求助！！

dujun1977

小弟正在看红皮书234，到关于schedtune这个命令的时候碰到了点很难懂的东西，麻烦那位dx给我解释一下好吗？
（ IBM ^ Certification Study Guide - AIX 5L Performance and System Tuning）

In AIX Version 4, the five possible values for the thread scheduling policy are asfollows:

SCHED_FIFO

This is a non-preemptive scheduling scheme. After a
thread with this policy is scheduled, it runs to completion
unless it is blocked, it voluntarily yields control of the
CPU, or a higher-priority thread becomes dispatchable.
Only fixed-priority threads can have a SCHED_FIFO
scheduling policy.

SCHED_RR

The thread has a fixed priority. When a SCHED_RR
thread has control at the end of the time slice, it moves to
the tail of the queue of dispatchable threads of its priority.
Only fixed-priority threads can have a SCHED_RR
scheduling policy.

SCHED_OTHER

This policy is defined by POSIX Standard 1003.4a as
implementation-defined. In AIX Version 4, this policy is
defined to be equivalent to SCHED_RR, except that it
applies to threads with nonfixed priority. The recalculation
of the running thread's priority at each clock interrupt
means that a thread may lose control because its priority
has risen above that of another dispatchable thread.

SCHED_FIFO2

This policy is the same as for SCHED_OTHER, except
that it allows a thread that has slept for only a short
amount of time to be put at the head of its run queue
when it is awakened. This policy is only available on AIX
Version 4.3.3 or later.

SCHED_FIFO3

A thread whose scheduling policy is set to
SCHED_FIFO3 is always put at the head of a run queue.
To prevent a thread belonging to the SCHED_FIFO2
scheduling policy from being put ahead of
SCHED_FIFO3, the run queue parameters are changed
when a SCHED_FIFO3 thread is enqueued, so that no
thread belonging to SCHED_FIFO2 will join the head of
the run queue. This policy is only available on AIX Version
4.3.3 or later.