The schedulable entity for the system is the light-weight process
Scheduling priorities and classes are attributes of LWPs
and not processes.
When scheduling system calls accept a process on which to operate,
the operation is applied to each LWP in the process.
The system scheduler determines when LWPs run.
It maintains priorities based on configuration parameters,
process behavior, and user requests, and it uses these priorities to assign
LWPs to the CPU.
By default, the scheduler uses a time-sharing policy
similar to the policy used in previous releases.
A time-sharing policy adjusts priorities dynamically in an
attempt to provide good response time to interactive LWPs
and good throughput to CPU-intensive LWPs.
A fixed class policy is now available that is similar to
the time-sharing policy except that it does not adjust priorities.
The scheduler also offers a fixed priority scheduling policy.
Fixed priority scheduling, previously known as real-time scheduling,
allows users to set fixed priorities on a per-process or LWP basis.
In the default configuration, the highest-priority fixed priority
LWP always gets the CPU
as soon as it is runnable, even if system processes are runnable.
An application can therefore specify the exact order in which
An application also can be written so that its
fixed priority LWPs
have a guaranteed response time from the system.
Users can have absolute control over the sequence in which
certain LWPs run, and the amount of time each
LWP can use the CPU before another
LWP can use the CPU.
For most environments, the default scheduler configuration works well
and no fixed priority LWPs are needed;
you should not change configuration parameters
and users should not change scheduler properties of their
However, for some applications with strict timing constraints,
fixed priority LWPs are the only way to guarantee
that the application's requirements are met.
Fixed priority LWPs used carelessly can have a dramatic
negative effect on the performance of time-sharing LWPs.
Administrators can manage the relationship between processes and processors.
Processors can be taken offline or brought online.
Processes can be bound to a specific processor and processors can be bound
exclusively by a process.
Note that exclusive binding applies only to user-level processes
and not to kernel drivers.
That is, the exclusive binding of a user-level process
excludes other user level processes, but not a kernel driver.
This topic is addressed to administrators
who need to adjust the default configuration for the scheduler.
You should understand the scheduler for these reasons:
For programming information on the scheduler, see
The primary user command for controlling process and LWP
scheduling is priocntl(1).
The primary function call for controlling process and
LWP scheduling is priocntl(2).
Note that aging is a page management concept
and is not affected by the scheduler.
The scheduler has an overriding effect on the performance and perceived
performance of a system.
The default scheduler is tuned to perform well in
representative work environments;
therefore, you must understand how it operates to know whether you
can reconfigure it to better suit local needs.
A bug in a fixed priority program or a malicious fixed priority user
can lock out all other processing, including kernel processing.
Users need privilege to create fixed priority processes,
and presumably only trustworthy users have this privilege.
But you still must be aware that the scheduler
functions introduce new ways to cause trouble
and should be prepared for accidents and for misuse of these functions.
The rest of this topic is organized as follows:
``Overview of the process scheduler''
tells what the scheduler does and how it does it.
It also introduces scheduler classes.
``Configuring the scheduler''
describes how you can control
the scheduler using tunable parameters and the scheduler parameter tables:
ts_dptbl(4) for time-sharing parameters,
fp_dptbl(4) for fixed priority parameters,
and fc_dptbl(4) for fixed class parameters.
``Changing scheduler parameters with dispadmin''
tells how to display or change scheduler parameters
in a running system.
These dispadmin changes do not survive a reboot.
To make permanent changes in scheduler configuration,
use idtune(1M) to change the
scheduler parameter tables in the /etc/conf/mtune.d files.
``Managing processors and processes''
describes how you can manage the
relationship between processors and processes.
It tells how you can take processors offline or bring them online.
You also can bind processes to processors.
Overview of the process scheduler
© 2004 The SCO Group, Inc. All rights reserved.
UnixWare 7 Release 7.1.4 - 22 April 2004