EPICS Channel Access Enhancements for LANSCE Timed and Flavored Data

1. J. Hill EPICS Channel Access Enhancements for LANSCE Timed and Flavored Data

2. Overview • Introduction • LANSCE Requirements • EPICS Event Queue • Event Queue Upgrade • Milestones

3. LANSCE Requirements LANSCE timing and flavoring of data Flavoring Logical configuration of beam gates Timing Time sampling (single element) within a beam pulse Many permutations Too many to, a priori, install records for all of them Channel Access client must somehow specify timing and flavoring when it subscribes

4. EPICS Fundamentals • Channels and Process Variables • Channel is virtual communication link to process variable • We can perform read, write, and subscribe operations over a channel • Publish and Subscribe • Clients subscribe for updates • Servers publish updates to multiple clients • Records post state change events to servers

5. EPICS Event Queue – A Pivotal Component • Tying high priority deterministically scheduled activities to the low priority load non-deterministic server component • The event queue guarntees that this value/alarm status/time stamp tuple are consistent • It is essential that we minimize what impact client induced load has on record processing threads

6. EPICS Event Queue – Theory of Operation • Records execute at relatively high priority, server executes at relatively low priority • Record processing never blocks for slow clients • Record is producer and client is consumer • Sever production rate might exceed consumption rate of client or network – queuing system needs to serve dual purposes • Burst • Attempts to avoid discarding updates • Sustained • Keep subscriptions current • Intentionally discard intermediate updates • Do not introduce time delays

7. Current Event Queue is Quite Inflexible • The only parameters that can be correlated together in time with any surety over Channel Access are • Scalar Value, Time Stamp, Alarm Status • The exception - after the fact correlation using the time stamp • This is very inflexible • It does not allow clients to specify their data capture constraints • Array value change events are not recorded on the event queue • It doesn’t allow for on the fly application driven experiments needed at LANSCE

8. Event Queue Upgrade Upgraded Record Specific Event Parameter Set Scalar or Vector Value Time Stamp Alarm Status … Upgraded Device Specific Event Parameter Set LANSCE Beam Gate Specification Legacy Fixed Event Parameter Set Scalar Value Time Stamp Alarm Status Record CA Server Event Queue

9. Event Queue Memory Management • Smart Pointers to Data Access Interfaced Objects • Reference counted • Increments when smart pointer is copied • Decrements when smart pointer is destroyed • Release callback when count decrements to zero • Memory Management • Handled by event posting agent • Can be free list based

10. TCP Circuit Event Filter Upgrade Record Event Filter Event Queue Event Filter Expression value < 10.0 and value > log ( currentMonitor0007.current ) and requireFlavor ( beamGateA | beamGateB ) and excludeFlavor ( beamGateC ) Discard

11. Event Filter Implementation • Could be based on the same library that the calc record uses • Would prefer however to base expressions on property names instead of {A, B, C, ..} as is currently the case with the CALC library • Many mainstream open source alternatives

12. Event Filter Upgrade Client Library Server Library Event Filter ModerateInterface Upgrade Moderate Subscription Protocol Upgrade

13. Array Index Metadata • Magnitude of zero-eth element index • Floating point • Magnitude of one index increment • Floating point • Units of these magnitudes • String

14. Data Access Interfaces for DBR Types Data Access Interfacing Adapter

15. Segmented Protocol Buffer • In R3.13 • Fixed 16KB protocol buffer limited array size • In R3.14 • Two protocol buffer sizes • Fixed 16KB protocol buffers • EPICS_CA_MAX_ARRAY_BYTES protocol buffers • In R3.15 • Segmented protocol buffer • Based on free list of fixed sized buffers • Improved memory management • In R3.14 two large EPICS_CA_MAX_ARRAY_BYTES protocol buffers dedicated to each client even if they are not currently using them • Potentially no array size limits • But, perhaps EPICS_CA_MAX_ARRAY_BYTES • Defines a quota, but with a large default value

16. portable server upgraded Data Access based interface Server Implementation Consolidation rsrv dbAccess snap-in database

17. LANSCE Capable Waveform Record Legacy db_post_events interface Legacy device support interface Ordinary Record Upgraded Event Queue Ordinary Device Support Data Access based event posting interface Data Access based event posting interface LANSCE Capable Record Data Access based device Support interface Upgraded Event Queue LANSCE Specific Device Support

18. Milestones Milestones Internal Design Review Interface Description Data Access Close Out Retrofit Event Queue to New Interface Retrofit Portable Server to New Interface Data Access to DBR_XXX Service Snap in for dbAccess Subscription Update Filter LANSCE Specific Waveform Record

19. Benefits for LANSCE • LANSCE style dynamic on-the-fly ad-hoc beam flavoring and beam timing experiments • But, in homogenous EPICS system • Tool based approach to LANSCE applications • Applications have abstract model of hardware • Incremental upgrades possible • Multi-element “Timed” data • COTS digitizer • Window in time selected

20. Benefits for EPICS Community • Flexible event snapshots • Parameters other than alarm status, time stamp, scalar value correlated on event queue • Arrays correlated on event queue with other parameters • Subscription update filtering • Expression (string) based means • Project and site independent – tool based approach • Minimally invasive for existing client side tools • Array index meta data • Expanding intersection of EPICS with data acquisition systems