Optimised for folding: GPUs and CPU threads

[Christopher_Ambrey]

Hi fellow folders,
I have two GPUs that I’m currently using for Folding@Home and I’m wondering if it is best to keep 2 CPU threads free? That is, one thread for each of the GPUs?
Geforce GT 610 (FahCore x15 only)
Geforce GTX 750 Ti (FahCore x18 at the moment, it depends)
I have Folding@Home using both of the CPU threads at the moment as well as the GPUs. The GPU usage according to GPU-Z is a generally at 99% but very briefly it goes down to 50% or even 0% GPU load. In contrast, when I have the two CPU threads free it dips down to 89% very briefly at most. I should mention here that I am using TTemp and my CPU and GPU have a temperature limit of 90 degrees Celsius (it’s usually only a binding limit for the CPU despite the all the fans and the watercooler). I don’t want to negatively impact the folding. Should I set two CPU threads aside for the GPUs? Or am I getting the most out of my PC doing this? The work units seem to be progressing along alright at the moment.
I’ve avoided using the automatic allocation setting ‘-1’ and set the CPU to pause-on-start because I am currently running BOINC with 6 of the CPU threads.
Keen to hear your thoughts when you get a chance.
Kindest regards,
Christopher

Further details about my system and situation...
SLI is disabled.
GPU-Z has ticks for OpenCL and CUDA for both GPUs.

Computer Specs:
CPU: Intel i7 CPU 950@3.07GHz
4 cores 8 threads
Memory: 12GB DDR3
Motherboard: ASUS P6T
Power Supply: 550W
OS: Windows 7
GPU0: Geforce GT 610
GPU1: Geforce GTX 750 Ti
Network Connection:
ADSL
Wireless network adapter in PCIe x16_3 slot
Overclocked?:
There is some automated overclocking controlled by the motherboard up to 3.2GHz or so. The CPU has a closed loop water cooler. There are plenty of fans in the case too (I can't fit any more in the case).
Stable?:
My system seems stable, except for that brief jumpiness in terms of the GPU load, fan speed etc… for the Geforce GTX 750 Ti which is only for a fraction of a second though. While the PC is certainly being pushed hard, it is stable.
Software:
I am using version 7.4.4 of the Folding@home client. I also have BOINC running on some of the threads (75% of the CPU resources, 6 threads), leaving two threads for the GPUs I am using with Folding@home.
WU details:
I'm not sure what to put here. My work units seem to be chugging along nicely and I think when I do have the two threads being used from the CPU for another work unit (FahCore 0xa4) I am actually getting more PPD than without it.

[davidcoton]

The only way to know what is best is to try it and see.

Standard advice is to leave a CPU thread free for each GPU that is folding -- normally this is optimal but YMMV.
It might work better with one thread free, as long as WUs are available for the CPU:7 configuration.
CPU:8 will get work more reliably, but will hold up the GPUs for longer, as you observed.

Whether the extra CPU ppd outweighs the lost GPU ppd can only be confirmed by experiment, and each period of experiment needs to be several days to give a chance of a fair balance of allocated WUs.

[toTOW]

SMP 6 is the best choice with two NV GPUs ...

[Christopher_Ambrey]

Thanks for the guidance davidcoton and toTOW. You've helped clear things up for me. I'll try it with one or two CPU threads free.

[bruce]

The last time I ran Core_15 by itself ie barely used any CPU resources. The 750TI (by itself) does a pretty good job of keeping 1 CPU busy although it does depend on the particular WU and the particular drivers you have. In the light of those estimates, 1 free CPU is pretty much required to keep the GPUs busy.

For set-it-and-forget-it operation I'd avoid CPU:7 but that's your choice. Another option would be two CPU slots (CPU:6 and CPU:1) together with two GPU slots. You'll have to experiment and see what works best for you (A) when you're using the machine and (B) when the machine is otherwise idle.

I've forgotten and I'm to lazy to look it up. Does NVIDIA recommend the same driver for both a GT 610 and a GTX 750 Ti?

The other choice you can make is which GPU drives your monitor. I'd probably hook the monitor up to the 610 unless that limits your video noticeably.