RTX slower than CUDA with a particular scene

[Alex_M]

An interesting thing happens with a scene I'm working on at the moment. RTX is consistently 5-10% slower than CUDA. I would expect it to be close to CUDA with some scenes, but slower? That can't be normal, right? Please check the render times in History panel:

[piotrus3333]

it does happen.

[Alex_M]

Well, evidently it does. The question is, is this to be expected with some scenes or is it a bug? I don't remember the official docs and the RTX marketing materials mentioning that it can be slower. In fact, they make it seem that if you have an RTX card, you should always use RTX and that there are no drawbacks performance wise.

Here's another example which is even more extreme. CUDA 30% faster than RTX:

[piotrus3333]

it is to be expected. it was discussed somewhere here. there is not a lot of raytracing needed in your scene. hence cuda is faster. more raytracing - rtx, more shading - cuda. one of the real world scenarios is for example a scene containing a lot of foliage with opacity maps.

[nikoleta.garkova]

Hi Alex, as piotrus3333 suggested, in some cases it may be expected. The benefit from RTX will vary depending upon how your scene is constructed and how 'heavy' its shaders are. There might be some CUDA optimizations that are weighing in this scenario, but you can, of course, share the scene so we can test it out further. You can also check the following article (particularly the Mileage varies paragraph) here.

[Alex_M]

Hi Alex, as piotrus3333 suggested, in some cases it may be expected. The benefit from RTX will vary depending upon how your scene is constructed and how 'heavy' its shaders are. There might be some CUDA optimizations that are weighing in this scenario, but you can, of course, share the scene so we can test it out further. You can also check the following article (particularly the Mileage varies paragraph) here.

Thanks for the clarification, Nikoleta. I've actually read this article a few times before (and I've read it again just now), along with the other couple of RTX articles on the website and none of them state or even allude to the fact that RTX can be slower. Please correct me if I'm wrong. I think it would have been a good idea if this was communicated clearly and upfront in these articles and the help docs that there are cases where RTX can be slower, ideally with some examples of such scenes. Statements like "just try the new RTX engine out, as there’s no risk" (from the linked RTX article), along with the fact that none of the examples show cases where RTX can be slower, further solidify the idea for the user that RTX will always be faster and that there are no drawbacks. That's why I was surprised with my findings and posted this thread thinking there must be a bug with Vray.