Real Time Operating Systems Lecture 10 David Andrews

What We Will Cover Today Operating Systems Services –Base Components Real Time Considerations –Task Management Time Triggered Event Triggered –Interprocess Communications Blocking Non-Blocking –Time Management Services Maintenance

Operating Systems Functionality Considerations –Footprint An Issue –Small Numbers of Programs –Virtual Memory Will Not be Required For Our Study Task Manager Scheduling Semaphores Timer Services Watchdog Countdown System Time IPC Synch Asynch File Mgt Virtual Mem Cache

Task Taxonomy Simple Task (S-task) No synchronization within task Executes from beginning to end, subject only to pre-emption by interrupts – Might be triggered by some other task initially – Might communicate results to some other task – But, no internal synchronization with other tasks Complex Task (C-task) Has some synchronization with other tasks – May have to wait for another task to process a request – May have to wait for I/O to complete – May have to wait for resource to become available (memory, network) Timing depends on the rest of the system – So, C-tasks are “bad” because they increase complexity – And, C-tasks make it more difficult to analyze system timing properties

C-Task Execution time S-Task execution time plus: Waiting for other processes… Not so bad if other tasks are S-Tasks and it is just a remote procedure call General case can be impractical to handle General solution: Break C-tasks into S-tasks separated by interactions Then, make sure that S-tasks are scheduled far enough apart for interaction to complete Great when it works...

Task Management Key component is scheduler: –Static: Schedule Built up Prior To Running Scheduler Develops Time Triggered Schedule Periodic/Predictable –Dynamic: Schedule Built During Run Time Can Be Based on Worst Case Execution Time (WCET) Scheduler Invoked by events –Timer, Interrupts, suspend, resume, etc.

Worst Case Execution Time (simple version)... Assume a simple task (S-task) that just executes its own code without coordinating with other threads or I/O How hard could it be to determine timing? Timing is once through the code + jitter due to: Non-fixed loop iteration count Non-fixed number of recursive calls Garbage collection Conditional statement execution asymmetry (if/else path timings) Effects of hardware non-determinism – Cache – Data-dependent instruction execution time – …

WACO … Plus S-Task Context Swapping Overhead from task switching: OS + Hardware … multiply by number of pre-emptions Gives WCAO (Worst-Case Administrative Overhead) due to pre-emptions Task A Save context OS Restore context Task B Save Context OS Restore context Task B

… Plus Effects From Nested Interrupts What if you have multiple prioritized interrupts Worst case execution time for interrupt at Priority X is = Execution time for interrupt at Priority 1 (perhaps multiple times) + Execution time for interrupt at Priority 2 (perhaps multiple times) + Execution time for interrupt at Priority 3 (perhaps multiple times) … + Execution time for interrupt at Priority X (perhaps multiple times) You can bound this as long as the period of each interrupt is bounded Same algorithm as for computing worst case CAN message latency(!) Scheduling on a CPU is a dual problem to scheduling on a network

Task Management Time Triggered –A priori build Task-Descriptor Table (TADL) TimeActionWCET Start T1 Send M5 Stop T1 Start T3 Send M Dispatcher

Interprocess Communications IPC can support Messages, or Common Data Regions… Messages –We have already discussed synch/asynch Asynchronous can be implemented in S-Tasks Synchronous (suspend) can be implemented in C-Tasks Common Data Regions –Implemented via shared memory Introduces critical regions Semaphore Operations needed –Test and Set

Time Management Time Management provides: –Clock synchronization –Time stamping –Watchdog Timers User Accessible Operating System

Real Time Operating Systems Unix “feel” QNX LinxOS “Smaller” RTOSs OS9, Microware VxWorks (be sure to get the memory protection extension) pSOS RTX, VenturCom A few research/freeware RTOS systems: KURT (KU real Time) RT-Mach, CMU …others Windows: WinCE; embedded NT, Windows + RT add-on products

Questions To Ask Your RTOS Vendor Real Time Operating System Purports to ensure timeliness of task execution Uses task priorities and a scheduling algorithm Hard questions: What is the tasking model (preemptive or non-preemptive)? Scheduler? What is the longest task switch latency? – Function of interrupt masking time How big is the footprint for a real system? (Not a stripped-down toy system) Does it have inter-task memory protection? How much of POSIX does it really support? How long does the system take to reboot? What is the runtime license fee? How robust is it (and what is the exception handling model?)