RT issues

Martin Pohlack mp26 at os.inf.tu-dresden.de
Thu Apr 14 12:17:30 CEST 2005

Rene Wittmann wrote:
> Hi,
> As I'm trying to implement a real-time application in DROPS, I still
> have some questions on this topic. Maybe there's someone who can answer
> them because I didn't find it described it the documentation...

You might want to read:


> 3. How can the preempter-thread tell the main-thread to stop it's work
> in case of a timeslice overrun or even a deadline miss?

You could synchronize both threads using shared mem or you could ex_regs
the worker thread from the preempter, whatever you like.

> 4. Does the system allow a thread's scheduling like this: 
> period-length  =  976 microsec, 
> timeslice_mand = 9760 microsec, 
> timeslice_opt  = 9760 microsec
> (which would mean that the system had to reserve timeslices that don't
> fit into one period...)
> OR 
> period-length  = 9760 microsec,
> timeslice_mand = 4380 microsec,
> timeslice_opt  = 4380 microsec
> (which would mean that the system had to reserve all resources for this
> thread..) 
> (what would happen if another thread comes with a period_length of also
> 9760 microsec and a timeslice_mand = 6000 microsec? What would be the
> system's behaviour?)

I think, the kernel does not prevent over-reservation, it will simply
generate *overrun messages at runtime.  Ressource admission should be
done in userland.  We currently have the package "cpu_reserve" in our
local repository.

> Other thing:
> File system support: 
> 1. which filesystems are supported (reiserfs?)? 

Common filessytems are only supported via l4linux, there is also an old
ext2 package here in our local repository.

> 2. Is there a way to have RT access to disk and accessing files this way? 
> If so, how?

You might want to read the paper http://os/drops/doc.html#rtss2003_das.

We describe how realtime access to disks in general does work in DROPS,
the work was focused on our real-time scsi disk driver.  There is a
simple real-time filesystem available in the package simplefs, which
however is not yet in the remote cvs.

Maybe Lars Reuther, the author of simplefs, can give more information.

Please reply to l4-hackers, as I'm on vacation in the next two weeks.


More information about the l4-hackers mailing list