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Subject: Re: Adding userTag to epicsTimeStamp
From: Marty Kraimer <mrkraimer@comcast.net>
To: core-talk@aps.anl.gov
Date: Tue, 10 Oct 2017 06:14:06 -0400
On 10/09/2017 11:09 AM, Kasemir, Kay wrote:
Not saying that this specific tag should not be 64 bits wide, but to change an existing approved design,
I would like to see a minimum amount of justification and discussion.
A time tag change from 32 to 64 bits makes no difference to those who simply use pvAccess via qsrv/pvaSrv since the EPICS database never provided a value, anyway.

And even though the time stamp is part of most updates, if I understand correctly this change will hardly affect the network package size because when you leave the tag at its default value of 0 and don't change, the monitor deltas won't include the tag, only the secs & nanosecs, correct?

Not quite but can be fixed.

I think the only problem is in the  timeStamp support code.
In particular
bool PVTimeStamp::set(TimeStamp const & timeStamp)
{
    if(pvSecs.get()==NULL) {
        throw std::logic_error(notAttached);
    }
    if(pvSecs->isImmutable() || pvNano->isImmutable()) return false;
    pvSecs->put(timeStamp.getSecondsPastEpoch());
    pvUserTag->put(timeStamp.getUserTag());
    pvNano->put(timeStamp.getNanoseconds());
    return true;
}

Note that it always sets userTag. The same problem exists in Java.

Both timeStamp and pvTimeStamp require changes in both Java and C++.

I agree that going to a 64 bit integer is a good idea.
But it should be a signed 64 bit integer for two reasons:
1) Keeps better compatibility between Java and C++.
2) The other fields of a timeStamp are signed, which is useful for computing time differences.

If we are going to make this change for first release of EPICS 7 changes must be made soon in both C++ and Java.
Will this also require changes to any of normativeTypes,  pvaPy, and p4p?
Will this cause compatibility problems?

Marty

The SNS beam line neutron data does use the user tag for a sequence number, incremented by server to allow double-checking for missing updates. So our software might need to be updated. I believe this could be done beam line by beam line, won't be a show stopper.

-Kay



Replies:
Re: Adding userTag to epicsTimeStamp Johnson, Andrew N.
References:
Adding userTag to epicsTimeStamp Johnson, Andrew N.
Re: Adding userTag to epicsTimeStamp Ralph Lange
Re: Adding userTag to epicsTimeStamp Marty Kraimer
RE: Adding userTag to epicsTimeStamp Mark Rivers
Re: Adding userTag to epicsTimeStamp Kasemir, Kay

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