写多个文件会在x秒后变慢

I have code which gets frames from a camera and then saves it to disk. The structure of the code is: multiple threads malloc and copy their frames into new memory, enqueue memory. Finally, another thread removes frames from queue and writes them (using ffmpeg API, raw video no compression) to their files (actually I'm using my own memory pool so malloc is only called when I need more buffers). I can have upto 8 files/cams open at the same time enqueing. I have code which gets frames from a camera and