Experimental Physics and
Industrial Control System

"Kasemir, Kay via Core-talk" <[email protected]> · Thu, 10 Jan 2019 14:22:24 +0000

Hi:

Is it possible that qsrv doesn't fully use the request information?

When I introspect and read a PV provided by the SNS neutron data server or the area detector NVPluginPVA, I can request specific fields:

$ pvinfo neutrons
neutrons structure
    time_t timeStamp
        long secondsPastEpoch
        int nanoseconds
        int userTag
    epics:nt/NTScalar:1.0 proton_charge
        double value
    epics:nt/NTScalarArray:1.0 time_of_flight
        uint[] value
    epics:nt/NTScalarArray:1.0 pixel
        uint[] value

$ pvget -r 'field(proton_charge)' neutrons
neutrons structure 
    epics:nt/NTScalar:1.0 proton_charge
        double value 1e+08

$ pvget -r timeStamp.secondsPastEpoch neutrons
neutrons structure 
    structure timeStamp
        long secondsPastEpoch 1547129085

$ pvinfo 13SIM1:Pva1:Image
13SIM1:Pva1:Image epics:nt/NTNDArray:1.0
    union value
        boolean[] booleanValue
    ..
    dimension_t[] dimension
        dimension_t
            int size
    ..
    display_t display
        double limitLow
    ..

$ pvget -r dimension 13SIM1:Pva1:Image
13SIM1:Pva1:Image structure 
    dimension_t[] dimension
        dimension_t 
            int size 1024
            int offset 0
            int fullSize 1024
            int binning 1
            boolean reverse false
        dimension_t 
            int size 1024
            int offset 0
            int fullSize 1024
            int binning 1
            boolean reverse false

For the SNS neutron data that was one argument for using PVAccess:
An XY histogram client may only need the pixel[],
a time-of-flight client may only need the time_of_flight[],
a proton charge accumulator may only need the proton_charge,
others may need a combination of elements up to the complete structure.

This access to sub-elements of the structure is also useful for CSS:
We cannot handle arbitrary structures like pva://neutrons, but we allow fetching sub-fields with a notation pva://neutrons/proton_charge.
The image widget can handle pva://13SIM1:Pva1:Image, but general tools like probe cannot. Still, probe can display pva://neutrons/proton_charge or pva://13SIM1:Pva1:Image/value which is just a scalar vs. number array.

When I use qsrv, however, I always seem to get the whole structure.
This is with EPICS 7.0.2:

$ pvinfo training:random
training:random epics:nt/NTScalar:1.0
    double value
    alarm_t alarm
        int severity
        int status
        string message
    time_t timeStamp
        long secondsPastEpoch
        int nanoseconds
        int userTag
    display_t display
        double limitLow
        double limitHigh
        string description
        string format
        string units
    control_t control
        double limitLow
        double limitHigh
        double minStep
    valueAlarm_t valueAlarm
        boolean active
        double lowAlarmLimit
        double lowWarningLimit
        double highWarningLimit
        double highAlarmLimit
        int lowAlarmSeverity
        int lowWarningSeverity
        int highWarningSeverity
        int highAlarmSeverity
        double hysteresis

$ pvget -r timeStamp.secondsPastEpoch training:random
training:random 2019-01-10 09:05:35.193  6.65934 

$ pvget -vv -r timeStamp.secondsPastEpoch training:random
training:random epics:nt/NTScalar:1.0 
    double value 4.50019
    alarm_t alarm 
        int severity 0
        int status 0
        string message NO_ALARM
    time_t timeStamp 2019-01-10 09:05:41.193  
        long secondsPastEpoch 1547129141
        int nanoseconds 193341059
        int userTag 0
    display_t display
        double limitLow 0
        double limitHigh 0
        string description 
        string format 
        string units 
    control_t control
        double limitLow 0
        double limitHigh 0
        double minStep 0
    valueAlarm_t valueAlarm
        boolean active false
        double lowAlarmLimit nan
        double lowWarningLimit nan
        double highWarningLimit nan
        double highAlarmLimit nan
        int lowAlarmSeverity 0
        int lowWarningSeverity 0
        int highWarningSeverity 0
        int highAlarmSeverity 0
        double hysteresis 0

It may not be an operational issue, because you typically want to get the complete NT* type for records served via qsrv.
But it's inconsistent, and it's unfortunate for training sessions where pvaSrv allowed me to say:
Look, you can read records via PVA and easily get just the units as  pva://record/display/units, because PVA generally allows you to fetch just what you want.

I think pvDatabaseCPP automatically supports requests for subsections of a structure.
Is qsrv not using pvDatabaseCPP for performance reasons?

Thanks,
Kay

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System