Re: Short sleep precision

[Jan Engelhardt]

>Running your program here I see even worse values than that on 2.6.11-mm4 
>and it's also interresting to see that for a lot of continuous runs the 
>values reported drop steadily and eventually settle around ~1100, but if I 
>insert a  sleep 1  between runs, then I see a steady ~1000 reported.
>This is all with HZ = 1000

That may be related to the new mix of USER_HZ. I can't really tell, 
just observed that there is such.

>Linux nanosleep() used to have a busywait loop for sleeps less than two
>milliseconds.  2.4.x still does.

Yes, I know. Linus threw it out during 2.5 because it was deemed to buggy.
That did not affect me, because that busywaiting is only possible in 
!SCHED_OTHER, which I cannot use, because tiny delays are needed in a 
user-runnable userspace app, and I don't want it suid and stuff.

(I've developed an "overhead correction" for accurate(*) realtime replay of 
logfiles. (*) with respect to the total runtime. Works well.)

>You can spin on the gettimeofday() result in userspace.

How can I use it? / What does it help me? I just have the gettimeofday() in 
the example script to measure the total time of nanosleep(). Sometimes, 
nanosleep completes in the same tick, sometimes (95%), another task is 
scheduled before returning. I am calling nanosleep repetedly to find out the 
_average_ time for a 0us-nanosleep(), usually 100/1000 us.



Jan Engelhardt
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