Re: nanosleep() for shorted than schedule slice

[Johnny Billquist <bqt%softjar.se@localhost>]

On 2017-07-02 23:24, Michael van Elst wrote:
> bqt%softjar.se@localhost (Johnny Billquist) writes:
>
> > I don't get it. What was the problem with using nanosleep for short
> > usleep's?
>
> usleep is just a wrapper around nanosleep. There is no difference
> except that nanosleep accepts higher precision delays.

So, the comment previously that it was not ok to use nanosleep for short 
usleeps seems to not hold much water.

> The kernel computes the number of ticks to sleep, schedules a callout
> that will wake the thread up and calls the scheduler to run other
> threads in the meantime. The callout is later dispatched by the
> clock interrupt.
>
> So you always have to wait for at least one tick, with HZ=100 that's 10ms.

Right. With the current implementation. And which means that "higher 
precision delays" are just imaginary. Our implementation does not 
actually give us any higher precision delays.

> A tickless kernel wouldn't run callouts from the regular clock interrupt
> but would use a hires timer to issue interrupts at arbitrary times.
> The callout API could then be changed to either accept timespec values or
> just fake a much higher HZ value.

Right. Not that I believe this have to be tied into tickless, but I 
suspect it might be easier to do it if we go tickless.

I think it's pretty bad that our timers only have the precision of 
ticks, even if people call something like nanosleep.
By the way, the fact that nanosleep also in the end uses ticks means 
that the OP suggestion of using nanosleep for short usleeps will not 
really improve the situation at all.

We really should be able to deal with shorter times, even if we have 
ticks. But I guess it all depends on the hardware (which it does if we 
go to tickless as well, though.)

Part of my confusion might also be slightly different uses of the term 
tickless. The kernels use of ticks for time keeping and some work does 
not necessarily force us to use them for callouts. It's just convenient. 
We could (maybe should?) use a different mechanism for the callout 
subsystem.

	Johnny

--
Johnny Billquist                  || "I'm on a bus
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