(Nearly) full tickless operation in 3.10

Posted May 8, 2013 16:24 UTC (Wed) by simlo (guest, #10866)
In reply to: (Nearly) full tickless operation in 3.10 by busterb
Parent article: (Nearly) full tickless operation in 3.10

After a little RTFC, I found that a HR-timer was used to calculate the next preemption point. I.e. instead of preempting on 100 Hz clock, it preempts exactly when the timeslot of the current process ends.

(Nearly) full tickless operation in 3.10

Posted May 8, 2013 18:50 UTC (Wed) by blitzkrieg3 (guest, #57873) [Link]

Okay, this makes absolutely perfect sense, but because of the way operating systems have been designed for the past 40 years, that solution completely eluded me.

(Nearly) full tickless operation in 3.10

Posted May 8, 2013 19:48 UTC (Wed) by intgr (subscriber, #39733) [Link] (3 responses)

I got the same impression as you. I think the wording in the article is unfortunate.

> Even then, a running CPU will only disable the timer tick if there is a single runnable process on that CPU. As soon as a second process appears, the tick is needed so that the scheduler can make the necessary time-slice decisions

It's misleading to call it "the tick" if it's not fixed to the HZ any more, seems more like a preemption timer.

(Nearly) full tickless operation in 3.10

Posted May 8, 2013 19:52 UTC (Wed) by corbet (editor, #1) [Link] (2 responses)

Confused. If two processes are running, the period tick will run just like it does now. What am I missing? What is misleading?

(Nearly) full tickless operation in 3.10

Posted May 8, 2013 20:23 UTC (Wed) by intgr (subscriber, #39733) [Link] (1 responses)

Sorry, it's much more likely that I am missing something.

AFAICT the scheduler doesn't switch tasks at every timer tick, even when there is contention for a CPU -- it has its own concept of timeslice length that changes with load. So why does a contended CPU need to run the timer tick if it's not going to switch tasks?

And grandparent wrote something that seemed to match that line of thinking:

> After a little RTFC, I found that a HR-timer was used to calculate the next preemption point. I.e. instead of preempting on 100 Hz clock, it preempts exactly when the timeslot of the current process ends.

(Nearly) full tickless operation in 3.10

Posted May 9, 2013 2:16 UTC (Thu) by nevets (subscriber, #11875) [Link]

It's confusing because there's two things at play here. There's the hrtick and the scheduler_tick.

The hrtick is used to denote exact time slices for the CFS scheduler to create more fairness. It really doesn't do much more than that. But this does not replace the scheduler_tick, which does among other things, keeps track of the SCHED_RR time slices, manages load balancing, and updates task timings.

But I'm sure in the future the hrtick may be used more to get rid of the periodic tick.