[Bug 1861612] Re: callbackRequestDelay not waiting for 1/60 sec on vxWorks

[mdavidsaver via Core-talk <core-talk at aps.anl.gov>]

@Ben, The short answer is the both Andrew and I have tried +=1 and it
doesn't work.

I think I can explain the reason by referring to the Timing diagram
(delay.png) I've attached to this issue.

Consider a case where D (requested timeout) is slightly less than T
(tick period), and A (time from call to next tick) is almost zero.

In this case the float to int conversion floor(D/T)==0.  So +=1 gives a
delay of 1.  This will wait until the next tick (time A) which is almost
zero.

The primitives we're working with on RTEMS and vxWorks seem to respond
to a requested non-zero timeout of 'd' ticks by waiting A+T*(d-1)
seconds.

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https://bugs.launchpad.net/bugs/1861612

Title:
  callbackRequestDelay not waiting for 1/60 sec on vxWorks

Status in EPICS Base:
  Triaged

Bug description:
  The problem is documented in this tech-talk thread:
  https://epics.anl.gov/tech-talk/2020/msg00308.php

  In 7.0.3.1, but not in 7.0.3, callbackRequestDelay(1/60) results in
  100% CPU time, most of the time in cbHigh.

  This happens on vxWorks 6.9.4.1 and was first seen with the scaler
  record with multiple types of scaler hardware and device support.

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