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Old 09-06-2012, 06:53 PM   #1
helenwoo
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Registered: Jul 2012
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Linux fair schedule timeslice 1-5ms?


I understand that Linux fair timeslice varies depending on the nr_running and the relative weight of this fair task, but through code studying, I figured out the main idea is to keep the timeslice 1 to 5 ms. Please correct me if I understand it wrong. I must be wrong here but I just cannot figure out how!

Also knowing that HZ, or the number of system ticks per s, or the number of timer interrupts every second, is normally 200 or 100 for arm machine (and most non-desktop machines too), which gives us a 5 to 10 ms tick rate.

Timeslice is put in action by starting rq->hrtick_timer in set_next_entity(), every time a fair task is scheduled to run, and invoking resched_task() in timeout callback function hrtick(). This timer is simply one of the queued timers that are processed by timer irq handler on every tick, timer_tick()...run_local_timer(). There seems no other hidden secret.

Then how we can get a timeslice shorter than 5 ms? Please help me understand this. Thank you very much!

The reason of asking this was really because of the interest to know how fine the granularity of the time service we can get at the most. Thanks!

Last edited by helenwoo; 09-07-2012 at 02:47 PM. Reason: wanted to make the title more attractive, but it seems that title cannot be modified, so give up.
 
  


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