# nice and priority (PRI in top)



## rambetter (Dec 12, 2008)

I have a game server process on my FreeBSD 7.0 server that needs to be the top priority process - nothing should get in its way.  So I used renice as root to set the nice value for the game server process to -20.  Now I have a few questions:

1.

Is this safe?  Let's say the process with nice value -20 becomes rogue and starts taking up all of the CPU (for example a tight infinite loop in the code).  Will I still be able to log in to the machine and kill the process?

2.

I'm looking at the output of top.  I see that the PRI column in top for the game server process is 76.  A lot of other running processes have priorities in the 96 range.  Does PRI mean the lower, the more important?  I have some httpd processes with PRI 4 and a  rtorrent process with PRI 4, and sshd process with PRI 4.  Why are these priorities so low?  The rtorrent and httpd processes with PRI 4?  What does PRI mean?  I looked at the man page for top but that didn't really explain it very well.


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## DutchDaemon (Dec 13, 2008)

nice/renice interfere with the system's priority scheduling (PRI), but they do not overrule it completely, so it's very possible that certain processes have a higher priority (based on nice value, cpu use, duration and some other factors), than processes you've reniced. In other words, a renice -20 does not mean every other process has to take a back seat to the process you're giving absolute priority. Therefor, it's quite unlikely that a rogue reniced process will lock you out of your system entirely -- slowdown may occur, of course.

See also man nice and man rtprio.

Oh, and yes: a lower PRI number is a higher scheduling priority.


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