V4 Design: Record Processing

EPICS V4: Record Processing

September 15 2005

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Overview

UNRESOLVED ISSUES

• Monitored data: What are the semantics for monitors that involve large amounts of memory?

This document described semantics for V4 record processing that are different than for V3. This includes the following:

• Semantics for Database and Channel Access Links.
• Semantics for record processing
• Record Locking
• Posting database monitors
• Status and Alarm Severity
• Access to array and structure fields.

This version is based on feedback from the EPICS core developer's meeting at ANL/APS on July 11th through July 14th. In addition discussions with Andrew Johnson and Eric Norum have led to many of the ideas described below. This wiki is a companion to the wiki "V4 Design: dbdInterfaces".

V3 record processing semantics do not work well for data acquisition applications. An example of data acquisition is:

• Move sample to new position.
  • Several motors may be involved
  • While moving post monitors to show current position, etc.
• Sample data
• Wait until CA client has fetched the data before next move.

SynAPPS provides the Scan, motor, etc records which help with data processing applications. These records process as follows:

• Record does not complete processing until all inputs, outputs, etc done. Not completing means that they do not call recGblFwdLink until all proccessing is complete. Note that recGblFwdLink is what causes the V3 ioc to complete record processing.
• While waiting for asynchronous events these records have PACT=0 but keep state that shows it is still active
• Can be scanned and issue monitors while internal state is active

Leaving PACT 0 but not calling recGblFwdLink was not anticipated when the V3 record processing semantics were created. For V4 a record can be processed while it is waiting for asynchronous processing to complete.

In V3, if an input link is asynchronous, Process Passive does not wait for asynchronous processing to complete before fetching data. This also complicated data acquisition applications.

V4 record processing is designed to make data acquisition easier. Both input and output links can wait for record processing. This is done, however, without record support blocking. If blocking makes it easier to implement record support, then the support can spawn a separate thread that does block. Record support can communicate with the separate thread to decide when to complete record processing.

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Goals for V4 Record Processing

V4 record processing has the following features not available in V3:

• Links to other records:
  • Can request that the linked record be processed. In V3 this was only available for puts and only if recordtype set pp(TRUE).
  • Can wait until the linked record completes processing. In V3 this was only possible for puts.
• While record support is active a record can still be processed and post monitors.
• Database links are processed via queue requests rather than recursive calls to dbProcess.
• Instead of lock sets, each record instance has a lock.
• Monitors are implemented by database access without help from record support
• Channel Access and Database Access can access structures

Link Semantics

The link semanics are similar for database and channel access links ( input, output, and process). All provide the ability to request processing and to wait until processing completes.

Processing Database Links

In V3 records linked via database links are processed via recursive calls to dbProcess. In V4 this will no longer be done. For V4 the following is done when a link requests processing:

1. If the record is active or already queued the request fails.
2. If the record does not allow link requests to cause processing the request fails.
3. If neither 1) or 2) are true than the record is queued for processing.

Record Locking

V3 implemented lock sets in order to prevent different threads from simultaneously accessing linked records.

Instead of lock sets V4:

• Implements a per record instance lock.
• Defines a rule that allows two records to be locked without deadlocks

Posting Monitors

In V3 monitors are posted when code (record support, device support, database access, etc) calls db_post_event. In V4 database access will itself handle the posting of monitors.

Monitors are posted by dbProcess when

• record support returns processActive.
• at the end of record processing.

Database Field Types

V4 allows Channel Access and Database Access to the following types of fields:

• primitive types
• string
• one dimensional array of primitive or string or struct
• struct with fields of following types:
  • primitive
  • string
  • struct
  • array of primitive or string or struct

Link Semantics

NOTES:

• Link just means access to a record or field. The requester can be:
  • A database link in a record
  • A channel access link in a record
  • A channel access client PV.
• Instead of "forward link" the name "process link" is used.

Links are processed as follows:

• Input Link: Two flavors are available
  • Monitor Link
    • Implemented via a monitor request on linked field
    • Optionally process record containing the link when monitor value is returned.
  • Input Link: Perform the following actions
    • Optionally request that record be processed.
    • Optionally wait for record to complete processing.
    • Get value.
• Output Link: Two varieties are available
  • Output Only: Perform the following actions
    • Put value
    • Optionally wait until the put completes
  • Output and Process: Perform the following actions
    • Put value and request that record be processed. Note that if record can not be processed the value is not changed.
    • Optionally wait until the put and process completes
• Process Link: Perform the following actions
  • request processing
  • Optionally wait until record completes processing.

The following rules apply when a process request is made:

• If record is already being processed the request fails.
• Record support can specify that links to particular fields not cause processing.
• A record can declare that it can not be processed via process links.
  • This can be specified in the Database Definition or the record instance.
• If a process request is allowed, it will be done regardless of the scan mechanism, i.e. the record does not have to be passive.

NOTES For Record links:

• wait does NOT mean to block. It means to save state, return, and resume at the saved state when called again.
• When a record has multiple input links the links can be processed in parallel or sequentially
• When a record has multiple output links the links can be processed in parallel or sequentially

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Record Processing

This section describes the semantics implemented by dbProcess.

dbProcess maintains a variable processState which is passed to iocRecord and to record support and can also be returned by them.

V4 Processing States

processState is passed to process (iocRecord or record support) and is also returned by process. processState defines the states:

processCancel

If record support is called with this state, it must terminate outstanding processing and prepare for being called with processIdle.

processIdle

record is not being processed

processStart

processing is starting

processInputActive

iocRecord is waiting for input links to complete

processActive

record support is waiting for completion of record specific activity

processOutputActive

iocRecord is waiting for output and process links to complete

processDone

process has completed.

NOTES:

• Record support can implement substates. For example the sequence record keeps state about which link set it is processing.
• Individual record types might use only a subset of the above states.
• V3 link instances had attributes like CA, CP, CPP. For V4 these are:
  • process - a request to process the record before gets or after puts.
  • wait - a request to wait until it has processed all it's input links
  • parallel - OK to allow simultaneous gets or puts

dbProcess

The functionality described in this section will be divided between iocCore and iocRecord. iocRecord is the V4 successor to the V3 dbCommon. For V4 it is a separate software component.

NOTE: FLNK (forward link) is replaced by an array of process links.

When dbProcess is called the action it takes depends on processState.

• processCancel
  • If iocRecord is active it is called with state processCancel
  • If record support is active it is called with state processCancel
  • prepare for state processIdle
• processIdle
  • prepare the record for processing
  • set state = processStart and goto processInputActive
• processInputActive
  • call iocRecord
    • If iocRecord returns processInputActive return
    • If iocRecord returns processActive set processState = processStart and goto processActive
    • If iocRecord returns processDone goto processDone
• processActive
  • Call record support
    • If record support returns processActive post monitors and return
    • If record support returns processDone goto processOutputActive
• processOutputActive
  • Call iocRecord
    • If iocRecord returns processOutputActive return
    • If iocRecord returns processDone goto processDone
• processDone
  • Perform final processing - See below for details.
  • set processState = processIdle
  • return

NOTES:

• Record support is called with state processStart or processActive. It can return processActive as often as it wants. This supports the semantics of both the motor and sequence records.
• The semantics support the following model
  • iocRecord processes all its input links and then
  • Record support stays active as long as desired and then
  • iocRecord processes all its output links and then
  • dbProcess does final processing and sets the record idle.

The remaining subsections provide more details about each processState. Keep in mind that some actions are performed by dbProcess, some by iocRecord, some by record support, and some by link support.

processState processCancel

When process (Record Support or iocRecord) is called with this state it just prepares for being called again with state processIdle.

An example of when process is called with this state is when a Channel Access request is made to modify a link or device field in a record instance that is not idle. If this happens, record support and/or iocRecord will be called to cancel any outstanding internal state. Process does not have to call link or device support because Database Access will call the cancel method of any link or device support attached to the record.

processState processIdle

When dbProcess is called with processIdle it:

• prepares record for processing
• calls iocRecord with processStart

processState processStart

When iocRecord is called with processStart it:

• prepares for alarmStatus and alarmSeverity. See Alarm Processing for details.
• starts processing it's input links. Details are given below.
• returns with processState one of the following:
  • processInputActive - dbProcess returns and when called again calls iocRecord.
  • processActive - dbProcess calls record support with processStart.
  • processDone - dbProcess completes processing. record support will NOT be called and iocRecord will not be called to handle output links

When record support is called with processStart it:

• starts processing
• returns with processState one of the following:
  • processActive - dbProcess returns and when called again calls record support.
  • processDone - dbProcess calls iocRecord with processDone.

processState processInputActive

Only iocRecord is called with or returns this state.

This state means that one or more input links handled by iocRecord have not completed.

When iocRecord detects that all it's input links have completed, it sets processState = processActive and returns. dbProcess calls record support to start processing.

iocRecord can also return processDone, In this case dbProcess will just do final cleanup and set state back to processIdle. This is done, for example if iocRecord finds the record disabled.

processState processActive

Only record support is called with this state. iocRecord and record support can return this state.

This state means that record support and associated device support are waiting for something to complete. Examples are:

• wait for links to complete
• wait for a motor to reach some position.

The record, however, can be processed so that it can report intermediate data values, e.g. the current motor position. For example the record can be periodically scanned.

When record support is called it returns one of:

• processActive - dbProcess post monitors and returns. When it is called again it calls record support.
• processDone - dbProcess calls iocRecord with processDone.

processState processOutputActive

Only iocRecord is called with or returns this state.

This state means that one or more output links handled by iocRecord have not completed.

iocRecord returns either

• processOutputActive - record support returns and when called again calls iocRecord with processOutputActive.
• processDone - dbProcess does final processing as described next.

processState processDone

Only iocRecord is called with or returns this state.

When iocRecord is called with processDone it starts processing its output links. It returns either

• processOutputActive - record support returns and when called again calls iocRecord with processOutputActive.
• processDone - dbProcess does final processing as described next.

When both record support and iocRecord have reported that they are done, dbProcess does some final steps and then sets the processState to processIdle.

The final steps are:

• If the request to process the record included a callback, the callback is called.
• If other record completion callback requests are present, call them.
• post monitors
• The record is set idle.

Links to Other Records

Links can be processed in parallel, sequentially, or a combination of these. iocRecord handles the links in iocRecord and record support handles record specific input links. The support is responsible for implementing sequential or parallel link processing. Link support performs the actual I/O. The Link support methods provide options specifying if the linked record should be processed and provide a callback to call when link processing completes. It is up to iocRecord or record support to again call dbProcess when all input links complete.

If is OK for dbProcess to be called multiple times while links are being processed.

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Mutual Exclusion

V3 implemented lock sets, i.e. when a record was being processed it and all records linked via database links were locked.

Lock sets solved two Mutual Exclusion problems:

• V3 allowed dbProcess to be called recursively.
  • process passive database links and forward links were implemented via recursive calls to dbProcess
  • Lock sets prevented deadly embrace problems for circular links.
• No puts could be done to an record in a lock set while record processing is active.
  • For synchronous records this guarantees that no fields are modified except by record processing itself.
  • For asynchronous records this guarantee is not valid.

V4 does not call dbProcess recursively. Instead a process request is queued. The request fails if either of the following is true:

• The record is active.
• The record is already queued.

Queuing requests to process rather than recursively calling dbProcess does not necessarily cause unnecessary context switches. For example if all linked records are passive and the same priority then all processing can occur without any context switches.

• A request is made to a thread to call dbProcess.
  • The request is put on a queue associated with the thread.
• Sometime later the request is dequeued and dbProcess is called.
  • For each link, with a request to process request, the request is put on the queue for the same thread
  • After all requests are queued dbProcess returns
• The thread takes each request off the queue and calls dbProcess for the associated linked record
  • When all linked records finish processing a request is queued to call dbProcess for the original record
• The request for the original record is removed from the queue and it's dbProcess is again called.
  • The record completes.

Question What about periodically scanned records? Needs thought.

This leaves the question of what should be done about mutual exclusion.

Each record instance has an associated lock maintained by dbAccess. When dbProcess is called it takes the lock. Before it returns it unlocks. Thus iocRecord and record support do not need to lock or unlock.

Other code that needs access to fields of a record instance must call dbLock before accessing any fields and dbUnlock after all accesses are complete.

This leaves the problem of how to access fields from two different record instances simultaneously. For example code that implements database access needs such access. Code that needs such access must call dbLockLink before accessing the linked record and dbUnlockLink after access.

dbLockLink is implemented as follows:

• Each Record instances has a unique ordinal number
• Assume Record X has a link to record Y
• If the ordinal number of X is less than that of Y
  • Just call dbLock for record Y
• If the ordinal numnber of X is greater than that of Y
  • call dbUnlock for X
  • call dbLock for Y
  • call dbLock for X

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Posting Modifications

The document "V4 Design: dbdIterfaces" provides a design that allows database access to handle all posting on monitors without requiring any help from the code that modifies database fields. It can do this because fields can be modified only via an interface implemented by database access.

Fields are modified in one of two ways:

• Changes made while record is being processed. For these posting is handled by dbProcess.
• Changes by other code. For these posting is handled by dbLock/dbUnlock.

Each field of a record instance has two associated "modified" bits. One is for changes handled by dbProcess. The other is for changes made by dbLock/dbUnlock.

dbProcess posts monitors when:

• When record support returns processActive.
• When it finishes record processing.

Other code can only change fields between calls to dbLock/dbUnlock

• When dbLock is called all its modify bits are false.
• Each field modification causes the associated modify bit to be set true.
• When dbUnlock is called it
  • posts all fields that have changed
  • sets the associated modify bits false

The following is the initial attempt to describe how monitors are implemented.

The following definitions are used:

• monitor - A client has asked to be notified when a field value changes.
• post - Create storage, copy the current value of the field into the storage, and pass the storage to the monitor subsystem.

Separate discussions is required for the following: 1) primitive fields, 2) string fields, 3) structured fields, and 4) array fields.

The following are just some thoughts.

• primitive fields - No problems.
• string fields - Memory is an issue.
  • Should there be a way to postpone posting until the monitor is ready to retrieve the value?
  • If so who decides?
• struct fields - Memory is again an issue.
  • Structure with only primitive field or string should be posted immediately but what about array fields?
  • Who decides?
  • How?
• array fields - memory is an issue
  • It should be possible to post small arrays of primitive type.
  • Large arrays should not be posted until monitor system is ready to accept the data.
    • It should also be possible to just notify the client that the array changed but the client must issue a read request to retrieve the data.

In any case the way monitors are posted must take into consideration the views defined in the database definition for each record.

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Alarm Processing

The V4 semantics for status and severity have the following goals:

• alarmSeverity has the same values as for V3
• alarmStatus is a string
• a record starts processing with alarmSeverity = "NO_ALARM" and status empty.
• status will ONLY be posted if it has changed since the last time it was posted.
• alarmSeverity will ONLY be posted if it has changed since the last time it was posted

The semantics, which are implemented by iocRecord, are:

• When iocRecord is called with processStart
  • A private variable newSeverity is set to NO_ALARM
  • A private variable prevSeverity is set to alarmSeverity
• When iocRecord completes its part of record processing
  • If newSeverity is NO_ALARM and prevSeverity has a different value
    • alarmSeverity is set to NO_ALARM
    • alarmStatus is set to empty

• When the iocRecord.setSeverity(sevr,status) is called
  • If sevr is less than or equal to newSeverity nothing is done
  • else
    • alarmSeverity is set to sevr
    • alarmStatus is set to status
    • newSeverity is set to sevr

NOTE: alarmStatus and alarmSeverity are written by the normal method. This means that their values will be posted.

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Channel Access/ Database Access field types

Overview

This section discusses how IOC database fields are accessed by channel access or via database links.

For the initial V4 implementation only the following types of data will be accessable.

• primitive types
• string
• enum
• one dimensional array of primitive or string or struct
• struct with fields of following types:
  • primitive
  • string
  • enum
  • struct (use recursion for allowed types)
  • one dimensional array of primitive or string or struct

The dbdInterfaces document defines database field types. It also defines a subset called basic types, which consists of primitive types, string, array, and structure. For external access only basic types can be accessed. This means that structures and arrays that do not consist of basic types are not accessable outside the record. For example a DbfLink is not accessable via channel access as a DbfLink.

Non basic fields will only be available via some combination of basic types.

For example a DbfLink field will appear as a string. If a Database Definition is

    struct(calcInpLink) {
        field(link,link(in))
        field(value,float64)
    };
    
    record(xxx) {
        field(link,link(in))
        field(value,float64)
    ...
        field(inp,array(struct(calcInpLink)[]))
    ...
    }

If a client attaches to

    record.field(link)

The data will be appear in the form:

    choiceName(pvname,...)

where choiceName is the name of the link support and the arguments are the values of the dataStruct associated with the support.

Then if a client specifies the process variable:

    record.field(inp[1].link)

Then it connects to the link field of the second calcInpLink of field inp.

primitive types

This includes DbfBool, DbfOctet, DbfInt16, ... , DbfFloat64. These do not present any problems. The scalar value is transfered.

string

A string is just a UTF_8 encoded character string

DbfMenu and DbfEnum

Handled as an enum

DbfLink and DbfDevice

Both of these have the following:

• choiceName A string that selects the link or device support.
• dataStruct Each support has an associated struct for configuration.

How should these be made available? Perhaps:

The field appears as a string with the following syntax:

choiceName(structAssignmentList)

where structAssignmentList has the same syntax as defined in "V4 DB Record Instance Syntax"

timeStamp

This is just transfered like a struct with two fields:

• int64 secondsSinceEpoch
• int32 nanoSeconds

struct

The database definition syntax allows a struct to be composed of fields of any DbfType. Database access, however, will only allow access to structs with fields of the following types:

• primitive - DbfBool,...,DbfFloat64
• DbfString
• DbfArray of primitive type or string
• DbfStruct - As long as its fields are valid types

The individual fields can be accessed directly so this discussion only involves access to the entire structure.

Access will only be available to clients that have knowledge of fields within the structure. For channel access clients this means that they provide a propertyCatalog that has some subset of the fields in the structure.

array

Arrays of the following types are accessable:

• primitive - DbfBool,...,DbfFloat64
• DbfString
• DbfStruct - As long as its fields are valid types

Note that arrays of structs are not accessable in the early V4 releases.

Arrays still need lots more discussion!!! The database definition syntax allow an array to be any of the basic types, i.e. DbfBool,...,DbfFloat64, DbfString, DbfArray, DbfStruct.

For the initial V4 version, database access only allows access to arrays of the following types.

• primitive - DbfBool,...,DbfFloat64
• DbfString
• ???? What else

An individual element of an array of type DbfStruct can be accessed directly if it satisfies the criteria for accessing a DbfStruct.

Question: Should database Access be implemented so that record/link/device support can transfer an array in segments. Examples:

• If the array is a circular buffer, it is presented in two segements
• If the array is stored in hardware, e.g. a transient recorder, the array can be read from the hardware and passed to the client in segments.

If an array is available only in segments then can we prevent two clients from trying to access the same array simultaneously?

Perhaps this is a problem that must be solved by an application:

• runControl records are one solution
• Other solutions can be envisioned

MDArray

Not implemented if the early V4 releases.

Associated Data

The typical example is a client that asks for timeStamp, severity, status, and data. These are handled automatically as described in "Posting Modifications"