Real-Time The Execution Performance Agent
An Hard/Soft Real-Time Digital Control and Continuous Media Experiment to Demonstrate Confidence-Based Scheduling With The RT-EPA Execution Control Interface
Sam Siewert [e-mail]
Emerging Soft Real-Time Applications
Motivation for RT-EPA
Virtual Reality, Multimedia, Flexible Automation and Control
Device Interfaces with RT Processing Between Them
Range of Device Bandwidths and Characteristics Aperiodic Bursty Periodic
Range of RT Performance Reliability Required Best-Effort Confidence-Based Guaranteed
Negotiated Service & Control of Kernel Processing Pipelines
Kernel Pipeline Efficiency Well-Demonstrated (Fall & Govindan) Use Real-Time Kernel Threads and Provide Trades between Utilization and Expected Number of Deadline Misses
Problems Addressed and Approach
Soft Real-Time - Confidence-Based Scheduling
Not Guaranteed, Not Best Effort -- Probabilistic Predictability Can Deadline Confidence Be Derived from Execution Time Confidence? -- Deadline Monotonic Confidence Extension Can WCET Hard Real-Time Requirement be Relaxed? -- Execution Time Model (Confidence Interval, Distribution) 99.9% of Deadlines are Met Given 99.9% Execution Time Confidence, Release Frequency, and Interference
Service Negotiation & Execution Control - EPA/PCIP
Can Execution Parameter Changes Be Handled Maintaining (Degrading) Deadline Confidences? -- Parametric Monitoring and Re-estimation of Confidences Can Application Modify Service On-Line? -- Dynamic Admission
RT EPA/PCIP Design Issues
Required RT-EPA Kernel Interface
Required Kernel Features (POSIX 1003.4 not sufficient)
Priority Preemptable Kernel-level Threads, Priority Inversion Control, VM Page Wiring, and Kernel-space Modules Simple Implementation in single-user RTOS (e.g. VxWorks) More complex Implementation in multi-user OS (e.g. Solaris) possible given required features
Portable RT Kernel-Ware modules (trusted like driver code)
EPA Service Negotiation Interface and Control
Soft and Termination Deadlines and Confidences (Computed) Execution Time Models (Distribution Assumption or Free) Output Hold (Buffering) for Isochrony Parameter Change Confidence Re-estimation (C(i), T(i), D(i)) Confidence and Deadline Management (Preserve Negotiated Confidences, Signal Inability to Maintain Negotiation)
RT EPA Interface Design
Low Frequency Interaction (Parameter Initialization and Modification)
kernel API Application
system call
High-Frequency Interaction and Per-Block Processing
HW / SW Interface Source Device
Execution Performance Agent
Device Interface
Pipe-Stage Filter
Device Interface
Sink Device
5 DOF Robot RTEPA Application
Robot Control
Execution Performance Agent
ARM
SHOULDER
WRIST
ELBOW
BASE
POS
RELAY
POS
RELAY
POS
RELAY
POS
RELAY
POS
RELAY
RACE RTEPA Application
RACE Control
Execution Performance Agent
RAMP RANGE
RAMP CONTROL
YAW CONTROL
YAW
CAMERA
68HC11
68HC11
DIGITAL COMPASS
RT EPA Confidence-Based Scheduling
release start time response utility earliest possible response earliest desired response desired optimal response latest desired response termination
response failure: dropout degradation time
response damage
utility curve
desired response interval WCET computation time Clow distribution , Dsoft signal Chigh , Dterm signal and abort
Cexpected best-case execution hold early response Rmin buffered response Ropt buffered response
context switch overhead
Extended Deadline Monotonic Equations
3 Basic Equations (sufficient but not necessary)
1) Clow and Chigh from distribution equation 2) EPA Deadline-monotonic admission test inequality given interference from current thread pool 3) Maximum Interference from current thread pool See www-sgc.colorado.edu/people/siewerts for detailed example
Termination deadline required for equations to work Pessimistic interference assumption (required for hard or mixed hard/soft real-time)
All interfering threads run to their termination deadline every release Worst case interference
Reliable interference assumption (for soft real-time only)
All interfering threads are assumed to run to high confidence Cexp
RT-EPA Interface
Pipeline task required inputs for admission
Entry point function pointer; Fptr Release source; ISR signal or interval timer RT POSIX signal Expected release period; Texp Expected periodic execution time; Cexp Execution time model; <sample array> or <noraml dist > Desired soft and hard termination deadlines; Dsoft and Dterm Desired soft and hard deadline confidences; Conf-soft, Conf-hard Output description and period; Dptr, size, Dest-addr, Tout
Returns
Soft and hard deadline confidence (actual or maximum possible given current thread pool)
Monitors & Controls
Actual execution time Actual release times Missed soft and hard deadlines Termination of threads at Dterm
RT-EPA Implementation and Evaluation
Demonstration RT EPA Implemented as VxWorks Task
EPA spawns and controls tasks (internal or interrupt release) Provides message queue interface for service negotiation Modeled as hard real-time periodic itself
RACE (Rail-guided, Air-powered, Control Experiment)
2 68HC11 microcontrollers (navigation and propulsion), 3 PWM...