Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Thu, 28 Apr 2011 11:20:56 -0500

Hi Ritesh,

On Thursday 28 April 2011 10:41:32 Ritesh Sugandhi wrote:
> Now , So I lower down the size to NLEM=65535,and kept the
> EPICS_CA_MAX_ARRAY_BYTES=90000 Now I receive the following output
> 
> *$ caget Py:char64k | more*
> *CA.Client.Exception...............................................*
> *    Warning: "The requested data transfer is greater than available memory
> or EPICS_CA_MAX_ARRAY_BYTES"*
> *    Context: "op=0, channel=Py:char64k, type=DBR_TIME_CHAR, count=65535,
> ctx="server unable to load read (or subscription update) response into
> protocol buffer PV="Py:char64k" max bytes=65560""*
> *    Source File: ../getCopy.cpp line 92*
> *    Current Time: Thu Apr 28 2011 17:31:05.277255000*
> *..................................................................*
> *Read operation timed out: some PV data was not read.*
> *Py:char64k 65535 0 0 0 0 0 0 0 0...... 0 0 0 *
> 
> I could not figure out the error.

On workstation machines that have virtual memory you should just set 
EPICS_CA_MAX_ARRAY_BYTES to some large number like 10000000 (maybe even bigger 
if you're going to be transferring images around over CA) and forget about 
trying to get the value exactly right.  Doing that really doesn't waste memory 
since the OS won't allocate physical RAM for the whole buffer until it's 
actually used.  The only place where you may need to be careful with the 
setting for EPICS_CA_MAX_ARRAY_BYTES is on an embedded system (vxWorks or 
RTEMS) that has a limited amount of RAM.

- Andrew
-- 
An error is only a mistake if you don't learn from it.
When you learn something from it, it becomes a lesson.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System