Friday, September 02, 2005

Know When to Be "nice" to OTher Users...and When Not to

39.9 Know When to Be "nice" to OTher Users...and When Not to
nice
The nice command modifies the scheduling priority of time-sharing
processes (for BSD and pre-V.4 releases of System V, all processes).
The GNU version is on the CD-ROM (the disc's install system will only
install nice if your system has the appropriate facilities).

If you're not familiar with UNIX, you will find its definition of
priority confusing - it's the opposite of what you would expect. A
process with a high nice number runs at low priority, getting
relatively little of the processor's attention; similarly, jobs with a
low nice number run at high priority. This is why the nice number is
usually called niceness: a job with a lot of niceness is very kind to
the other users of your system (i.e., it runs at low priority), while
a job with little niceness will hog the CPU. The term "niceness" is
awkward, like the priority system itself. Unfortunately, it's the only
term that is both accurate (nice numbers are used to compute
priorities but are not the priorities themselves) and avoids horrible
circumlocutions ("increasing the priority means lowering the
priority...").

Many supposedly experienced users claim that nice has virtually no
effect. Don't listen to them. As a general rule, reducing the priority
of an I/O-bound job (a job that's waiting for I/O a lot of the time)
won't change things very much. The system rewards jobs that spend most
of their time waiting for I/O by increasing their priority. But
reducing the priority of a CPU-bound process can have a significant
effect. Compilations, batch typesetting programs (troff, TeX, etc.),
applications that do a lot of math, and similar programs are good
candidates for nice. On a moderately loaded system, I have found that
nice typically makes a CPU-intensive job roughly 30 percent slower and
consequently frees that much time for higher priority jobs. You can
often significantly improve keyboard response by running CPU-intensive
jobs at low priority.

Note that System V Release 4 has a much more complex priority system,
including real-time priorities. Priorities are managed with the
priocntl command. The older nice command is available for
compatibility. Other UNIX implementations (including HP and
Concurrent) support real-time scheduling. These implementations have
their own tools for managing the scheduler.

The nice command sets a job's niceness, which is used to compute its
priority. It may be one of the most non-uniform commands in the
universe. There are four versions, each slightly different from the
others. BSD UNIX has one nice that is built into the C shell, and
another standalone version can be used by other shells. System V also
has one nice that is built into the C shell and a separate standalone
version.

Under BSD UNIX, you must also know about the renice(8) command
(39.11); this lets you change the niceness of a job after it is
running. Under System V, you can't modify a job's niceness once it has
started, so there is no equivalent.

NOTE: Think carefully before you nice an interactive job like a
text editor. See article 39.10.

We'll tackle the different variations of nice in order.
39.9.1 BSD C Shell nice

Under BSD UNIX, nice numbers run from -20 to 20. The -20 designation
corresponds to the highest priority; 20 corresponds to the lowest. By
default, UNIX assigns the nice number 0 to user-executed jobs. The
lowest nice numbers (-20 to -17) are unofficially reserved for system
processes. Assigning a user's job to these nice numbers can cause
problems. Users can always request a higher nice number (i.e., a lower
priority) for their jobs. Only the superuser (1.24) can raise a job's
priority.

To submit a job at a greater niceness, precede it with the modifier
nice. For example, the command:

% nice awk -f proc.awk datafile > awk.out

runs an awk command at low priority. By default, csh version of nice
will submit this job with a nice level of 4. To submit a job with an
arbitrary nice number, use nice one of these ways:

% nice +n command
% nice -n command

where n is an integer between 0 and 20. The +n designation requests a
positive nice number (low priority); -n request a negative nice
number. Only a superuser may request a negative nice number.
39.9.2 BSD Standalone nice

The standalone version of nice differs from C shell nice in that it is
a separate program, not a command built in to the C shell. You can
therefore use the standalone version in any situation: within
makefiles (28.13), when you are running the Bourne shell, etc. The
principles are the same. nice numbers run from -20 to 20, with the
default being zero. Only the syntax has been changed to confuse you.
For the standalone version, -n requests a positive nice number (lower
priority) and --n requests a negative nice number (higher
priority-superuser only). Consider these commands:

$ nice -6 awk -f proc.awk datafile > awk.out
# nice --6 awk -f proc.awk datafile > awk.out

The first command runs awk with a high nice number (i.e., 6). The
second command, which can be issued only by a superuser, runs awk with
a low nice number (i.e., -6). If no level is specified, the default
argument is -10.
39.9.3 System V C Shell nice

System V takes a slightly different view of nice numbers. nice levels
run from 0 to 39; the default is 20. The numbers are different but
their meanings are the same: 39 corresponds to the lowest possible
priority, and 0 is the highest. A few System V implementations support
real-time submission via nice. Jobs submitted by root with extremely
low nice numbers (-20 or below) allegedly get all of the CPU's time.
Systems on which this works properly are very rare and usually
advertise support for real-time processing. In any case, running jobs
this way will destroy multiuser performance. This feature is
completely different from real-time priorities in System V Release 4.

With these exceptions, the C shell version of nice is the same as its
BSD cousin. To submit a job at a low priority, use the command:

% nice command

This increases the command's niceness by the default amount (4, the
same as BSD UNIX); command will run at nice level 24. To run a job at
an arbitrary priority, use one of the following commands:

% nice +n command
% nice -n command

where n is an integer between 0 and 19. The +n entry requests a higher
nice level (a decreased priority), while -n requests a lower nice
level (a higher priority). Again, this is similar to BSD UNIX, with
one important difference: n is now relative to the default nice level.
That is, the command:

% nice +6 awk -f proc.awk datafile > awk.out

runs awk at nice level 26.
39.9.4 System V Standalone nice

Once again, the standalone version of nice is useful if you are
writing makefiles or shell scripts or if you use the Bourne shell as
your interactive shell. It is similar to the C shell version, with
these differences:

*

With no arguments, standalone nice increases the nice number by
10 instead of by 4; this is a significantly greater reduction in the
program's priority.
*

With the argument -n, nice increases the nice number by n
(reducing priority).
*

With the argument - -n, nice decreases the nice number by n
(increasing priority; superuser only).

Consider these commands:

$ nice -6 awk -f proc.awk datafile > awk.out
# nice --6 awk -f proc.awk datafile > awk.out

The first command runs awk at a higher nice level (i.e., 26, which
corresponds to a lower priority). The second command, which can be
given only by the superuser, runs awk at a lower nice level (i.e.,
14).

- ML from O'Reilly & Associates' System Performance Tuning, Chapter 3

39.10 A nice Gotcha

NOTE: It's NOT a good idea to nice a foreground job (12.1). If the
system gets busy, your terminal could "freeze" waiting to get enough
CPU time to do something. You may not even be able to kill (38.9) a
nice'd job on a very busy system because the CPU may never give the
process enough CPU time to recognize the signal waiting for it! And,
of course, don't nice an interactive program like a text editor unless
you like to wait... :-)

- JP

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