On Thursday 18 July 2002 01:40 am, Amit Kucheria wrote:
> On Wed, 17 Jul 2002, Karthik M wrote:
> > hi everyone,
> >
> > as a follow up to the previous posting regarding
> > effects of measurment, I was testing gettimeofday().
> > The results that i got out of my test program were not
> > consistent and were varying. Wanted to get an opinion
> > abt it.
> >
> > ----snip-----
> >
> > the values are varying a lot and did'nt have a clue
> > why it was the case. Did a little search on google but
> > could'nt find an answer.
>
> IMHO "Scheduling granularity" is the key here. The linux system on x86
> architectures has a heartbeat of 100Hz = 10ms. What this means is that
> the system tries to complete its range of tasks such as scheduling
> various subsystems, checking on blocked I/O, sending & receiving packets
> etc etc. every 10ms. Since some tasks have more priority over others,
> they might cause a delay for these low priority tasks which are
> generally user processes.
>
If you include
#include <sched.h>
struct sched_param param;
param.sched_priority = sched_get_priority_max(SCHED_FIFO);
if( sched_setscheduler(0, SCHED_FIFO, ¶m) < 0)
{
printf("ERROR: Could not become realtime\n");
exit(0);
}
You should see a lot less variation. In the tens or hundereds rather than
thousands because no other normal processes can go pefore yours.
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