[Qemu-devel] how qemu do io throttling?

[yue]

hi ,all
i now know there are 2 ways to implement io throttling,
one is cgroup, but it is only targeted at block device.
another is qemu itself.
i want to know the theory how qemu itself do io throttling? and in that case if 
it depends on the type of back storage(nfs,file,rbd, other posix fs)?
it better tell me where is the code .
 
thanks