Experimental Physics and
Industrial Control System

Matt Newville <[email protected]> · Thu, 3 Nov 2011 15:08:54 -0500

Hi,

I'm seeing an issue with Channel Access getting "large data arrays"
and hope someone can provide some insight.  First, this is not an
issue with EPICS_CA_MAX_ARRAY_BYTES, which is set large enough.
Rather the issue I'm seeing seems to raise the naive question:

    Once I do a ca_array_get(type, count, chid, pvalue), when can I
successfully read the data in pvalue?

That is, how do I know how long to wait in ca_pend_event() and
ca_pend_io()?    In most cases of a fast network and scalar values or
small arrays, values on the order of
   ca_pend_event(1.e-3);
   ca_pend_io(1.0);

seem to work well.   But, if the array is "large" or the network
"slow", I have a hard time predicting these values, and accessing the
data may give incorrect values unless I've waited "long enough".
Using either a slow network or pend_event/io times that is clearly
"too short",  say,
   ca_pend_event(1.e-5);
   ca_pend_io(0.01);

leaves the data at pvalue incorrect (all zeros).  Trying to access
this data from python, I can get python to segfault for large enough
data (say, 4.2M ints).  I'm not sure I fully understand why python is
crashing -- the segfault happens well after ca_array_get() returns,
but can happen in ca_pend_event() a short time later.  If I save but
don't try to access the data in pvalue, or wait "long enough", it
seems I never have a crash.

Related questions are:  What is supposed to happen if ca_pend_event()
and/or ca_pend_io() do time out?  and: Is there a way to tell if
pend_event() or pend_io() have timed out or if there are events that
are pending?

I was hoping to be able to do something like
   ret_ev = ca_pend_event(1.e-3);
   ret_io = ca_pend_io(1.);

   while (ret_io == ECA_TIMEOUT) {
      ret_ev = ca_pend_event(1.e-3);
      ret_io = ca_pend_io(1.);
   }

Unfortunately, it seems that checking the return values are not so
helpful, as pend_event() always(?) returns ECA_TIMEOUT and pend_io()
seems to return ECA_TIMEOUT no more than one time before returning
ECA_NORMAL.

For what it's worth, I'm using base 3.14-12.1.   Thanks in advance for
any insight.

--Matt Newville <newville at cars.uchicago.edu> 630-252-0431

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System