Pre-emption and Priority Calculation

A thread is pre-empted when higher priority work becomes ready to process. Under normal circumstances the pre-empting thread will run then give up its time-slice and eventually the original thread will be re-scheduled.

It is possible for a thread not to be re-scheduled if a higher priority thread will not give up the processor. However, the OS/2 scheduler applies dynamic boost to priorities according to resource requirements and makes priority comparisons based on a calculated priority. The elements involved in the priority calculation are the following:

TCBPriClass

The thread's priority class. There are four classes, which in order of priority are:
• Time-critical

• Foreground Server (or fixed high)

• Regular

• Idle
TCBPriLevel
The priority delta which may range from 0x00 to 0x1f.
TCBPriClassMod
The priority boosts which may be any of the combined values:

0x04

Keyboard Boost
0x08
CPU Starvation Boost
0x10
Device I/O Boost
0x20
Foreground Boost
0x40
Window Boost
0x80
VDM Simulated Interrupt.
TCBPriorityMin
The minimum allowed priority. Normally 0 but set when priority inversion becomes a possibility. This is discussed later.

Priority is calculated by forming an index by ORing TCBPriClass and TCBPriClassMod and reading a constant value from the priority table. The low byte of this is then further ORed with the TCBPriLevel.

The following diagram shows the priority table.

┌───────────────────────────────────────────────────────────────────────────┐│     Table Index = (TCBPriClass | TCBPriClassMod)                          │
│                                                                           │
│     Starved         08 --------------------------------------------------+│
│     Device I/O      10 -------------------------------------------------+|│
│     Foreground      20 ------------------------------------------------+||│
│     Window          40 -----------------------------------------------+|||│
│     VDM Interrupt   80 ----------------------------------------------+||||│
│                                                                      |||||│
│ +-- TCBPriClass                                                      |||||│
│ |                                                                    |||||│
│ |   ============================================================     |||||│
│ |   Not Keyboard                ||  Keyboard                         |||||│
│ |                               ||                                        │
│ +-> Server  Idle    Regular TC  ||  Server  Idle    Regular TC       IWFDS│
│                                                                           │
│                                                                           │
│     0x300,  0x100,  0x200,  0x800,  0x300,  0x100,  0x200,  0x800,// -----│
│     0x62f,  0x100,  0x61f,  0x800,  0x62f,  0x100,  0x61f,  0x800,// ----S│
│     0x72f,  0x100,  0x71f,  0x800,  0x72f,  0x100,  0x71f,  0x800,// ---D-│
│     0x72f,  0x100,  0x71f,  0x800,  0x72f,  0x100,  0x71f,  0x800,// ---DS│
│     0x300,  0x100,  0x300,  0x800,  0x300,  0x100,  0x400,  0x800,// --F--│
│     0x62f,  0x100,  0x61f,  0x800,  0x62f,  0x100,  0x61f,  0x800,// --F-S│
│     0x74f,  0x100,  0x73f,  0x800,  0x74f,  0x100,  0x73f,  0x800,// --FD-│
│     0x74f,  0x100,  0x73f,  0x800,  0x74f,  0x100,  0x73f,  0x800,// --FDS│
│     0x500,  0x100,  0x500,  0x800,  0x500,  0x100,  0x500,  0x800,// -W---│
│     0x62f,  0x100,  0x61f,  0x800,  0x62f,  0x100,  0x61f,  0x800,// -W--S│
│     0x74f,  0x100,  0x73f,  0x800,  0x74f,  0x100,  0x73f,  0x800,// -W-D-│
│     0x74f,  0x100,  0x73f,  0x800,  0x74f,  0x100,  0x73f,  0x800,// -W-DS│
│     0x500,  0x100,  0x500,  0x800,  0x500,  0x100,  0x500,  0x800,// -WF--│
│     0x62f,  0x100,  0x61f,  0x800,  0x62f,  0x100,  0x61f,  0x800,// -WF-S│
│     0x74f,  0x100,  0x73f,  0x800,  0x74f,  0x100,  0x73f,  0x800,// -WFD-│
│     0x74f,  0x100,  0x73f,  0x800,  0x74f,  0x100,  0x73f,  0x800,// -WFDS│
└───────────────────────────────────────────────────────────────────────────┘

Notes

VDM Simulated interrupts always result in a value of 0x800

Foreground server class is not affected by the keyboard boost.

Time-critical class is not affected by any boosts.

Idle class is not affected by any boosts.

By examining the priority table it is clear that idle class will always be pre-empted by any other class.

Time-critical class can never be pre-empted by any other class.

Time-critical threads can only be pre-empted by other time-critical threads with a higher delta.

Server and Regular class threads may pre-empt each other depending on priority boosts and delta.

The key to looking at pre-emption problems is to look for other CPU bound threads of a higher priority. In particular time-critical threads.

.P displays the current calculated priority for each thread.

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