🚀 go-pugleaf

Jason S Birch (jasonb@cs.uwa.edu.au) writes:
>mlelstv@serpens.rhein.de (Michael van Elst) writes:
>>jasonb@cs.uwa.edu.au (Jason S Birch) writes:
>>Of course you can drop the priority of all these tasks to the same
>>level. But apparently this is not what happens with Executive.
>
>Executive dynamically adjusts the priorities every second so that, over
>time, several CPU-intensive tasks (originally at the same priority)
>will indeed get equal amounts of CPU time. This satisfies your
>requirement for timeslicing, except the timeslices tend to be in
>quantums of one second rather than four VBLs. 

  This depends on the used scheduler. The standard-scheduler in
  Executive does this, but the better ones, especially the queues-
  scheduler works differently. It mimics BSD-style multilevel
  feedback queue scheduler, although tasks won't migrate between
  queues until priorities are recalculated every second. Let
  me quote from Executive documentation:

  ------
    This scheduler divides the dynamic priority range to eight priority
    levels and runs several tasks with the same priority. Tasks that use
    approximately the same amount of CPU time get same priority.

    Exec will Round-Robin tasks that have same priority. Each task is
    run for a time-slice called Quantum and then the next task gets
    chance to run. Exec normally uses a fixed Quantum value 4, but
    this scheduler adjusts the Quantum value for each task to maximize
    throughput of CPU-intensive tasks and to make interactive tasks
    more responsive. Here are the Quantum values given to tasks with
    different priorities:

      PRIORITY                           QUANTUM
       above dynamic range                  4
       scheduled (inside dynamic range)    2-10
       below dynamic range                  12

    For scheduled tasks the Quantum is the largest (10) when task's
    priority is close to dynamicmax priority (by default -100).

    This scheduler tries to eliminate "wobbling" priorities by not
    letting a task migrate upwards immediately. Task has to wait
    for one second before its priority can be raised.

    This is currently the best scheduler in Executive.
  ------

  When using this scheduler, tasks that use approximately the same
  amout of CPU time, will soon get the same priority and will be
  "round-robined" by Exec. For example, this Ps-client's output
  is sorted by task priority:

  PID  PRI RPRI NICE TYPE STATE      TIME  IDLE    CPU% NAME              
  295  -50    1    0 proc wait     0.801s    3m   0.00% CygnusEd          
  287  -50    1    0 proc wait    22.707s    6s   0.38% CygnusEd          
    1  -62    0    0 bcli wait     2.530s         4.94% [find]            
   14  -62    0    0 bcli wait    13.008s         7.09% [find]            
   10  -75    0    0 bcli ready    4.592s        13.91% [lha]             
   15  -75    0    0 bcli ready   22.401s        20.26% [lha]             
-- 
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       Petri Nordlund             __///         petrin@megabaud.fi
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