Re: nanosleep with small value

[Eric Piel <Eric.Piel@lifl.fr>]

11/17/2005 05:55 PM, linux-os (Dick Johnson) wrote/a écrit:
> On Thu, 17 Nov 2005, Dag Nygren wrote:
:
>>
>>real    0m8.000s
>>user    0m0.000s
>>sys     0m0.000s
> 
> 
> On an unprivilged account, I get this with
> version 2.6.13.4
> 
> Script started on Thu 17 Nov 2005 11:44:47 AM EST
> LINUX> time ./xxx
> sched_setscheduler returned: -1
> Operation not permitted
> 
> real	0m2.000s
> user	0m0.000s
> sys	0m0.001s
> LINUX> uname -r
> 2.6.13.4
> LINUX> exit
> Script done on Thu 17 Nov 2005 11:45:07 AM EST
> 
>>From the root account where the scheduler could be set:
> 
> Script started on Thu 17 Nov 2005 11:45:29 AM EST
> [root@chaos root]# time /tmp/xxx
> 
> real	0m2.001s
> user	0m0.000s
> sys	0m0.001s
> [root@chaos root]# exit
> 
> Script done on Thu 17 Nov 2005 11:45:53 AM EST
> 
> ... essentially the same thing. And 2 seconds, not 8.
> 
> The HZ value for my kernel is 1000. It you are at 100 HZ,
> that might explain it.
No, it means he is at 250 HZ.


> 
> Note that nanosleep() doesn't claim to be able to sleep
> less than the resolution of some kernel timer. It just takes
> parameters in seconds and nanoseconds.
> 
In general small nanosleep() returns between 1/HZ and 2/HZ. In this 
code, the sleep is (indirectly) synchronised on a tick so it's always 
2/HZ, the worse!

1000 small nanosleeps take therefore 1000 * 2/HZ s = 8s :-)

Eric