Hopefully others who know much more about PVAccess and PVData respond in detail. I am not an expert on that by any means.
> Does the statement "a client-server shared memory system optimized for efficiency (optimal zero-copy etc) “ mean that a client can essentially access the memory one the server(IOC) side and request whatever data is available?
PVAccess lets you access IOC data in a structured way, while Channel Access has scalers and very limited arrays only. For example with areaDetector an NDArray object and all of the associated NDAttributes are transmitted atomically over the network with PVAccess. With Channel Access each items must be fetched separately, with no guarantee they are all in an internally consistent state at the time they are read.
You can define such structures at run-time, so your client code can decide what it wants to access with a single read, or write/read.
But I don't think you can get it to do the averaging you were looking for with temperature, for example. Others may correct me.
Perhaps it would be helpful if you could outline what you would like to do.
Mark
________________________________
From: Valentyn Stadnytskyi <v.stadnytskyi at gmail.com>
Sent: Sunday, July 19, 2020 5:24 PM
To: Mark Rivers
Cc: tech-talk at aps.anl.gov
Subject: Re: data acquisition strategies
Mark,
I have been looking into PVAccess and PVData
“ pvData is the control data interface of EPICS V4 endpoints, such as user agent software and EPICS V4 IOCs. Together with pvAccess, they support essentially a client-server shared memory system optimized for efficiency (optimal zero-copy etc) and control (PV locking, alarms etc). “ - http://epics-pvdata.sourceforge.net/pvAccess_Protocol_Specification.html
Does the statement "a client-server shared memory system optimized for efficiency (optimal zero-copy etc) “ mean that a client can essentially access the memory one the server(IOC) side and request whatever data is available?
The section 10.10 describes Channel put-get and section 10.12 described Channel array. These feature of PvAccess sound like what I need if I want to have a way to retrieve custom data from a server(IOC)
10.10 Channel put-get (0x0C)
A "channel put-get" set of messages are used to set (put) data to the channel and then immediately retrieve data from the channel. Channels are usually "processed" or "updated" by their host between put and get, so that the get reflects changes in the process variable's state.
10.12 Channel array (0x0E)
A "channel array" set of messages are used to handle remote array values. Requests allow a client agent to: retrieve (get) and set (put) data from/to the array, and to change the array's length (number of valid elements in the array).
Do I understand it right?
Valentyn
On Jul 19, 2020, at 5:49 PM, Valentyn Stadnytskyi <v.stadnytskyi at gmail.com<mailto:v.stadnytskyi at gmail.com>> wrote:
Hi Mark,
Thank you for the explanation and examples.
In the example you provided there are 10 images and 10 values of the ring current. Are those instantaneous values captured at a time of image was generated? What if one has long integration time and there are multiple values of the ring current available? In this case you need to have additional code in the main program to accumulate multiple values of the ring current and save the mean value and standard deviation (or whatever one wants to save). Is that correct? So because EPICS has distributed database and publish-subscribe communication protocol their is no way to retrieve short history from IOC (ring current in our example) without developing a middle man that would accumulate this history and make it available on demand. I know there is an archiver in EPICS but it doesn’t archive every value (especially if data acquisition unit is capable of high sampling frequency). For example, I want to know what was the temperature of a sample measured at 2 Hz while I integrated xray image for 2 seconds. If I have IOC that publishes temperature, I will know only the last known value, or I can have a custom PV that give me average of the last 4 known value(but this becomes to specific and not extensible).
Valentyn
On Jul 17, 2020, at 11:45 AM, Mark Rivers <rivers at cars.uchicago.edu<mailto:rivers at cars.uchicago.edu>> wrote:
Hi Valentyn,
> I can imagine two different peripheral data collection strategies.
- First, my image collecting software subscribes to the PVs, and whenever I get the new image I save it and all last known values of PVs with it.
- Second, I sent a request to an archiver(a place where all PV values are saved with according timestamps) that returns values at the requested time. The second one requires additional infrastructure, some kind of server(a middle man) that will collect all PV values and process request from clients.
Many beamlines use strategy #1 because it is very well supported in the EPICS areaDetector software. It works like this:
- The areaDetector driver (marCCD, Pilatus, Point Grey, Eiger, etc.) collects data into NDArray class objects.
- NDArrays objects have a list of NDAttributes attached to them that contain the metadata. NDAttributes can get their values from any EPICS PV, in addition to other sources.
- Most areaDetector file saving plugins can save both the NDArray data as well as the NDAttribute metadata. This includes the HDF5, Nexus, netCDF, and TIFF plugins.
The list of NDAttributes to be attached to each NDArray is specified in an XML file.
This is an example of part of such an XML file:
(base) corvette:CARS/iocBoot/ioc13bmd_pg1>more tomoDetectorAttributes.xml
<?xml version="1.0" standalone="no" ?>
<!-- Attributes -->
<Attributes>
<Attribute name="DetectorManufacturer" type="EPICS_PV" source="$(DET)cam1:Manufacturer_RBV" dbrtype="DBR_STRING" description="Detector manufacturer"/>
<Attribute name="DetectorModel" type="EPICS_PV" source="$(DET)cam1:Model_RBV" dbrtype="DBR_STRING" description="Detector model"/>
<Attribute name="SerialNumber" type="EPICS_PV" source="$(DET)cam1:SerialNumber_RBV" dbrtype="DBR_STRING" description="Detector serial number"/>
<Attribute name="FirmwareVersion" type="EPICS_PV" source="$(DET)cam1:FirmwareVersion_RBV" dbrtype="DBR_STRING" description="Detector firmware version"/>
<Attribute name="SDKVersion" type="EPICS_PV" source="$(DET)cam1:SDKVersion_RBV" dbrtype="DBR_STRING" description="Detector vendor SDK version"/>
<Attribute name="DriverVersion" type="EPICS_PV" source="$(DET)cam1:DriverVersion_RBV" dbrtype="DBR_STRING" description="areaDetector driver version"/>
<Attribute name="ADCoreVersion" type="EPICS_PV" source="$(DET)cam1:ADCoreVersion_RBV" dbrtype="DBR_STRING" description="areaDetector ADCore version"/>
…
<Attribute name="AcquireTime" type="EPICS_PV" source="$(DET)cam1:AcquireTime_RBV" dbrtype="DBR_NATIVE" description="Exposure time"/>
<Attribute name="AcquirePeriod" type="EPICS_PV" source="$(DET)cam1:AcquirePeriod_RBV" dbrtype="DBR_NATIVE" description="Exposure period"/>
<Attribute name="ExposurePeriod" type="EPICS_PV" source="13BMD:SIS1:Dwell" dbrtype="DBR_NATIVE" description="SIS Dwell=exposure period"/>
<Attribute name="FrameRate" type="EPICS_PV" source="$(DET)cam1:FrameRate_RBV" dbrtype="DBR_NATIVE" description="Frame rate"/>
<Attribute name="FrameRateEnable" type="EPICS_PV" source="$(DET)cam1:FrameRateEnable_RBV" dbrtype="DBR_STRING" description="Frame rate enable"/>
<Attribute name="Gain" type="EPICS_PV" source="$(DET)cam1:Gain_RBV" dbrtype="DBR_NATIVE" description="Gain"/>
<Attribute name="GainAuto" type="EPICS_PV" source="$(DET)cam1:GainAuto_RBV" dbrtype="DBR_NATIVE" description="Gain auto"/>
<Attribute name="PixelFormat" type="EPICS_PV" source="$(DET)cam1:PixelFormat_RBV" dbrtype="DBR_STRING" description="Readout pixel format"/>
<Attribute name="ConvertPixelFormat" type="EPICS_PV" source="$(DET)cam1:ConvertPixelFormat_RBV" dbrtype="DBR_STRING" description="Converted pixel format"/>
<Attribute name="ArrayCounter" type="EPICS_PV" source="$(DET)cam1:ArrayCounter_RBV" dbrtype="DBR_NATIVE" description="Frames collected to date"/>
<Attribute name="DetectorTemperature" type="EPICS_PV" source="$(DET)cam1:TemperatureActual" dbrtype="DBR_NATIVE" description="Detector temperature"/>
…
<Attribute name="DateTimeStart" type="EPICS_PV" source="S:IOC:timeOfDayForm1SI" dbrtype="DBR_STRING" description="Date and time at start"/>
<Attribute name="DateTimeEnd" type="EPICS_PV" source="S:IOC:timeOfDayForm1SI" dbrtype="DBR_STRING" description="Date and time at end"/>
<Attribute name="RingCurrent" type="EPICS_PV" source="S:SRcurrentAI" dbrtype="DBR_NATIVE" description="Ring current"/>
<Attribute name="TopUpStatus" type="EPICS_PV" source="S:TopUpStatus" dbrtype="DBR_STRING" description="Top-up status"/>
<Attribute name="BeamMode" type="EPICS_PV" source="OPS:message3" dbrtype="DBR_STRING" description="Beam mode"/>
<Attribute name="SampleX" type="EPICS_PV" source="13BMD:m85.RBV" dbrtype="DBR_NATIVE" description="Bottom X stage translation"/>
<Attribute name="SampleOmega" type="EPICS_PV" source="13BMD:m38.RBV" dbrtype="DBR_NATIVE" description="Sample rotation"/>
<Attribute name="SampleY" type="EPICS_PV" source="13BMD:m90.VAL" dbrtype="DBR_NATIVE" description="Sample vertical height"/>
<Attribute name="SampleXCent" type="EPICS_PV" source="13BMD:m92.VAL" dbrtype="DBR_NATIVE" description="Sample X centering"/>
<Attribute name="SampleZCent" type="EPICS_PV" source="13BMD:m93.VAL" dbrtype="DBR_NATIVE" description="Sample Z centering"/>
…
<Attribute name="HDF5FrameLocation" type="EPICS_PV" source="$(TS)HDF5Location" dbrtype="DBR_STRING" description="Where to store frame in HDF5 file"/>
</Attributes>
This is part of the output of the ncdump command on a netCDF file saved by areaDetector, looking at the value of the ring current attribute:
(base) corvette:~/tomo_data/NaCl_tomoscan>ncdump -v Attr_RingCurrent NaCl_C1.nc
netcdf NaCl_C1 {
dimensions:
numArrays = UNLIMITED ; // (10 currently)
dim0 = 1200 ;
dim1 = 1920 ;
attrStringSize = 256 ;
variables:
int uniqueId(numArrays) ;
double timeStamp(numArrays) ;
int epicsTSSec(numArrays) ;
int epicsTSNsec(numArrays) ;
short array_data(numArrays, dim0, dim1) ;
char Attr_DetectorManufacturer(numArrays, attrStringSize) ;
…
data:
Attr_RingCurrent = 101.876796241609, 101.876058881609, 101.876288921609,
101.876337641609, 101.876575521609, 101.875805921609, 101.874051801609,
101.875392481609, 101.874212281609, 101.874040321609 ;
In this case there are 10 values of the ring current because this file contains 10 images, and the ring current attribute is captured and stored for each one.
This is part of the output of the h5dump command on an HDF5 file saved by areaDetector, looking at the value of the ring current attribute:
(base) corvette:~/scratch>h5dump --dataset /RingCurrent hdf5_test_097.h5
HDF5 "hdf5_test_097.h5" {
DATASET "/RingCurrent" {
DATATYPE H5T_IEEE_F64LE
DATASPACE SIMPLE { ( 10 ) / ( H5S_UNLIMITED ) }
DATA {
(0): 102.062, 102.062, 102.062, 102.062, 102.062, 102.063, 102.063,
(7): 102.063, 102.063, 102.063
}
ATTRIBUTE "NDAttrDescription" {
DATATYPE H5T_STRING {
STRSIZE 20;
STRPAD H5T_STR_NULLTERM;
CSET H5T_CSET_ASCII;
CTYPE H5T_C_S1;
}
DATASPACE SCALAR
DATA {
(0): "Storage ring current"
}
}
ATTRIBUTE "NDAttrName" {
DATATYPE H5T_STRING {
STRSIZE 11;
STRPAD H5T_STR_NULLTERM;
CSET H5T_CSET_ASCII;
CTYPE H5T_C_S1;
}
DATASPACE SCALAR
DATA {
(0): "RingCurrent"
}
}
ATTRIBUTE "NDAttrSource" {
DATATYPE H5T_STRING {
STRSIZE 13;
STRPAD H5T_STR_NULLTERM;
CSET H5T_CSET_ASCII;
CTYPE H5T_C_S1;
}
DATASPACE SCALAR
DATA {
(0): "S:SRcurrentAI"
}
}
ATTRIBUTE "NDAttrSourceType" {
DATATYPE H5T_STRING {
STRSIZE 19;
STRPAD H5T_STR_NULLTERM;
CSET H5T_CSET_ASCII;
CTYPE H5T_C_S1;
}
DATASPACE SCALAR
DATA {
(0): "NDAttrSourceEPICSPV"
}
}
}
}
In this case there are also 10 values of the ring current because this file contains 10 images, and the ring current attribute is captured and stored for each one.
Mark
-----Original Message-----
From: Tech-talk <tech-talk-bounces at aps.anl.gov<mailto:tech-talk-bounces at aps.anl.gov>> On Behalf Of Valentyn Stadnytskyi via Tech-talk
Sent: Friday, July 17, 2020 10:13 AM
To: tech-talk at aps.anl.gov<mailto:tech-talk at aps.anl.gov>
Subject: data acquisition strategies
Hello All,
I hope you all are doing well.
I am a user at BioCars. I have a few questions about data acquisition strategies. But before I ask them let me briefly describe a situation which might be very familiar to all of you.
I am collecting X-ray images (of whatever sample/ technique). I want to know and save other data associated with the image. For example, I want to save temperature, humidity, motor1 position, motor2 position, laser intensity and the pressure(and list can go on and on) at which the image was collected. Let us assume that all of these devices and DAQs are on the network and have PVs associated with the values I am seeking. I can imagine two different peripheral data collection strategies.
- First, my image collecting software subscribes to the PVs, and whenever I get the new image I save it and all last known values of PVs with it.
- Second, I sent a request to an archiver(a place where all PV values are saved with according timestamps) that returns values at the requested time. The second one requires additional infrastructure, some kind of server(a middle man) that will collect all PV values and process request from clients.
How do you collect data from all additional devices\motors etc. Do you use strategy 1 or 2 or else? If you use strategy 1, how do you ensure that you don’t get very old PV value or you don’t get NAN. If subscription to PVs works well my main program should always have the most recent value. Is it correct?
Can someone recommend something to read on how a beamline handles data collection?
Valentyn
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