring. Does anyone know why this is happening and what I can do to fix it?

Also with my Nvidia setup I found that I got a better benchmark when I booted to shell prompt and ran Pyrit without X server. Is there a way to run Pyrit without X Server for ATI Cards using Cal++? When I tried to run Pyrit without X Server, my GPUs werent recognized, just the CPU cores. If not, what other things can be done to optimize performance? Thanks for the help guys.

[Bloodrain]

im afraid theres nothing you can do with that, I think its about the decreased gain explained by Amdahl's law http://en.wikipedia.org/wiki/Amdahl%27s_law

[cramrod]

Thanks for the reply Bloodrain. Ive decided to just add more PCs to work around this problem.

Does anyone know if there is there a way to run Pyrit without X Server for ATI Cards using Cal++? I think X is slowing me down a lot.