Issues in Real-Time CORBA

1
Issues in Real-Time CORBA

• Requirements for Real-Time CORBA
• A. Inselberg and P.Krupp
• Requirements for Real-Time CORBA. Milcom97
• A System
• D. Schmidt et al. TAO a Middleware Framework
  for Real-Time ORB Endsystems, IEEE Workshop on
  Middleware for Distributed Real-Time Systems and
  Services, 1997.

2
Requirements for Real-Time CORBA

• Applications domains with real-time needs
• DOD/Aerospace
• Manufacturing
• Electronic Commerce
• Process Control
• Finance
• Telecommunications

• Characteristics of distributed real-time systems
• Response Time
• Reliability
• Correctness and Completenes
• Concurrency
• Distribution

3
Real-Time Requirements for CORBA

• Functional Requirements
• Ability to track events (object startup,
  completion, creation, invocation, )
• Availability of global clock.
• Support for variety of scheduling policies.
• Priority based queueing and priority inheritance
• Support for multithreaded clients and servers.
• Support for multiple communication protocols.
• Support for failure notification (timing
  failures, operational failures)
• Operational Requirements
• Make available performance times of CORBA
  functions.
• Shared memory
• Predictable garbage collection
• Support large number of concurrent connections to
  remote objects.
• Implementation Requirements
• Small memory footprint
• Thread-safe and reentrant

4
Impact on CORBA

• Modification of ORB and its Services
• Dynamic Invocation Interface?
• Thread abstraction layer.
• Priority based queueing for components and
  services
• Priority inheritance, priority ceiling, etc.
• Time awareness (e.g. in Event Service)
• Publish worst-case execution time for all
  functions.
• TCP/IP?
• Object mobility, distributed transparency?
• Addition of New Services for Real-Time
• New Real-Time Services, e.g. Scheduling Service
• Timing exceptions, both at client and at server
• Extensions to IDL
• Timing constraints in invocation vs. as part of
  IDL.

5
TAO (D. Schmidt et al)

• Trends in Distributed, Real-Time System
  Development
• programming applications vs. integrating reusable
  components
• remote method invocation to integrate distributet
  application components
• efforts to define standard software
  infrastructure in heterogeneous environments.
• demand for QoS guarantees in and systems what
  about network? rb
• gt CORBA/DCOM/Java RMI
• Problems
• lack of QoS specification interfaces
• lack of QoS enforcement
• lack of real-time programming features
• lack of performance optimizations

6
TAO Overview

• Real-time IDL stubs and Skeletons
• specify and enforce timing requirements
  end-to-end.
• Real-time object adapter (ROA)
• in addition to demultiplexing requests, ROA
  dispatches requests in accordance with scheduling
  strategies.
• ORB QoS Interface
• Real-time I/O subsystem
• admission control and priority assignment
• High-speed network adapters
• ATM on steroids
• Originally static priority (communication
  20Hz,10Hz,5Hz,1Hz)

7
A Real-Time CORBA Scheduling Service in TAO

• Example Avionics mission computing
• deterministic real-time requirements e.g. weapon
  release, navigation
• statistical real-time requirements e.g. built-in
  tests, low-priority display queues.
• Typical application characteristics
• bounded executions
• bounded rates
• known operations

8
Limitations of Static Scheduling

• Traditionally RMS
• Inefficient handling of non-periodic processing
• Needs to treat aperiodic requests as if periodic,
  i.e., as if occuring at maximum rate (e.g.
  sporadic server, etc.)
• Utilization phasing penalty for non-harmonic
  periods
• High utilization possible only for harmonic
  workloads
• Otherwise schedulability bound slightly larger
  than 69
• Inflexible handling of invocation-to-invocation
  variation in resource requirements
• Mode changes are very expensive.

9
Overcoming Limitations Dynamic Scheduling

• For example EDF(Earliest Deadline First), MLF
  (Minimum Laxity First), MUF (Maximum Urgency
  First)
• Problems
• Many dynamic scheduling strategies do not offer a
  priori guarantees
• Purely dynamic scheduled systems can behave
  non-deterministically under heavy loads.
• EDF
• Operations dispatched based on time-to-deadline.
• Limitation Operation with earliest deadline is
  dispatched whether or not there is sufficient
  time to complete its execution. Fact that
  operation cannot meet its deadline only detected
  after the deadline has expired.
• MLF
• Operations dispatched in laxity order.
• Can detect deadline violation prior to deadline
  itself.

10
Priority Mapping in TAO

• Priority fields in RT_Info of task
• static priority
• dynamic subpriority
• static subpriority
• Real-time information about task
• criticality
• execution time
• period
• dependencies
• importance
• Various mapping of info to priority fields allow
  for variety of scheduling algorithms.