How to render a 30K image with cycles? I have to render an image of 27000 x 7000 for a 150dpi print. The scene is quite big, but cycles preview and a render in 2700x7000 works as expected.

If I hit render my memory raises up to 20gb, but my cpu usage is at 5%. Why cycles does not using the full cpu power during the render process?

Here are my rendersettings:

Pathtracing
Samples:3000
Tilesize: 64x64
Bounces: Max 12 | Min 3
Diffuse: 4
Glossy: 4
Transmission: 12
Volume: 1


Is there a way to optimize things?
With very large image resolutions Blender can run out of RAM. The operating system will then start using the hard disk for memory which is slow and will cause low CPU usage (swapping).

To reduce memory usage for very high resolution renders:


Enable Save Buffers in the Performance panel so Blender does not keep the full render in memory during render
Render from the command line so no display buffer must be kept in memory
Output to an OpenEXR file so one tile can be written at a time