CPU change - slower gpu iray render?

Da_green1977_aa156fc426

Hello all,

Have a rig with two gpu's, 2080ti and 1080ti, 32 Gb ram.

With my CPU I5 8400 (6 cores) both my gpu's was loaded around 95-100% when render iray.

When building a computer to my sisterson i gave the i5 CPU to him and put in a i3 9100f (4 cores) thinking it didn't matter to me since i use gpu render.

Now when render my gpu's are at 80-90%.
 

Does the CPU matters that much or is it something else?

BR// Daniel

 

JD_Mortal

80-90% what? Cuda processing?

Are you using the same scene to render with the same settings as your test comparison?

Windows and Daz and video-cards have had a lot of updates recently. Could be that it is running more efficiently, or simply reporting more accurate numbers. Then again, your RAM-speed and PCIe lanes are tied together by your CPU speed, since the CPU now controls those data-busses. Are you running with a slower core-clock?

None of my GPUs (CUDA Processes), have ever run more than about 85%. (Windows only reports 1% use, so I have to visually estimate. There are gaps between peaks of 90% use, which are about 1/8 the width of the 90% use. Nothing in windows reports anything GPU related, correctly, because IRAY isn't using "Windows", to do the processing in the GPU, it is talking directly to the GPU, outside of windows. We are lucky it even reports CUDA levels at all.)

You have RTX and the latest nVidia drivers are making RTX more efficient. (Translation, doing the same work in less time, or with less resources being wasted in the process.) So, I would be willing to bet that the 5-10% apparent loss is actually just a 5-10% gain of "efficient processing", unless you have a time-demo from before and after, indicating that the i3 is taking 5-10% longer.)

Could it also be that the other processor was draining more physical power, or causing more heat build-up in your case? If your card is thermally throttling, due to heat, it will be at 100% where a cooler card, unthrottled, is easily doing that same work in less time, thus, seeming to run 5-10% faster, or 10-20% faster. It can also power-throttle, if there is inadequate power being supplied. (When a video card hits a thermal limit, it lowers its internal clock-speeds, doing less, so it creates less heat. When it would normally be "done", it is still processing, and isn't done until more work is sent. An unthrottled card will be done, and sit and wait for the next work-order, having more time to cool and seem to be doing less, when it is actually doing more, or the same work.)

By the way, I have a i9 36 core CPU... running at 4.2Ghz (Nothing crazy), but Daz uses all cores when rendering, but the average is only around 50%. This is with CPU rendering turned-off. If you have CPU rendering turned-on, then it may be "waiting for work", until the CPU has time to send the card more work, in its "free time". I assume it is preparing chunks of work, but I am not 100% sure. There could be a factor related to available CPU cores and the maximum ability to reach IRAY GPU rendering maximum potential. I have just never seen any of my four cards use 100%, when rendering anything, ever. (Games, yes, rendering, never.)

P.S. I have 2x Titan-V and 2x Titan-Xp, no RTX cards, just the Volta(Tensor)/CUDA components that RTX cards have, in the Titan-V (Same components, but it is all one large chunk, unlike the RTX which is the same setup in smaller chunks, tied to the RTX components.)

I don't use CPU rendering, because, even though I have a lot of cores, it only shaves off seconds from a render that still takes 10-30 minutes. Not worth the cost of power to use it and burn my CPU up, for a few seconds of time.

Da_green1977_aa156fc426

Thanks for the answer.

I monitor the 80-90% gpu load with gpu-z. I notice the lower usage there. 
But you are right, i havent render the same exact scene...so maybe the result isn't usefull.

My cards run hot, both are above 80C when rendering. 
 

Thanks again.

 

BR // Daniel

andy vron

Da_green1977_aa156fc426 said:

Thanks for the answer.

I monitor the 80-90% gpu load with gpu-z. I notice the lower usage there. 
But you are right, i havent render the same exact scene...so maybe the result isn't usefull.

My cards run hot, both are above 80C when rendering. 
 

Thanks again.

 

BR // Daniel

thats a bit too hot for long life ...try and add as many fans as possible that the case will take ...make two thirds intake and a third extract and set them in the bios to only kick in when needed otherwise it will be noisy when idle....

i run my 2080 at 70 full load but i have just the one ...two cards together can be a real issue as they heat each other up ...

kenshaw011267

Anything above the high 60's C will result in the cards downclocking. That's how Nvidia makes them now. So you really should try and get more airflow. If the front is closed off, or mostly closed off, try rendering with the side panel completely off.

As to your question, you might be seeing a drop in performance but until you run a benchmark in the two scenarios you can't know for certain.