Re: time drift in camonitor timestamps

[Eric Norum <wenorum@lbl.gov>]

This seems like it would be pretty easy to fix.

--- dbScan.c 2011-12-12 12:14:45.000000000 -0800

+++ dbScan.c.proposed 2013-01-08 09:43:23.000000000 -0800

@@ -87,6 +87,7 @@

     double              period;

     volatile enum ctl   scanCtl;

     epicsEventId        loopEvent;

+    unsigned long       overrunCount;

 } periodic_scan_list;

 

 static int nPeriodic = 0;

@@ -542,18 +543,23 @@

 {

     periodic_scan_list *ppsl = (periodic_scan_list *)arg;

 

-    epicsTimeStamp start_time, end_time;

+    epicsTimeStamp nextScan, now;

     double delay;

 

     taskwdInsert(0, NULL, NULL);

     epicsEventSignal(startStopEvent);

 

+    epicsTimeGetCurrent(&nextScan);

     while (ppsl->scanCtl != ctlExit) {

-        epicsTimeGetCurrent(&start_time);

         if (ppsl->scanCtl == ctlRun) scanList(&ppsl->scan_list);

-        epicsTimeGetCurrent(&end_time);

-        delay = ppsl->period - epicsTimeDiffInSeconds(&end_time, &start_time);

-        if (delay <= 0.0) delay = 0.1;

+        nextScan += ppsl->period;

+        epicsTimeGetCurrent(&now);

+        delay = epicsTimeDiffInSeconds(&nextScan, &now);

+        if (delay <= 0.0) {

+            ppsl->overrunCount++;

+            delay = 0.01;

+            epicsTimeGetCurrent(&nextScan);

+        }

         epicsEventWaitWithTimeout(ppsl->loopEvent, delay);

     }

On Jan 8, 2013, at 8:51 AM, Ralph Lange <Ralph.Lange@gmx.de> wrote:

To answer your original question...

Yes, this problem is known and documented since ~2.5 years, but not fixed yet [1].

Even though people have been running into this issue every now and then (there was even support added to devIOCstats [2] to measure the error), no-one's pain was bad enough to invest the time to track it down and fix it.

For >99% of the applications, it just does not matter if the scan period has an error in the order of a few promille.

~Ralph

[1] https://bugs.launchpad.net/epics-base/+bug/597054
[2] http://www.slac.stanford.edu/grp/cd/soft/epics/site/devIocStats/

-- 
Eric Norum
wenorum@lbl.gov