Help - Losing GPU Power

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Eggz

Eggz

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Sep 3, 2013
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I've been having graphical issues, so I ran a few tests. If I ramp up my CPU (via Prime 95) while running a GPU stress test (MSI Kombustor), I notice that the FPS drops from about 100 to 4. It looks like I'm losing power to the GPU whenever the CPU's draws a lot of power. It's like there's only enough power for one or the other, but the PSU never goes above 50% - 60% . . . weird!

Using the PSU's power meter software (Corsair Link), I noticed this funny behavior on the 12V rail. When I'm stressing the video only, two PCI-e power plugs are delivering power to the video card while some power also goes to the CPU's 8-pin plug, which is normal. But as soon as I stress the CPU, way more power goes to the CPU's plug (as expected) but none goes to the GPU. All power shifts to the CPU, and it seems like the GPU starts drawing power almost entirely from the PCI-e slot on the motherboard.

Is there a setting to control this? The CPU is overclocked, but it has passed all stability testing (Prime 95 overnight).

The PSU is also new (AX 860i), and it has been able to run multiple stress tests at the same time. It stopped working after I ran an SLI test with a second video card. The PSU never maxed out, but now it won't power both the CPU and GPU on max at the same time.

If you can tell me whether there is a setting for this, I would appreciate being able to try it before doing an RMA on the PSU. I've already tried an RMA on the GPU, but both the original and the replacement showed the same issue, so I don't think it's the GPU. Please chime in if you have ideas.

Thanks!
 
Solution
Cristi72
Hello,

Is it the system from your signature? Please try only the 750Ti in the first slot to see if it changes the behaviour. It so, it could be a problem with the motherboard. I wouldn't blame the PSU as it is a very good one and has a huge +12V/76 Amp rail. 780Ti peaking power is 250W in gaming, a little over 20 Amps, so plenty of power.
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Cristi72

Cristi72

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Hello,

Is it the system from your signature? Please try only the 750Ti in the first slot to see if it changes the behaviour. It so, it could be a problem with the motherboard. I wouldn't blame the PSU as it is a very good one and has a huge +12V/76 Amp rail. 780Ti peaking power is 250W in gaming, a little over 20 Amps, so plenty of power.
 
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Solution
Eggz

Eggz

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That was actually an excellent idea, and it did help me rule out the motherboard. Swapping PCI-e slots and running the same tests replicated the results. That tells me it isn't dependent on which slot the cards are plugged into. I suspect it might be the PSU because it won't deliver more than 480 watts under any circumstance. If that is a limit that has somehow been placed due to a malfunction, then it would make sense that the computer would prioritize the CPU over the GPU when it comes to delivering limited power.

[EDIT] - I found that limiting Prim 95 to 6 threads allows everything to run, and the power will go up past 600 watts during that time. Putting it higher, takes away from the GPU usage, and it will do almost nothing while running 12 threads on Prime 95.

I'm not sure what the deal is. I have done Prime 95 before with all 12 threads while running a GPU stress test at the same time (to check max power consumption). Maybe I had different OC settings or something, but I don't think so. Very strange.
 
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Kari

Kari

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Sounds like the prime95 is using up all the cpu cycles so the gpu stress test doesn't get enough cpu time to keep the gpu fed with work. Limiting it to 6 threads alleviates it by leaving the HT treads available.
See what happens if you lower the priority of prime95 in task manager to be lower than that of kombustor. So kombustor would get all the time it needs and prime95 gets whats left over basically.
 
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Eggz

Eggz

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Kari, I actually had to look up that bit about priority in Task Manager - pretty cool! Nonetheless, even setting Prime 95's priority all the way down and Kombustor's all the way up to "realtime", the same behavior occurred. Either one of (or both of) the programs doesn't respond properly to Task manager's priority settings, or it something else (maybe a power issue?).

Not sure how to diagnose whether it's actually power (don't think it should be). I have another power supply, but it's only 600 watts max, and the system goes up to 610 under load.
 
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Cristi72

Cristi72

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Use only the 780Ti with the other PSU.

A good and new power supply should have the possibility to cope with loads over their ratings (10% over their limits is OK).

780Ti: 250W (let's say 300W Furmark), so 25Amps; i7: 130W TDP (let's say 175W full) another 15 Amps. A good 50W for the MB makes another 4 Amps. For every HDD add another 1 Amp. You need at least 50 Amps on +12V rail or minimum 500W for that rail. For +3.3V and +5V let's say 15 Amps each. Are the specs for the other PSU close to the requirements?
 
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