You can choose either the iray renderer or the mental ray renderer to be the ActiveShade renderer.
Viewport being rendered in an iray ActiveShade Floater
While an iray or mental ray ActiveShade window or viewport is open, you can't start a Production rendering. Because of this restriction, sample slots and material/map previews in the Material Editor, and the Exposure Control Render Preview window don't render while the ActiveShade window or viewport is open, either.
To resume using the Production renderer, close the ActiveShade Floater (or change the viewport to a different mode) and then use the Render flyout to choose (Render Production).
When the iray renderer is assigned to ActiveShade, you can use the Hardware Resources rollout to specify how many CPUs (Central Processing Units) and GPUs (Graphics Processing Units) are used by iray ActiveShade rendering.
The ActiveShade Floater or viewport updates interactively to show the following kinds of changes to the scene:
The iray ActiveShade Floater does not update to show the following kinds of changes to the scene:
To see these kinds of changes, you need to restart ActiveShade. Close the ActiveShade window or viewport, then on the main toolbar click (ActiveShade) once more. Or if the Render Setup dialog is open, you can restart ActiveShade by clicking the ActiveShade button.
Lighting levels, shadows, and reflections are physically accurate, provided the scene uses physically based materials and photometric lighting. This is always the case with the iray and mental ray renderers.
When you draw an area to render (right-click the window of the ActiveShade Floater and choose Draw Region), the iray or mental ray renderer renders only that region, but it doesn't clear the rest of the window the way the scanline renderer does.
Draw Region with iray ActiveShade
Only the Water - Pool material has changed.