free -m
内存算法 used=total-free 即 total=used+free
实际内存占用：used-buffers-cached 即 total-free-buffers-cached
实际可用内存：buffers+cached+free
free -m
内存算法 used=total-free 即 total=used+free
实际内存占用：used-buffers-cached 即 total-free-buffers-cached
实际可用内存：buffers+cached+free