Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Wed, 05 Apr 2006 13:51:47 -0500

john sinclair wrote:

After adding two additional scans: 0.01 and 0.005, it seems to me that the
EPICS scan loops use delays instead of timers. Is this true? If so, can this
be changed easily? If so ...

The code that runs in the periodic scan tasks is in src/db/dbScan.c:
static void periodicTask(void *arg)
{
    scan_list *psl = (scan_list *)arg;
    epicsTimeStamp	start_time,end_time;
    double	diff;
    double	delay;
    taskwdInsert(epicsThreadGetIdSelf(), NULL, NULL);
    epicsTimeGetCurrent(&start_time);
    while(TRUE) {
	if (interruptAccept) scanList(psl);
        epicsTimeGetCurrent(&end_time);
        diff = epicsTimeDiffInSeconds(&end_time, &start_time);
	delay = psl->period - diff;
        delay = (delay <= 0.0) ? .1 : delay;
	epicsThreadSleep(delay);
        epicsTimeGetCurrent(&start_time);
    }
}
Hmm, I suspect that the magic ".1" number in the delay calculation could
probably be reduced to something a little smaller nowadays, but the
correct value is likely to be processor-specific.
If you have any alternative robust cross-platform suggestions for this
code I'd be interested in seeing them. In particular, it must behave
sensibly if the scanList() call that processes the records takes longer
than a whole scan period to execute - the above delay calculation
ensures that the slower scan periods will always get some time to
execute even if the faster scans always overrun their periods. This is
most important on OSs where threads are scheduled strictly by priority
so the slower scans could suffer from CPU starvation.
- Andrew
--
There is no S in exprexxo.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System