SetPriorityClass equivalent on Linux

Question

I have a daemon-like application that does some disk-intensive processing at initialization. To avoid slowing down other tasks I do something like this on Windows:

SetPriorityClass(GetCurrentProcess(), PROCESS_MODE_BACKGROUND_BEGIN);

// initialization tasks

SetPriorityClass(GetCurrentProcess(), PROCESS_MODE_BACKGROUND_END);

// daemon is ready and running at normal priority

AFAIK, on Unices I can call nice or setpriority and lower the process priority but I can't raise it back to what it was at process creation (i.e. there's no equivalent to the second SetPriorityClass invocation) unless I have superuser privileges. Is there by any chance another way of doing it that I'm missing? (I know I can create an initialization thread that runs at low priority and wait for it to complete on the main thread, but I'd rather prefer avoiding it)

edit: Bonus points for the equivalent SetThreadPriority(GetCurrentThread(), THREAD_MODE_BACKGROUND_BEGIN); and SetThreadPriority(GetCurrentThread(), THREAD_MODE_BACKGROUND_END);

Answer 1

You've said that your processing is disk intensive, so solutions using nice won't work. nice handles the priority of CPU access, not I/O access. PROCESS_MODE_BACKGROUND_BEGIN lowers I/O priority as well as CPU priority, and requires kernel features that don't exist in XP and older.

Controlling I/O priority is not portable across Unices, but there is a solution on modern Linux kernels. You'll need CAP_SYS_ADMIN to lower I/O priority to IO_PRIO_CLASS_IDLE, but it is possible to lower and raise priority within the best effort class without this.

The key function call is ioprio_set, which you'll have to call via a syscall wrapper:

static int ioprio_set(int which, int who, int ioprio)
{
    return syscall(SYS_ioprio_set, which, who, ioprio);
}

For full example source, see here.

Depending on permissions, your entry to background mode is either IOPRIO_PRIO_VALUE(IO_PRIO_CLASS_IDLE,0) or IOPRIO_PRIO_VALUE(IO_PRIO_CLASS_BE,7). The sequence should then be:

#define IOPRIO_PRIO_VALUE(class, data)  (((class) << IOPRIO_CLASS_SHIFT) | data)

ioprio_set(IOPRIO_WHO_PROCESS, 0, IOPRIO_PRIO_VALUE(IO_PRIO_CLASS_BE,7));
// Do work
ioprio_set(IOPRIO_WHO_PROCESS, 0, IOPRIO_PRIO_VALUE(IO_PRIO_CLASS_BE,4));

Note that you many not have permission to return to your original io priority, so you'll need to return to another best effort value.