AIX 5L Problem Determination - IBM Redbooks

schedtune limitations
The schedtune command can only be executed by root. Changes made by the
schedtune command last until the next reboot of the system. If a permanent
change in VMM or time-slice parameters is needed, an appropriate schedtune
command should be put in the /etc/inittab file.
schedtune example one
The following command will result in SCHED_R = 0 and SCHED_D = 0.5:
# /usr/samples/kernel/schedtune -r 0
This would mean that the CPU penalty was always 0, making priority absolute.
No background process would get any CPU time unless there were no
dispatchable foreground processes at all, as background processes in ksh are
started with adding 4 to the nice value of the parent shell. The priority values of
the threads would effectively be constant, although they would not technically be
fixed-priority threads.
schedtune example two
The following command would result in an effective SCHED_R = 1 and
SCHED_D = 1:
# /usr/samples/kernel/schedtune -r 32 -d 32
This would mean that long-running threads would reach a C value of 120 and
remain there, contending on the basis of their nice values. New threads would
have priority, regardless of their nice value, until they had accumulated enough
time slices to bring them within the priority value range of the existing threads.
schedtune example three
The most likely reason to manipulate the values would be to make sure that
background processes do not compete with foreground processes. By making
SCHED_R smaller, you can restrict the range of possible priority values. For
example:
# /usr/samples/kernel/schedtune -r 5
(SCHED_R = 0.15625, SCHED_D = 0.5) would mean that a foreground process
would never have to compete with a background process started with the
command nice -n 20. The limit of 120 CPU time slices accumulated would mean
that the maximum CPU penalty for the foreground process would be 18. In
Figure 10-2 on page 257, this relationship is graphically shown. Because the
CPU penalty will get a maximum value of 18, the foreground process with a nice
value of 20 will always, when it needs, get CPU. On the other hand, the
background process, with a nice value of 40, will use CPU only when the
foreground process does not need the CPU.
256 IBM ^ Certification Study Guide - AIX 5L Problem Determination Tools and Techniques