Hi!

I've got a port up and running a demo with the
following tasks and their priorities:

--------------------
Task Name  Priority
--------------------
Tmr Svc    4
BTest1     2
QConsB1    2
QProdB4    2
BTest2     1
IntMath    0
QConsB3    0
QConsB6    0
QProdB2    0
QProdB5    0
Reg1       0
Reg2       0
IDLE       0
Stats      0
--------------------

I've implemented a statistics task that prints the
statistics gathered every 60 seconds on the serial
console. The system tick is running at an interval
of 1ms, and the stats timer at 100us.

After letting the system run for some time we get the
following result:

-------------- RUN TIME STATISTICS ---------------
--------------------------------------------------
Task Name       1/10 Ticks      Percentage
--------------------------------------------------
IDLE            24828           <1%
IntMath         27909887        33%
QProdB2         88840           <1%
QConsB3         94174           <1%
QProdB5         1431541         1%
QConsB6         1447645         1%
Reg1            19473930        23%
Stats           8326            <1%
Reg2            27934058        33%
BTest2          64616           <1%
QConsB1         2738151         3%
QProdB4         2724660         3%
Tmr Svc         2801            <1%
BTest1          56573           <1%
--------------------------------------------------
TOTAL TICKS     8400000
STATS EXE TICKS 7
PRINT EXE TICKS 2301
--------------------------------------------------

After intuitively replacing "<1%" with 0,5%, we get
the following result:

--------------------------------------------------
Task Name   1/10 Ticks   Percentage   Priority
--------------------------------------------------
IntMath     27909887     33,00%       0
Reg2        27934058     33,00%       0
Reg1        19473930     23,00%       0
QConsB1      2738151      3,00%       2
QProdB4      2724660      3,00%       2
QConsB6      1447645      1,00%       0
QProdB5      1431541      1,00%       0
BTest1         56573      0,50%       2
BTest2         64616      0,50%       1
IDLE           24828      0,50%       0
QConsB3        94174      0,50%       0
QProdB2        88840      0,50%       0
Stats           8326      0,50%       0
Tmr Svc         2801      0,50%       4
--------------------------------------------------
SUM         84000030    100,50%     
--------------------------------------------------

The time divided by 10 pretty much adds up to 8400000
and the percentage almost adds up to 100% as well.

BUT it seems all other tasks are only stealing time
from task "Reg1". The tasks "Reg2" and "IntMath" seem
to get their fair share of the CPU.

Is this a result of an unfairness of the scheduler or
where is this result possibly coming from?

Can someone possibly shed some light on this?

Regards
Friedl

Some tasks use their entire time slice, other tasks do something very briefly then yield (be it explicitly by calling taskYIELD() or by blocking on a queue or semaphore, or even by posting to a queue or semaphore that unblocks a higher priority task resulting in preemption).

In this artificial demo task environment, tasks that execute after a task that only used part of a time slice will only ever get part of a time slice themselves. If they start half way through a time slice then they will get swapped out again at the end of the time slice. Real applications do not normally result in this type of execution pattern because real tasks are performing real functionality.

Regards.