1. RTOS Modelling
Naren Bala

2. Problem
Want to know if we can design a real-time system using a given RTOS
Expensive Simulation tools  for popular RTOS
performance metrics from some commercial RTOS vendors
RTOS Modelling – estimate the overhead and computation time for the given problem
Target RTOS – VxWorks
Applicable to all RTOS

3. VxWorks features Tasks share memory
Supports pre-emptive scheduling and round robin scheduling
No concept of kernel space and user space
Kernel clock – default of 60 Hz ~ 17 ms
minimum and maximum values for kernel clock depending on target hardware

4. RTOS modelling Task set {Ti (period), Ci (WCET)}
Priority Pi, Deadline Di
response time Ri = Ci + Bi
Overhead in using a RTOS

5. Measure Overhead High Precision measurements
Requires timestamp drivers
Timestamp driver – 40 ns
VxWorks default timers – 17 ms to 100 μs

6. Overhead in RTOS Models
Overhead in using a RTOS
Task overhead
Blocking overhead
Pre-emptive Kernel overhead

7. Task Overhead Creation and deletion of tasks 44 μs 0.2 μs
Allocation and deallocation of memory
0.72 μs μs
Context Switching
14 μs (2 empty tasks) 18 μs (loaded task)
Interrupt Latency

8. Blocking Overhead Disable and re-enable interrupts
< 100 ns < 100 ns
Message queues
Avg = 33 μs Stdev = 24 μs
Semaphores (binary, mutex)
0.2 μs (take) μs (release)
Priority avoidance protocol 174 μs

9. Pre-emptive Kernel Blockage due to pre-emption
Difficult to track the specific usage of time
Easier to measure “idle time”
Create an idle task with the lowest priority
Kernel Timers not precise
Require precise hardware timers for periodic tasks

10. Summary and Future Work
VxWorks is fast and deterministic
Same measurements can be easily repeated for most RTOS
Evaluate LM Bench on VxWorks
Need comparison with QNX, real-time Linux and Integrity
Interrupt latency measurements using a logic analyzer or oscilloscope

11. References Windriver systems, Tornado User Guide
Windriver systems, VxWorks Programmer Guide
Windriver systems, BSP Developer Guide
QNX Neutrino Real-time OS: Kernel Benchmark Methodology
Real-Time Concepts for Embedded systems, Qing Li, Caroline Yao, 2003