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SubjectRe: severe jitter experienced with "select()" in linux 2.6.14-rt22
Steven Rostedt wrote:

>Please always CC Ingo Molnar on all -rt related issues. And Thomas
>Gleixner and John Stultz on timer issues with -rt. (CC John on timer
>issues in mainline too).
>
>
Will do so in future, did not know anyone else well enough.

>
>Is there any requirement that select must sleep the full time? At least
>have you checked the value of the timeout to see if there was reported
>time left? The return value wont be zero. I believe that the select my
>return early with reported time left.
>
>
>

Yes, select is permitted to return before full timeout value, but on an
idle, fast machine one hopes this does not happen too too often. And one
also hopes that overruns are not too frequent. However, the results I
get were, as can be seen from the select histogram, rather all over.

>The simple answer is that the select system call uses the non high
>resolution timers. There's really no reason to use select for timing.
>That's really just a side effect of the system call. If you need
>accurate timing, that's what nanosleep is for. IIRC, others on LKML
>have stated that it is considered bad programming to use select as a
>timer when nanosleep is available.
>
>
well, there are a large number of applications that we have that use
select(). These include CORBA ORBs etc and we would like to run them and
get the benefits of excellent RT properties of -rt kernel. It would be
too too difficult for us, at least for now, to rewrite an ORB in such a
way that it does not use any select() but instead uses nanosleep(). I
assume high resolution timers must be more expensive - that is why they
do not get used by select(). But there are cases where I don't mind
paying the extra overhead, if any, to obtain good behaviour out of
select().

>So, what you show is what I would expect.
>
>
Sigh, but thanks for the comments.

>-- Steve
>
>

Gautam


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