Priority Guidelines

Within the two most common priority classes-time-critical and regular-there are certain broad guidelines recommended for choosing the priority level for a program.

• TIME-CRITICAL CLASS

  The guidelines for level within the time critical class are set to maximize the number of different applications that can successfully multitask in an OS/2 system. The guidelines are described in the following table.

  Recommended Priority Levels for Time-Critical Threads

  ┌────────────────────┬────────────────────┬────────────────────────────────────────────────────────────┐
  │Activity            │Range of Recommended│Justification/Comments                                      │
  │                    │Priority Levels     │                                                            │
  ├────────────────────┼────────────────────┼────────────────────────────────────────────────────────────┤
  │Robotics/Real time  │20-31               │OS/2 systems can be used on manufacturing lines to control  │
  │process control     │                    │equipment or in other real time process control             │
  │                    │                    │applications. In this case a slow response from the PC could│
  │                    │                    │cause expensive damage to equipment or even human injury.   │
  │                    │                    │Therefore the highest priority levels should be reserved for│
  │                    │                    │these applications.                                         │
  ├────────────────────┼────────────────────┼────────────────────────────────────────────────────────────┤
  │Communications      │10-19               │In communications, the inability to get the processor could │
  │                    │                    │cause a loss of data or communications sessions. Therefore  │
  │                    │                    │this class of applications is next highest.                 │
  ├────────────────────┼────────────────────┼────────────────────────────────────────────────────────────┤
  │Other               │0-09                │Other threads might need to preempt the foreground in       │
  │                    │                    │special cases (for example, Control-Break). These should be │
  │                    │                    │set below the other 2 classes.                              │
  └────────────────────┴────────────────────┴────────────────────────────────────────────────────────────┘
  

  In general, application performance is not a good reason to make a thread time critical.

• REGULAR CLASS

  In cases where explicit priority levels are set, they should follow the guidelines listed below.

  Recommended Priority Levels for Regular Priority Threads

  ┌────────────────────┬────────────────────┬──────────────────────────────────────────────────┐
  │Activity            │Range of Recommended│Justification                                     │
  │                    │Priority Level      │                                                  │
  ├────────────────────┼────────────────────┼──────────────────────────────────────────────────┤
  │Communications      │26-31               │Communications should take priority over other    │
  │                    │                    │background processing to increase overlap with    │
  │                    │                    │transmission and processing on the partner PC or  │
  │                    │                    │host.  This gives the best system performance.    │
  ├────────────────────┼────────────────────┼──────────────────────────────────────────────────┤
  │Other               │0-25.               │If an application has multiple threads it might be│
  │                    │                    │necessary to set them to several different        │
  │                    │                    │priorities to optimize the order in which they    │
  │                    │                    │run. A range of priority levels is provided to    │
  │                    │                    │facilitate this. (The default priority is regular │
  │                    │                    │class, level = 0.)                                │
  └────────────────────┴────────────────────┴──────────────────────────────────────────────────┘