Learn Linux, 101
Process execution priorities
Keeping your eye on what's going on
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Overview
This article grounds you in the basic Linux techniques for managing execution process priorities. Learn to:
- Understand process priorities
- Set process priorities
- Change process priorities
This article helps you prepare for Objective 103.6 in Topic 103 of the Linux Professional Institute's Junior Level Administration (LPIC-1) exam 101. The objective has a weight of 2.
Prerequisites
To get the most from the articles in this series, you should have a basic knowledge of Linux and a working Linux system on which you can practice the commands covered in this article. Sometimes different versions of a program will format output differently, so your results may not always look exactly like the listings and figures shown here. The results in the examples shown here were obtained on a Ubuntu 9.10 (Karmic Koala) distribution. This article builds on the concepts discussed in the previous article "Learn Linux 101: Create, monitor, and kill processes."
Knowing your priorities
Linux, like most modern operating systems, can run multiple processes. It does this by sharing the CPU and other resources among the processes. If one process can use 100% of the CPU, then other processes may become unresponsive.
If you run the top command, its
default is to display processes in decreasing order according to their CPU
usage, as shown in Listing 1. In the previous article, "Learn Linux 101: Create, monitor,
and kill processes," we showed a Poor
Man's Clock script, which prints the time on the console every 30 seconds and
does nothing for the rest of the time. If we ran that process, it probably
wouldn't make it onto the output list from top
because the process spends most of its time not using the CPU.
Listing 1. Typical output from top on a Linux workstation
top - 08:00:52 up 1 day, 10:20, 5 users, load average: 0.04, 0.08, 0.04
Tasks: 172 total, 1 running, 171 sleeping, 0 stopped, 0 zombie
Cpu(s): 3.7%us, 0.3%sy, 0.0%ni, 95.6%id, 0.0%wa, 0.0%hi, 0.3%si, 0.0%st
Mem: 4057976k total, 1777976k used, 2280000k free, 225808k buffers
Swap: 10241428k total, 0k used, 10241428k free, 655796k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
11220 ian 20 0 555m 106m 27m S 8 2.7 36:06.16 firefox
7 root 15 -5 0 0 0 S 1 0.0 10:59.36 ksoftirqd/1
10849 ian 20 0 212m 15m 10m S 0 0.4 0:08.11 gnome-terminal
1 root 20 0 19584 1888 1196 S 0 0.0 0:00.83 init
2 root 15 -5 0 0 0 S 0 0.0 0:00.01 kthreadd
3 root RT -5 0 0 0 S 0 0.0 0:00.02 migration/0
4 root 15 -5 0 0 0 S 0 0.0 0:01.08 ksoftirqd/0
5 root RT -5 0 0 0 S 0 0.0 0:00.00 watchdog/0
6 root RT -5 0 0 0 S 0 0.0 0:00.03 migration/1Your system may have many commands that are capable of using lots of CPU. Examples include movie editing tools, and programs to convert between different image types or between different sound encoding, such as mp3 to ogg.
When you only have one or a limited number of CPUs, you need to decide how to share those limited CPU resources among several competing processes. This is generally done by selecting one process for execution and letting it run for a short period (called a timeslice), or until it needs to wait for some event, such as IO to complete. To ensure that important processes don't get starved out by CPU hogs, the selection is done based on a scheduling priority. The NI column in Listing 1 above, shows the scheduling priority or niceness of each process. Niceness generally ranges from -20 to 19, with -20 being the most favorable or highest priority for scheduling and 19 being the least favorable or lowest priority.
Using ps to find niceness
In addition to the top command, you can also
display niceness values using the ps command.
You can either customize the output as you saw in the article
"Learn Linux 101: Create, monitor,
and kill processes,"
or you can just use the -l option to get a long
listing. The output of ps -l is
shown in Listing 2. As with top, look for the
niceness value in the NI column.
Listing 2. Using ps to find niceness
ian@attic4:~$ ps -l F S UID PID PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD 0 S 1000 26502 26501 0 80 0 - 5368 wait pts/4 00:00:00 bash 0 R 1000 27046 26502 0 80 0 - 1684 - pts/4 00:00:00 ps
Default niceness
You may have guessed from Listing 1 or Listing 2 that the default niceness,
at least for processes started by regular users, is 0. This is usually the
case on current Linux systems. You can verify the value for your shell and
system by running the nice command with no
parameters as shown in Listing 3.
Listing 3. Checking default niceness
ian@attic4:~$ nice 0
Setting priorities
Before we look at how to set or change niceness values, let's build a little CPU-intensive script that will show how niceness really works.
A CPU-intensive script
We'll create a small script that just uses CPU and does little else. The script takes two inputs, a count and a label. It prints the label and the current date and time, then spins, decrementing the count till it reaches 0, and finally prints the label and the date again. This script shown in Listing 4 has no error checking and is not very robust, but it illustrates our point.
Listing 4. CPU-intensive script
ian@attic4:~$ echo 'x="$1"'>count1.sh ian@attic4:~$ echo 'echo "$2" $(date)'>>count1.sh ian@attic4:~$ echo 'while [ $x -gt 0 ]; do x=$(( x-1 ));done'>>count1.sh ian@attic4:~$ echo 'echo "$2" $(date)'>>count1.sh ian@attic4:~$ cat count1.sh x="$1" echo "$2" $(date) while [ $x -gt 0 ]; do x=$(( x-1 ));done echo "$2" $(date)
If you run this on your own system, you might see output similar to Listing 5. Depending on the speed of your system, you may have to increase the count value to even see a difference in the times. This script uses lots of CPU, as we'll see in a moment. If your default shell is not Bash, and if the script does not work for you, then use the second form of calling shown below. If you are not using your own workstation, make sure that it is okay to use lots of CPU before you run the script.
Listing 5. Running count1.sh
ian@attic4:~$ sh count1.sh 10000 A A Wed Jan 20 08:34:16 EST 2010 A Wed Jan 20 08:34:16 EST 2010 ian@attic4:~$ bash count1.sh 99000 A A Wed Jan 20 08:34:20 EST 2010 A Wed Jan 20 08:34:22 EST 2010
So far, so good. Now let's create a command list to run the script
in background and launch the top command to see
how much CPU the script is using. (See the previous article
"Learn Linux 101: The Linux command
line"
for a refresher on command lists.) The command list is shown in
Listing 6 and the output from top in Listing 7.
Listing 6. Running count1.sh and top
ian@attic4:~$ (sh count1.sh 5000000 A&);top
Listing 7. Using lots of CPU
top - 15:41:15 up 1 day, 17:59, 6 users, load average: 0.20, 0.06, 0.02
Tasks: 169 total, 2 running, 167 sleeping, 0 stopped, 0 zombie
Cpu(s): 52.1%us, 0.7%sy, 0.0%ni, 47.3%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 4057976k total, 1393772k used, 2664204k free, 235596k buffers
Swap: 10241428k total, 0k used, 10241428k free, 662592k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
26756 ian 20 0 4004 588 496 R 100 0.0 0:03.53 sh
11220 ian 20 0 555m 101m 27m S 5 2.6 57:58.07 firefox
26757 ian 20 0 19132 1364 980 R 0 0.0 0:00.03 top
1 root 20 0 19584 1888 1196 S 0 0.0 0:00.89 init
2 root 15 -5 0 0 0 S 0 0.0 0:00.01 kthreaddNot bad. We are using 100% of one of the CPUs on this system with just a
simple script. If you want to stress multiple CPUs, you can add an extra
invocation of count1.sh to the command list. If
we had a long running job such as this, we might find that it interfered
with our ability (or the ability of other users) to do other work on our
system.
Using nice to set priorities
Now that we can keep a CPU busy for a while, we'll see how to set a priority for a process. To summarize what we've learned so far:
- Linux and UNIX® systems use a priority system with 40 priorities, ranging from -20 (highest priority) to 19 (lowest priority.
- Processes started by regular users usually have priority 0.
- The
pscommand can display the priority (nice, or NI, level, for example) using the-loption. - The
nicecommand displays our default priority.
The nice command can also be used to start a
process with a different priority. You use the
-n or (--adjustment)
option with a positive value to increase the priority value and a negative
value to decrease it. Remember that processes with the lowest priority
value run at highest scheduling priority, so think of increasing the
priority value as being nice to other processes. Note that you
usually need to be the superuser (root) to specify negative priority
adjustments. In other words, regular users can usually only make their
processes nicer.
To demonstrate the use of nice to set
priorities, let's start two copies of the
count1.sh script in different subshells at the
same time, but give one the maximum niceness of 19. After a second we'll
use ps -l to display the process
status, including niceness. Finally, we'll add an arbitrary 30-second
sleep to ensure the command sequence finishes after the two subshells do.
That way, we won't get a new prompt while we're still waiting for output.
The result is shown in Listing 8.
Listing 8. Using nice to set priorities for a pair of processes
ian@attic4:~$ (sh count1.sh 2000000 A&);(nice -n 19 sh count1.sh 2000000 B&);\ > sleep 1;ps -l;sleep 10 A Thu Jan 21 14:38:39 EST 2010 B Thu Jan 21 14:38:39 EST 2010 F S UID PID PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD 0 R 1000 946 1 99 80 0 - 1001 - pts/3 00:00:01 sh 0 R 1000 948 1 99 99 19 - 1001 - pts/3 00:00:01 sh 0 R 1000 952 32408 0 80 0 - 1684 - pts/3 00:00:00 ps 0 S 1000 32408 32407 0 80 0 - 5368 wait pts/3 00:00:02 bash A Thu Jan 21 14:38:45 EST 2010 B Thu Jan 21 14:38:45 EST 2010
Are you surprised that the two jobs finished at the same time? What happened to our priority setting? Remember that the script occupied one of our CPUs. This particular system runs on an AMD Athlon™ 7750 dual-core processor, which is very lightly loaded, so each core ran one process, and there wasn't any need to prioritize them.
So let's try starting four processes at four different niceness levels (0, 6, 12, and18) and see what happens. We'll increase the busy count parameter for each so they run a little longer. Before you look at Listing 9, think about what you might expect, given what you've already seen.
Listing 9. Using nice to set priorities for four of processes
ian@attic4:~$ (sh count1.sh 5000000 A&);(nice -n 6 sh count1.sh 5000000 B&);\ > (nice -n 12 sh count1.sh 5000000 C&);(nice -n 18 sh count1.sh 5000000 D&);\ > sleep 1;ps -l;sleep 30 A Thu Jan 21 16:06:00 EST 2010 C Thu Jan 21 16:06:00 EST 2010 D Thu Jan 21 16:06:00 EST 2010 B Thu Jan 21 16:06:00 EST 2010 F S UID PID PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD 0 R 1000 1422 1 94 80 0 - 1001 - pts/3 00:00:00 sh 0 R 1000 1424 1 42 86 6 - 1001 - pts/3 00:00:00 sh 0 R 1000 1427 1 56 92 12 - 1001 - pts/3 00:00:00 sh 0 R 1000 1431 1 14 98 18 - 1001 - pts/3 00:00:00 sh 0 R 1000 1435 32408 0 80 0 - 1684 - pts/3 00:00:00 ps 0 S 1000 32408 32407 0 80 0 - 5368 wait pts/3 00:00:02 bash A Thu Jan 21 16:06:14 EST 2010 B Thu Jan 21 16:06:17 EST 2010 C Thu Jan 21 16:06:26 EST 2010 D Thu Jan 21 16:06:30 EST 2010
With four different priorities, we see the effect of the different niceness values as each job finishes in priority order. Try experimenting with different nice values to demonstrate the different possibilities for yourself.
A final note on starting processes with nice; as
with the nohup command, you cannot use a
command list or a pipeline as the argument of
nice.
Changing priorities
renice
If you happen to start a process and realize that it should run at a
different priority, there is a way to change it after it has started,
using the renice command. You specify an
absolute priority (and not an adjustment) for the process or processes to
be changed as shown in Listing 10.
Listing 10. Using renice to change priorities
ian@attic4:~$ sh count1.sh 10000000 A& [1] 1537 ian@attic4:~$ A Thu Jan 21 16:17:16 EST 2010 sh count1.sh 1renice 1 1537;ps -l 1537 1537: old priority 0, new priority 1 F S UID PID PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD 0 R 1000 1537 32408 99 81 1 - 1001 - pts/3 0:13 sh count1.sh 100 ian@attic4:~$ renice +3 1537;ps -l 1537 1537: old priority 1, new priority 3 F S UID PID PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD 0 R 1000 1537 32408 99 83 3 - 1001 - pts/3 0:18 sh count1.sh 100
Remember that you have to be the superuser to give your processes higher scheduling priority and make them less nice.
You can find more information on nice and
renice in the man pages.
Downloadable resources
Related topics
- Use the developerWorks roadmap for LPIC-1 to find the developerWorks articles to help you study for LPIC-1 certification based on the April 2009 objectives.
- At the LPIC Program site, find detailed objectives, task lists, and sample questions for the three levels of the Linux Professional Institute's Linux system administration certification. In particular, see their April 2009 objectives for LPI exam 101 and LPI exam 102. Always refer to the LPIC Program site for the latest objectives.
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- Review the entire LPI exam prep series on developerWorks to learn Linux fundamentals and prepare for system administrator certification based on earlier LPI exam objectives prior to April 2009.
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- The Linux Documentation Project has a variety of useful documents, especially its HOWTOs.
- In the developerWorks Linux zone, find more resources for Linux developers, and scan our most popular articles and tutorials.
- See all Linux tutorials and Linux tips on developerWorks.