Noisy scene even with 3K pass

Question

I know many many people have asked this before, but i really can't understand what is wrong...
I've tried everything: raised passes up to 3k, added denoising, indirect clamping, multi sample resolution, raised and lowered light bounces, doubled the resolution in order to scale it down later, removed all light sources except the external, the light path node for the glass material then removed glass from windows...

Nothing worked, i can't get a nice image...

So, i'm asking you to have a look at my file (a corrected copy can be found in the answer)...
The HDR image can be found here.
All other textures should be generated with noises

Result with 2000 Passes after taking advantage of light portals

Answer 1

Since this is an interior scene with windows being the primary light source, you will want to use a feature called Light Portals.

Basically, you add an area lamp into each window, align it so that the plane of the lamp is in line with the plane of the window, scale the lamp up (using the size value in the lamp settings) to be the same size as the window, and then check the Portal option that is also found in the lamp settings.

These portals help Cycles figure out where the light is coming from, and can greatly reduce noise.

Here's a good video that goes over setting them up and how they work: https://www.youtube.com/watch?v=1LjLyTBbl6s.

EDIT: Here's an actual comparison of your scene. I added a portal at each window/door/skylight, and used the exact same settings as you had. The only difference was that I changed the dimension scaling from 200% to 50% and the samples from 2000 to 50, just so I could do this quickly.

Here is with no Portals and 50 samples. It took 38s to render on my GPU.

Now here is the result with a portal at each light source (even the ones outside the camera view). It took only 4 seconds longer at just over 42 seconds.

And I think those results speak for themselves... :)

Here's the file too if you want to look at what I did. You will have to relocate the HDRi though, as I had to set it to a different path on my system.

EDIT #2 I also noticed that you have a large number of light bounces. Since the portals really brighten up the image as well as reduce the noise, you can lower the bounces back down, and it will make your image even brighter, as well as render a bit faster. A max of 8 bounces and min of 3 gave this result and shaved just over a second off the time as well.

Do keep in mind that this is an interior scene, and the nature of interior scenes is that there are a lot of light bounces, especially when using multiple light sources. Keeping the bounces low will help with noise, brightness and render times, although for a good final render a few thousand samples will still be necessary.