Re: Best Kernel Timers?

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 28 Sep 2005, Simon White wrote:

> Hi,
>
> Wondered if anyone could provide information about where to look for
> suitable kernel timers?

Have you taken a look at what is being done by Thomas Gleixner?

http://lwn.net/Articles/152363/

Also you may be interested in Ingo Molnar's RT kernel.

http://people.redhat.com/mingo/realtime-preempt/

As well as the work being done by the HRT folks.

http://sourceforge.net/projects/high-res-timers

-- Steve

>
> For a while I have been working on supporting the hardsid/catweasel
> cards on Linux (and Windows).  Although undesirable the original
> implementations required real usec delays in the OS
> (this requirement being fixed in the very latest hardware).  I know
> Linux is not realtime so the original drivers were designed to
> queue hardware writes to a realtime thread that busy waited and
> recovered as best it could from errors (on the whole this worked
> pretty well).  Also another version of the code was written to use
> rtlinux/rtai that was capable of non busy waiting.
>
> More recently with the release of the new buffering hardware the
> driver was redesigned from the realtime posix code.  Due to these
> changes the busy waiting (for the old cards) can nolonger occur
> and the delays have to happen asynchronusly notifing the realtime
> thread when the delay has expired.  The code uses the posix
> timer_set, etc calls with realtime clock with absolute delays and
> flags a semaphore when the signal occurs (works great under
> realtime systems).
>
> Now as an alternative it is again desired that a version (although
> wont perfectly work) be available to a vanilla 2.6 kernel (possibly
> 2.4) with similiar limitations as before.  Its a shame the posix
> calls appear to not be supported in kernel for drivers so I have
> wrapped the calls for semaphores/mutexs/threads to kernel
> equivalents.
>
> However I have no idea what to do for the timers.  Is there
> something suitable inkernel that would provide an async callback
> to pre-empt a realtime thread and provide better resolution than
> HZ a far amount of the time?  Or do I have to run a seperate lower
> priority busy waiting thread to wakeup the realtime one?
>
> Advice appreciated.
> Simon
>
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