Does the scheduler run every time jiffies is incremented?

Does the scheduler run every time jiffies is incremented?

[Francis Litterio]

I'm reading Rubini's "Linux Device Drivers" and it isn't clear to me
whether the scheduler runs every time the timer interrupt increments
jiffies or less frequently.

Does the scheduler run every time jiffies is incremented?
--
Francis Litterio
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Re: Does the scheduler run every time jiffies is incremented?

[Rik van Riel]

[please remove linux-kernel from the CC and move it to the
 more appropriate kernelnewbies list]

On 22 Apr 2001, Francis Litterio wrote:

> I'm reading Rubini's "Linux Device Drivers" and it isn't clear to me
> whether the scheduler runs every time the timer interrupt increments
> jiffies or less frequently.
> 
> Does the scheduler run every time jiffies is incremented?

No.  Every timer interrupt a bunch of functions in
kernel/timer.c is run, amongst them the following lines
of code (from update_process_times):

                if (--p->counter <= 0) {
                        p->counter = 0;
                        p->need_resched = 1;
                }

As you can see, we will only reschedule when the time slice
of the currently running process is over.

regards,

Rik
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Re: Does the scheduler run every time jiffies is incremented?

[Russell King]

On Sun, Apr 22, 2001 at 10:52:15AM -0400, Francis Litterio wrote:
> Does the scheduler run every time jiffies is incremented?

No, it runs when something needs to be rescheduled (ie, when
the current tasks need_resched element (current->need_resched) is set).
This typically happens when someone wakes up, or the task reaches the
end of its allotted quantum.

--
Russell King (rmk@arm.linux.org.uk)                The developer of ARM Linux
             http://www.arm.linux.org.uk/personal/aboutme.html