CEC 450 Real-Time Systems

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1 CEC 450 Real-Time Systems Lecture 7 Review October 9, 2017 Sam Siewert

2 Coming Next Finish Up with Recount of Mars Pathfinder and Unbounded Priority Inversion Mike Jone s Page (Microsoft) Glenn Reeves on Mike Jone s Account Robot Science and Technology Account Review Take Practice Quiz (Wednesday or Monday 10/16) Exam Next Week Wed and Friday, 10/18, 10/20 Monday Q&A Session and Quiz Solutions Overview Wed Knowledge, Concepts, Short Answer Friday Problem Solving Sam Siewert 2

3 The Scenario Mars Pathfinder Story The Debugging Process The Root Cause The Fix Sam Siewert 3

4 Exam #1 Revew RTECS Chapters 1 to 6 Ch. 1 Introduction, RT Correctness, Definitions Ch. 2 System Resources, CPU, I/O, Memory Bound Ch. 3 Processing, RM LUB Derivation, WC Analysis Ch. 4 I/O Resources, Start/End I/O, Execution Efficiency Ch. 5 Memory (Review from Microprocessors) Ch. 6 Multi-Resource Services, Unbounded Inversion, Deadlock, Mars Pathfinder Assignments #1, #2 (graded), #3 grades posted on Tuesday Cheddar Tool Jetson Fundamentals (Using with NUC, Native - not Cross) POSIX RT Extensions Threads, Synchronization, Timeouts, Processor Affinity, SCHED_FIFO Sam Siewert 4

5 Typical RTOS Scheduling Policies Fixed Priority Preemptive Tasks State Based on Resources Required Needs CPU Only = Ready Non-CPU Resource Required = Pending Task Context Switch on System Call or Interrupt Dispatch from Ready Queue Dispatch with Rate Monotonic Priority RM LUB Feasibility Feasibility Tests Scheduling Point Algorithm Completion Test Algorithm Over LCM of Periods Dynamic Priority Premptive Earliest Deadline First Least Laxity First Wait on Resource (semtake()) Pending semgive() Dispatch Resume (taskresume() ) Note: Different from Chapter 7 In VxWorks Kernel Programmer s Guide, Ref Manual Executing Ready Suspended, +Delayed, +Pending Suspend (unhandled exception e.g. division by 0!, tasksuspend() ) Interference (preemption) Timeout Yield with Wakeup (taskdelay()) Delayed Sam Siewert 5

6 POSIX RT Scheduling Policies Pthreads with SCHED_FIFO Pthread_create Pthread attributes Processor Affinity Pthread join Dispatch from Ready Queue Dispatch with Rate RT Max to Min Priority System Calls Interference or Yield pthread_yield() Blocking/Pending sigwait(), pthread_mutex_lock() Delay usleep(), nanosleep(), pause() Suspension, Resume sigsuspend(), sigqueue() Wait on Resource (sigwait(), pthread_mutex_lock()) pending Dispatch pthread_mutex_unlock() executing ready Suspend (unhandled exception such as division by 0!, sigsuspend()) Suspended, +Delayed, +Pending Interference or Yield (preemption, sched_yield()) Yield with Wakeup (usleep(), pause()) delayed Sam Siewert 6

7 Exam #1 Review Fundamentals of RT Analysis RT Correctness Before Deadline and Correct Result Best Effort, Hard RT, Isochronal RT, and Soft RT Utility Curves for each of the above CPU, I/O, and memory resource space (CPU-bound, I/O-bound, memory/cpu trade-offs) Basic Timing Diagrams Theorem 1 RM Least Upper Bound Theorem 2 (Lehoczky, Shah, Ding) If deadline(s) are met over longest period (or better yet, LCM) from C.I., then system is feasible Fixed priority, preemptive, run-to-completion scheduling Services and interference (interrupts and higher prio services) Necessary and Sufficient feasibility means? Sam Siewert 7

8 Exam #1 Review Deadline Monotonic Theory DM priority assignment policy Simple sufficient feasibility test Improved (more necessary) feasibility test Differences between RM and DM theory T not equal to D Priority assignment policy Iterative feasibility test Sam Siewert 8

9 Exam #1 Review Rate Monotonic Theory RM priority assignment policy (optimality is?) Liu and Layland Assumptions C = WCET T=D Critical Instant Others? RM Feasibility test is? Derivation of 2 task RM sufficient LUB LUB is O(n+c) complexity equation Sufficient, but not Necessary and Sufficient Scheduling Point O(n 3 ), but N&S Completion Test - O(n 3 ), but N&S Dynamic Priority Theories EDF Least Laxity Sam Siewert 9

10 Exam #1 Review Linux and POSIX RT Extensions Phtread create and join SCHED_FIFO Priorities RT Max, RT Min Attributes SCHED_OTHER CPU Affinity Real-Time Clock [Relative and Absolute Time] Blocking and Timeouts Timespec struct Timed wait logmsg versus printf Printf I/O delays caller and can t be called in kernel/isr context logmsg performs output in slack time via tlogtask and message queue interface Sam Siewert 10

11 Exam #1 Review RT Synchronization Priority Inversion Unbounded Priority Inversion Priority Inheritance Priority Ceiling Necessary 3 Conditions for Unbounded Prio Inversion 3 or more tasks (H, M, L priority) H and L tasks involved in mutex 1 or more M tasks not involved in mutex cause interference Mars Pathfinder Story What went wrong? Why? How was it fixed? Sam Siewert 11

12 Exam #1 Review Linux POSIX RT Extensions Message queues Priority enqueue and dequeue Same priority? Blocking vs. non-blocking send and receive Real-time signals Signals that queue why? Passing data how? Real-Time interval timers and clocks Systems tick and interval timer jiffy Hardware interval timer operation Virtual timers built on hardware interval timer(s) Sam Siewert 12

13 Exam #1 Review Service Efficiency Concepts Blocking is Evil Ideally Block Only on Service Release Mechanism After Release, Run to Completion Other than Interference Avoid I/O During Release Input, Release, Output Path Length Driven by Algorithm Driven by Compiler Code Generation Path Execution Efficiency Cache Misses Pipeline Stalls Intermediate I/O Overlapping Intermediate I/O with CPU Hiding Latency CPU-bound? IO-bound? Sam Siewert 13

14 Exam #1 - Code Walkthroughs POSIX RT Threads Simple Thread Improved Thread POSIX RT Examples Clock Linux demo Message Queues RT Clock Lab #1 - Sequencer Better, More Generic Sequencer [in preparation] Synch Sam Siewert 14

CEC 450 Real-Time Systems

CEC 450 Real-Time Systems Lecture 2 Introduction Part 1 August 31, 2015 Sam Siewert So Why SW for HRT Systems? ASIC and FPGA State-Machine Solutions Offer Hardware Clocked Deterministic Solutions FPGAs