I am rendering a scene where the middle of the image is simple Diffuse material, but the outer edges have a lot of Volumetric material.
When the render tiles start in the middle of the image, they are ALL therefore only rendering Diffuse. At this point overall CPU usage is running at 100%.
When the circle of tiles widens, some tiles are still rendering simple Diffuse and some are now rendering Volumetrics. At this point overall CPU usage drops as low as 50%.
Yet once all tiles have widened into the volumetric areas (all Diffuse has been done), At this point overall CPU usage goes back up to 90%-100%.
It seems to me that all tiles can only go as fast as the slowest tile. That is to say that if, by itself, a Diffuse tile takes 1 minute, and by itself as Volumetric tile takes 4mins, then if you have a Diffuse and a Volumetric tile rendering at the same time, then the Volumetric will take 4mins but the Diffuse will be stretched out to also take 4mins albeit at 25% burden.
Does this sound correct? If not, why is this happening, and in any case, what can be done to max out CPU at all times thereby speeding rendering?
Like Zeealpal said, anytime Cycles shoots a ray into a volumetric domain (especially a heterogeneous one) it has to do the volumetric ray marching, even if there’s no actual volume visible at that point.
If the center diffuse area does not have a volume domain then you may be dealing with a bug that should be reported.
What we have is a camera looking fairly downwardly at rock floor (Diffuse) with some areas of low level mist over it (Volumetric) - mist that clings to the floor like in horror films. However there is a character (Diffuse) standing on the rock floor with his feet in the mist, but the rest of him rises out of the mist. As the character is quite bulky, this means that 50% of what takes up the camera view is the character, as he obscures a lot of the mist below him.