https://www.nixtutor.com/linux/changing-priority-on-linux-processes/
Changing Priority on Linux Processes
Wed, Apr 22, 2009
What is Priority and Why Should I Care?
When talking about processes priority is all about managing processor time. The Processor or CPU is like a human juggling multiple tasks at the same time. Sometimes we can have enough room to take on multiple projects. Sometimes we can only focus on one thing at a time. Other times something important pops up and we want to devote all of our energy into solving that problem while putting less important tasks on the back burner.
In Linux we can set guidelines for the CPU to follow when it is looking at all the tasks it has to do. These guidelines are called niceness or nice value. The Linux niceness scale goes from -20 to 19. The lower the number the more priority that task gets. If the niceness value is high number like 19 the task will be set to the lowest priority and the CPU will process it whenever it gets a chance. The default nice value is zero.
By using this scale we can allocate our CPU resources more appropriately. Lower priority programs that are not important can be set to a higher nice value, while high priority programs like daemons and services can be set to receive more of the CPU’s focus. You can even give a specific user a lower nice value for all of his/her processes so you can limit their ability to slow down the computer’s core services.
Checking the Priority of Running Processes
The easiest way to get a quick picture of what the current niceness priority on a process is to open up the top processes with:
top
top
q
h
ps -o pid,comm,nice -p 123
Setting priority on new processes
At this point you are probably wondering how you can set your own priority levels on processes. To change the priority when issuing a new command you do nice -n [nice value] [command]:
nice -n 10 apt-get upgrade
This will increment the default nice value by a positive 10 for the command, ‘apt-get upgrade’ This is often useful for times when you want to upgrade apps but don’t want the extra process burden at the given time. Remember a positive number is gives less priority for a process.
renice 10 -p 21827
This will increment the priority of the process with an id of 21827 to 10.
Note: Only root can apply negative nice values.
Setting Priority on Existing Processes
Obviously at some point you are going to want to alter the nice value of something that is already running. Just the other day I was doing an upgrade to Ubuntu Jaunty and Firefox started to become unusably slow. A quick renice command rescheduled the upgrade to a lower priority and I was back to surfing in no time.
To change the priority of an existing process just do renice [nice value] -p [process id]:
renice 10 -p 21827
This will increment the priority of the process with an id of 21827 to 10.
Note: Only root can apply negative nice values.
Setting Permanent Priority on all Processes for a Specific User
Sometimes it is helpful to give specific users lower priority than others to keep system resources allocated in the proper places like core services and other programs.
You can set the default nice value of a particular user or group in the /etc/security/limits.conf file.
/etc/security/limits.conf
It uses this syntax: [username] [hard|soft] priority [nice value]
backupuser hard priority 11 Fields Management for window 1:Def, whose current sort field is %CPU 2 Navigate with Up/Dn, Right selects for move then <Enter> or Left commits, 3 'd' or <Space> toggles display, 's' sets sort. Use 'q' or <Esc> to end! 4 5 * PID = Process Id 6 * USER = Effective User Name 7 * PR = Priority 8 * NI = Nice Value 9 * VIRT = Virtual Image (KiB) 10 * RES = Resident Size (KiB) 11 * SHR = Shared Memory (KiB) 12 * S = Process Status 13 * %CPU = CPU Usage 14 * %MEM = Memory Usage (RES) 15 * TIME+ = CPU Time, hundredths 16 * COMMAND = Command Name/Line 17 PPID = Parent Process pid 18 UID = Effective User Id 19 RUID = Real User Id 20 RUSER = Real User Name 21 SUID = Saved User Id 22 SUSER = Saved User Name 23 GID = Group Id 24 GROUP = Group Name 25 PGRP = Process Group Id 26 TTY = Controlling Tty 27 TPGID = Tty Process Grp Id 28 SID = Session Id 29 nTH = Number of Threads 30 P = Last Used Cpu (SMP) 31 TIME = CPU Time 32 SWAP = Swapped Size (KiB) 33 CODE = Code Size (KiB) 34 DATA = Data+Stack (KiB) 35 nMaj = Major Page Faults 36 nMin = Minor Page Faults 37 nDRT = Dirty Pages Count 38 WCHAN = Sleeping in Function 39 Flags = Task Flags <sched.h> 40 CGROUPS = Control Groups 41 SUPGIDS = Supp Groups IDs 42 SUPGRPS = Supp Groups Names 43 TGID = Thread Group Id 44 ENVIRON = Environment vars 45 vMj = Major Faults delta 46 vMn = Minor Faults delta 47 USED = Res+Swap Size (KiB) 48 nsIPC = IPC namespace Inode 49 nsMNT = MNT namespace Inode 50 nsNET = NET namespace Inode 51 nsPID = PID namespace Inode 52 nsUSER = USER namespace Inode 53 nsUTS = UTS namespace Inode