Re: nanosleep() for shorter than schedule slice

[Frank Kardel <kardel%kardel.name@localhost>]

Hi !


I am pretty skeptical by the hardwired constant of 20 (clockticks). At 
the usual/common clock interrupt rate of 100 Hz this is 200ms.

So any nanosleep below 200ms will be a kernel busy loop...

For sleeps less than 1 (at most two) clock ticks this would be a crude 
workaround until we have better mechanisms like timed 
interrupts/tickless kernel.

With today processors 20ms is a LONG time, busy waiting wastes a lot of 
otherwise useful cycles/power. Setting up a timed interrupt should be 
well below 100us using todays HW and taking the interrupt also (save for 
16MHz 68020 machines and similar ones).

Frank


On 12/09/17 15:48, maya%netbsd.org@localhost wrote:
> On Sun, Jul 02, 2017 at 10:33:58AM +0200, Emmanuel Dreyfus wrote:
> > but that kind of performance problem could exist in many other softwares.
> With Go:
> go test -short -v -run=TestWaitGroupMisuse2 -count=5000 sync
>
> Performance was so bad they made a bug report for it (10-100s runtime
> vs. 0.2s on linux: https://github.com/golang/go/issues/22944
>
> I initially got units confused and used this, it's not the scheduler
> timeslice (20ms) but 20hz. switching to scheduler timeslice made it as
> bad as before. (??)
>
> this gets the go example to run in 0.3-0.01 s
>
> Index: kern_time.c
> ===================================================================
> RCS file: /cvsroot/src/sys/kern/kern_time.c,v
> retrieving revision 1.189
> diff -u -p -r1.189 kern_time.c
> --- kern_time.c	11 Nov 2016 15:29:36 -0000	1.189
> +++ kern_time.c	9 Dec 2017 14:46:08 -0000
> @@ -343,11 +343,12 @@ nanosleep1(struct lwp *l, clockid_t cloc
>   		return error;
>   	}
>   
> -	/*
> -	 * Avoid inadvertently sleeping forever
> -	 */
> -	if (timo == 0)
> -		timo = 1;
> +	/* Can't do anything useful in so little time */
> +	if (timo < 20) {
> +		DELAY(hztoms(timo));
> +		return 0;
> +	}
> +
>   again:
>   	error = kpause("nanoslp", true, timo, NULL);
>   	if (rmt != NULL || error == 0) {