Problem with multiple V-Ray skylights

[Mnietek2707]

I have problems with scene with multiple V-Ray light set to skylight mode.
There are three skylights. When all of them are ON lightcache calculation takes ages. When one of them is deleted/turned off lightcache is computed normally. Whats going on here???
Scene is here: skylight problem_01.zip

[hermit.crab]

In your scenario, the portals do not seem to be used correctly (rendering with those lights disabled gives the same result) which may be the reason for the excess Light Cache calculation time. Their point is to transfer (brighten) light from the environment to an interior scene (through an opening) for example. Also, note that skylight portals have become obsolete with the introduction of the Adaptive Dome Light. What is the reason you're using skylight portals? Maybe there is another way to achieve your desired effect.

[^Lele^]

I changed shadow casting and the SSS glass shader a bit.
I also turned off the two portals around the elongated light and over the spot: as Alex mentioned, they aren't used properly, and create (much) slower renders and more noise for the same amount of sampling.
It seems to be much quicker for much the same result, have a go and see if it's fine by you (i saved the file as max 2018, and ofc i do not have your ICC profile, so i see the image differently in the VFB.).

[Mnietek2707]

Thank you for your effort (and sorry for late response).
I thought that encapsulating light fixture with skylight portal is the way to instruct V-Ray: behind this light portal is light source.

Below is example of light fixtures from Cosmos library.
Surface of this lamp is 2-sided material and V-Ray treats this surface as any other. I think this is the reason that rendering is very slow. Is there any way to speed things up?

[^Lele^]

I would say that for a material which needs to light, one would have to go for a lightMtl with "direct lighting" active, which will sample it as efficiently as if it was a geometric area light.
In the specific fixture's case, i disapprove of the setup: as it's a single mesh, one can't make the paper not cast shadows, and so every single ray crossing them is a stupendously expensive caustic ray.
You'd do well to import the fixture as mesh, and after extracting the paper bit, set it to not cast shadows or, as there is really nothing to see inside it, make it a light material and be done.

Thanks for finding this out, will flag this to the cosmos devs.

EDIT: i attached a 20s render (to noise threshold, at that!) of the fixed feature, versus a 30 seconds of the original one. The difference in paper look can largely be fixed by raising the translucency color's value a bit.

EDIT 2: The light fixtures are indeed being looked into and optimised for what is possible. Notice that some changes may require more than the ideal amount of time, at this stage, as some changes may have to be made to more than the assets alone.