[PLUG] Linux process scheduling

[Jason R. Martin]

On 2/15/06, Larry Brigman <larry.brigman at gmail.com> wrote:
> On 2/15/06, Terry Griffin <griffint at pobox.com> wrote:
> > Larry Brigman inquired:
> > > I have a problem that I have spent at least two days searching on google
> > > and
> > > not found a solution.
> > >
> > > Problem background.
> > >
> > > I want to have a process wake up every x time period and send a network
> > > packet.
> > > Doing this is not hard but what seems hard to me is that I need this x
> > > period to be
> > > close to the same every time.  Minor variations are allowed and can be
> > > compensated
> > > in the program.  Basically keep a constant packet rate, whatever rate
> > > is required
> > >
> > > When using select or nanosleep (stock kernel 2.6.15-1.1830_FC4smp), I get
> > > wild
> > > variations of output.  Example: request sleep for 2.4 msec. get
> > > 7.9msec.;  then have
> > > to send three packets to make up for the time the process did not send
> > > because it
> > > was sleeping.
> > >
> > > The original program that I was using is iperf but when I want to use
> > > two different rates
> > > using two copies of the program running at the same time they conflict
> > > as the program
> > > is using a spin loop to get the timing correct.  I replace this spin
> > > loop with nanosleep
> > > and the program output is no longer constant.
> > >
> > > The really strange thing is when I increase the packet rate (decrease
> > > the sleep period)
> > > the programs using nanosleep or select don't get as much error ( 1
> > > msec. request get
> > > 980 usec. to 1020 usec.).
> > >
> > > The programs use posix threads (which is NPTL) and don't do any
> > > special calls for
> > > scheduling or priority settings.
> > >
> >
> > Try:
> >   pthread_attr_init()
> >   pthread_attr_setschedpolicy() with a policy of SCHED_RR.
> >   pthread_create() using the attribute initialized above.
> >
> > The process will have to be started as root for this to work,
> > but after the new thread is running you may setuid() to a
> > not-root user and retain the SCHED_RR policy.
> >
> > Man pages: pthread_attr_init(3), pthread_create(3), and setuid(2).
> >
>
> Tried SCHED_RR and SCHED_FIFO. No help.
> Is there a default process time slice (tick) that can be adjusted down?
>
> Would a low latency/preemptive kernel make a difference?

Perhaps HZ is set too low on your system?  There was a fairly recent
change to allow HZ to be configurable (2.6.15? 14?  don't
remember...), so perhaps you need to set it back to 1000.

Jason