Whenever I render a video using the blender video editor only one core is ever used at any time. I have an 8 core CPU(FX-9590) yet regardless of whether blender is set to auto detect or a fixed 8 threads only one core ever sits at 100% at any given time. The rest of the cores sit and really low usage as if blender is completely ignoring them. I'm running blender 2.77a on Linux. I'm using Theora as the codec with an ogg container and Vorbis audio. How do I get blender to use all my cores?
First and foremost, there is a significant difference between multi-threading and multi-core. Depending on the task that needs to be processed, multiple threads are also possible on a single core.
Blender VSE got some improvements during the Gooseberry Project, unfortunately most of them were just cosmetics and multiple threads on multiple cores was not on the roadmap. AFAIK there's nothing planned to change that in the near future.
You can use Pulverize to do multi-process rendering from Blender's Video Sequence Editor:
Pulverize will render multiple parts of your video in separate processes, then concatenate them into one video file using FFMPEG. It uses the Blender command line to select frame ranges for each render.
There is a Python script that splits video then renders it on mulitple cores and then stitches result.
Author says it is cross platform.