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What does 'U<s' mean in ps output on Mac?

I see the following ps output. What does U<s mean? How to force it running?

root              3531   0.0  0.4  4737660  34172   ??  U<s  Wed09AM 189:36.15 /System/Library/CoreServices/backupd.bundle/Contents/Resources/backupd
Solution

  • U is uninterruptible wait

    < is high priority

    s is session leader

    The last 2 are in common with other systems; I've never seen U before (uninterruptible wait is usually 'D' since disk I/O is a common case).

    It can't be "forced" to run except by completion of whatever it's waiting for, which is not apparent from just the state.

    How do you know it's not running? I imagine that a backup process is I/O bound so it will quite likely be in an I/O wait when you look at it - but that doesn't mean it's been waiting all alone; it could be flipping between waiting and running quite rapidly.