Set ionice for a multi-threaded application

I have a program that spawns multiple threads, all of which do fairly intensive IO, running on the background. I want to set the scheduling class to idle so that it doesn't clog up the system; however, ionice -c3 -p , where ` is the process ID, does not have the desired effect. Although the scheduling class for process is changed, when I launched iotop`, all the threads it had spawned still had the default priority (best-effort level 4). How do I change the IO priority of a program and all the threads or processes it has spawned on Linux?