Chapter 15 Specification of Software Components

1. Chapter 15 Specification of Software Components Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

2. Overview • Introduction • Windows NT as a real-time OS • Real-time features of windows NT • Observations and recommendations • Windows NT in a real-time setting • Summary Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

3. Introduction • Windows NT is presented from the perspective of real-time constraints. • Address the following questions: • What type of applications can use Windows NT, as is? • Can the unpredictable parts of NT be “masked”? Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

4. Windows NT as a Real-Time OS • The original design of NT was based on a micro-kernel, common to many real-time operating systems. • During the evolution a limited number of real-time features have been incorporated: • IDLE, NORMAL, HIGH and REALTIME. • The REALTIME priority class is available as a support for real-time applications. • The base priority for the REALTIME class includes levels 16 to 31. Levels 17 to 21 and 27 to 30 are currently not used by thescheduler. Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

5. Windows NT Priority Structure Interrupts Deferred Procedure Calls (DPC) Time Critical NA Highest Above Normal Normal Below Normal Lowest NA Idle 31 26 25 24 23 22 16 15 0 Real-Time Priority Class Time Critical Normal Foreground Normal Background Below Normal Idle Idle Thread Dynamic-Time Priority Classes Decreasing Priority Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

6. Scheduling • Priority based preemptive scheduling: • The highest priority active task is chosen for execution possibly interrupting the execution of another task. • Base priority • REALTIME: non-degradable priorities • NORMAL/HIGH can be delayed • For this reason the latter priority classes are commonly referred to as dynamic classes. Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

7. Deferred Procedure Call • Interrupt service routine (ISR) • Deferred procedure call (DPC) • DPCs are queued in the system DPC queue, in a first in first out (FIFO) manner. • Separation of ISR and DPC • Allows quick response to any further interrupts, • The priority structure at the interrupt level is not maintained in the DPC queues. • A DPC is not preemptable by another DPC, • A DPC be preempted by an (unimportant) interrupt. Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

8. Real-Time Features of Windows NT • Empirical Characterization of NT’s Features which affect Real-Time Activities • The first set of experiments was targeted toward the behavior of threads at REALTIME priority class and their effect on the I/O Subsystem, and visa versa. • To this end, we used two threads with the same thread priority in the REALTIME class, one performing I/O and another, a CPU-intensive thread, performing a continuous “for loop”. Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

9. Experiment 1 • To study the effect on keyboard and mouse I/O, the I/O thread was made to read from the keyboard/mouse. • When the CPU-intensive thread was running, no I/O activity was observed. • After the CPU-intensive thread completed, all the keyboard inputs were processed. • This showed that the CPU-intensive real-time thread essentially shuts out keyboard/mouse I/O even when this I/O is from/to a real-time thread. Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

10. Experiment 2 • To study the effect on disk I/O, the I/O thread was made to write a file with 40,000 64-bit values. • The time-stamps for the I/O and CPU-intensive activities were found to be interleaved, this indicating time-sharing between the two threads. • This showed that a CPU-intensive real-time thread did not shut out disk I/O. Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

11. Experiment 3 • To study the effect on network I/O, the I/O thread was made to read data from a remote server using Windows Sockets API. • The time-stamps for the two activities were found to be interleaved, this indicating time-sharing between the two threads. • This showed that a CPU-intensive real-time thread has no adverse impact on network I/O. Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

12. Explaining the Observations • The I/O manager sends the request in the form of an I/O request packet (IRP) to the device driver. The driver starts the I/O operation. • The device completes the I/O operation and interrupts. The device driver then services the interrupt. (This involves execution of ISR and queuing of a DPC.) • The I/O manager completes the I/O request. Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

13. Win32API Function Name Time (msecs) CreateProcess() 2600 SetPriorityClass() - from normal to real-time priority class 240 SetPriorityClass() - for all others combinations 125 SetThreadPriority() - for a thread to set its own priority 9 SetThreadPriority() - for a thread to set priority of another thread of the same process 10 QueryPerformanceCounter() - to obtain the current time-stamp 6 Time Taken for System Operations Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

14. Observations and Recommendations • Potential blocking time • If processes or threads are performing network or disk I/O, the effect of system-wide FIFO DPC queues may lead to unbounded response times. • If the duration of I/O activity in a given period can be characterized, it may be possible to compute the response times conservatively. • One should not depend on the Windows NT scheduler to achieve correct “fair sharing” behavior in cases where screen, keyboard and mouse interactions are at the same level of priority as the other real-time CPU-intensive tasks. Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

15. Windows NT in a Real-Time Setting • Operator input • Incoming sensor data • Incoming video streams Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

16. Design of the Real-Time Application • Efficiency through threads • Achieving periodicity Heart beat is a highest prio process • Periodically enables user-level threads • User-level threads can execute for 1 time unit only Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

17. Software Architecture of Industrial Control Prototype Receiver Buffer Consumer Producer Heartbeat Ti Heartbeat Ti Operator input Real Video Operator ack Acknowledgement Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

18. Software Architecture of Industrial Control Prototype • Besides the Heartbeat timer at each node, the main entities at the remote server are: • Producer • Acknowledgment • The main entities at the operator workstation are: • Receiver • Consumer • Operator input • Operator ack Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

19. Summary • Using HIGH or REAL_TIME priority alone significantly reduces the variability in response times, without any observable degradation in system performance, so: • IF the application tasks do not monopolize the CPU for long durations, • AND there is sufficient CPU capacity, • THEN using these priority assignments may be sufficient to meet the performance requirements of these processes – even when I/O is involved. Building Reliable Component-based SystemsChapter 15 - Specification of Software Components

20. Summary Continued • However: • IF the periodic workload per process is high, or • IF a process is a COTS application whose workload varies, • THEN it will be necessary to impose some additional controls on the amount of time allocated to a task. Our findings indicate that a scheduling approach combined with an admission Building Reliable Component-based SystemsChapter 15 - Specification of Software Components