I see the same thing, so I always leave at least one hyperthreaded core idle so maximize mfaktx throughput.

[QUOTE=petrw1;511247]I have a 2080Ti GPU running mfaktc
on a i7-7820X with 32GB of 3600DDR4 RAM running Large P-1 on all 8 cores.

The CPU is running at 60 degrees F and the GPU at 81 degrees F.

The GPU is at about 3,900 GHZDays/Day
but if I stop Prime95 the GPU thruput immediately goes to about 4,250.
The GPU stays at 81 degrees F.
If I restart Prime95 the GPU stays at 4,250 until about the time all 8 cores are started, have the RAM allocated and are running the P-1 again.

In other words the total thruput of the rig is LOWER when the CPU is busy. It does about 75 GhzDays/Day of P1 while the GPU loses about 300.

I don't know if the impact would be the same if I was running LL instead of P-1 (much less RAM); though my guess is it would be about the same impact.[/QUOTE]
60[B]F[/B] and 81[B]F[/B]? What's ambient temperature where this system is located?
My systems are typically 70-85[B]C[/B] cpu cores, 80-90[B]C[/B] gpu, even in a system with TEN or more fans. HDs are ~29[B]C[/B]. Ambient 20-27C typ.
It's common for modern gpus to clock faster at GIMPS gpu application startup, then dial back as the gpu warms up and gpu fan speed goes up (even if the cpu is idle for the duration). Pretty much the same goes for the cpu cores; cpus thermally regulate by changing clock rate. Perhaps you've allowed for the effect of thermal time constants by providing plenty of time stagger in your throughput test, but that was not apparent to me in your post.
Another consideration is primality test and P-1 are one kind of calculation, and TF another which gpus are very good at. GhzD/day in primality and P-1 on a gpu are much lower than the same gpu's TF throughput. Another way to think about that is TF Ghz are cheap, primality and P-1 more valuable, since the cost of ~63 GhzD primality or P-1, or ~1016 GhzD TF, are about the same: a GTX1080-day, ~16:1 exchange rate. The "exchange rate" is even more extreme on the RTX20xx, around 40:1 as I recall; -300/40 = -7.5, not bad for +75. from your cpu.
The tradeoff is worse when using igp's on laptops to do TF. The igp uses part of the package power budget, reducing primality or P-1 throughput on the cpu side of the chip by around half. It can still be worthwhile on the raw GhzD/day basis, but not necessarily if allowing for the exchange rate.

[QUOTE=petrw1;511247]I have a 2080Ti GPU running mfaktc
on a i7-7820X with 32GB of 3600DDR4 RAM running Large P-1 on all 8 cores.

The CPU is running at 60 degrees F and the GPU at 81 degrees F.

The GPU is at about 3,900 GHZDays/Day
but if I stop Prime95 the GPU thruput immediately goes to about 4,250.
The GPU stays at 81 degrees F.
If I restart Prime95 the GPU stays at 4,250 until about the time all 8 cores are started, have the RAM allocated and are running the P-1 again.

In other words the total thruput of the rig is LOWER when the CPU is busy. It does about 75 GhzDays/Day of P1 while the GPU loses about 300.

I don't know if the impact would be the same if I was running LL instead of P-1 (much less RAM); though my guess is it would be about the same impact.[/QUOTE]

For my rig (RYZEN 5 1600X, 16GB RAM, GTX960) it didn't do any difference with prime95 on or off. But in my rig it's the gpu that's bottlenecking troughput. And I run LL testing without hyperthreding on 6 cores. So i have spare cycles to take care of gpu demands on cpu/memory.

Oops yes, C not F degrees

8 worker windows X 1 core (i.e. NOT using Hyper threading)

[QUOTE=Thecmaster;511254]For my rig (RYZEN 5 1600X, 16GB RAM, GTX960) it didn't do any difference with prime95 on or off. But in my rig it's the gpu that's bottlenecking troughput. And I run LL testing without hyperthreding on 6 cores. So i have spare cycles to take care of gpu demands on cpu/memory.[/QUOTE]Prime95 or its linux twin mprime are great for yielding cpu cores for serving the occasional needs of gpu GIMPS apps. One thing to watch out for is P-1 gcd in CUDAPm1 is done on a cpu core, which stops a whole prime95 worker (with however many cores involved for that worker) in its tracks for the duration of the gcd computation. This favors fewer cores per prime95 worker. But on a system capable of hyperthreading, even if hyperthreading is not being used in the prime95 application, it softens the blow from gpu P-1 on the cpu prime95 throughput.

[QUOTE=James Heinrich;511248]I see the same thing, so I always leave at least one hyperthreaded core idle so maximize mfaktx throughput.[/QUOTE]

Interesting...stopping core 8 and GPU goes form 3,850 to 4,150.
Also stop core 7 and GPU goes to 4,250.

In both cases within a couple seconds so I don't think it is temperature related of either GPU or CPU.

[QUOTE=petrw1;511261]In both cases within a couple seconds so I don't think it is temperature related of either GPU or CPU.[/QUOTE]

I would not make that assumption.

[QUOTE=petrw1;511261]Interesting...stopping core 8 and GPU goes form 3,850 to 4,150.
Also stop core 7 and GPU goes to 4,250.

In both cases within a couple seconds so I don't think it is temperature related of either GPU or CPU.[/QUOTE]
If you run gpu-z for the gpu, what do you see in PerfCap reasons and how does it change?

[QUOTE=kriesel;511263]If you run gpu-z for the gpu, what do you see in PerfCap reasons and how does it change?[/QUOTE]

Always Pwr

[QUOTE=PhilF;511262]I would not make that assumption.[/QUOTE]I would. I can only speak from my own experience with mfakto and ultra-fast runtime (<4s) assignments but runtime goes up by ~15% when the CPU is fully loaded on all threads compared to having even a single hyperthreaded core available. [i]petrw1[/i] may have a small delay before seeing the performance change kick in if the runtime per class is several seconds.

[QUOTE=James Heinrich;511265][i]petrw1[/i] may have a small delay before seeing the performance change kick in if the runtime per class is several seconds.[/QUOTE]

1.5 seconds