DOCUMENT:Q106253  08-DEC-1993  [W_NT]
TITLE   :Program priority and Multithreaded applications.
PRODUCT :Windows NT
PROD/VER:3.10
OPER/SYS:WINDOWS
KEYWORDS:

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The information in this article applies to:
 - Microsoft Windows NT operating system version 3.1
 - Microsoft Windows NT Advanced Server version 3.1
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SUMMARY
=======

When a program is started under Windows NT it will have a priority
class that is either Normal or equal to the priority class specified
in the command line when using the START command (Idle, Normal, High
or Realtime). The priority class determines which priority (from 1 to
31) the program will run at. In a multithreaded application, threads
spawned by the program can be lower or higher than the current
priority class of the program; however all thread priorities will be
relative to the priority class of the parent program. While a program
can change it’s priority class after starting, all of its threads will
still have the same relative priority.

MORE INFORMATION
================

When a process spawns a thread, one of the parameters in the function
call sets the thread priority relative to the priority of the parent
process. This parameter can have one of the following values: Idle,
Lowest, Below Normal, Normal, Above Normal, Highest and Time Critical.
The starting priority set for the thread will be based upon that
parameter and the current priority of the process starting the thread.
The priorities are as follows:

RELATIVE                  PROCESS PRIORITY CLASS
THREAD                Normal, in      Normal, in
PRIORITY       Idle   Background      Foreground    High    Realtime
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Time Critical   15        15              15         15        31
Highest          6         9              11         15        26
Above Normal     5         8              10         14        25
Normal           4         7               9         13        24
Below Normal     3         6               8         12        23
Lowest           2         5               7         11        22
Idle             1         1               1          1        16

The table above lists the base priority of a thread which is set by
calling the function SetThreadPriority, however a thread’s priority
can change as the thread executes. The system can boost a thread’s
priority higher as time goes on and then reduce the priority back down
to the base, however Windows NT will never reduce a thread lower than
it’s base priority. The system will boost a thread’s priority when a
user interacts directly with a thread, including mouse and keyboard
input or simply bringing the thread to the foreground.

For more information on process priorities, query on the following key
words:

priority and process

Additional reference words: 1.00
KBCategory:
KBSubCategory: pertune

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Copyright Microsoft Corporation 1993.