Why does cycles max out 1 CPU core in GPU mode?

Edit: Sorry for the auto correct fails.

When I render a scene In GPU compute mode in Cycles it will use my GPU and 100% of one of my CPU cores. I thought it was a bottleneck in the CPU but when I overclock my CPU to 5ghz it did not give any performance gain. I heard it could be a bug in the Ubuntu system monitor or maybe it might be some kind of hybrid CPU+GPU render.

  • 3
    $\begingroup$ When using GPU render blender doesn't use the CPU for rendering. The CPU usage you see is probably for things like shuffling memory around/loading things from disk, etc. In many cases the CPU isn't at 100% at all. To render using the GPU and CPU simultaneously, see blender.stackexchange.com/q/3232/599 $\endgroup$
    – gandalf3
    Commented Feb 24, 2015 at 2:18
  • $\begingroup$ On the other hand, it isn't too uncommon to see high (easily could be 100% in one core) CPU usage while GPU rendering. Depending on the scene a lot of memory accesses etc might need to be done. I'm quite sure the CPU isn't being used for rendering directly in addition to the GPU, as cycles doesn't support this out of the box (afaik). $\endgroup$
    – gandalf3
    Commented Feb 27, 2015 at 20:27
  • $\begingroup$ Will this affect my performance since the cpu usege is only in one core and my cpu is 5ghz I can't get any better single core performance $\endgroup$ Commented Feb 27, 2015 at 23:01


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