使用pthread来利用多个核心的问题

I'm doing a simple ray tracer in C++ using SDL for graphics and pthread for threading. And I have a problem making my program utilizing two cores, the threads work, they just don't drive both cores to 100%. To interface SDL I write directly to it's memory, SDL_Surface.pixels, so I assume that it can't be SDL locking me.I'm doing a simple ray tracer in C++ using SDL