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Achieving (hard) Real-Time with General Purpose Operating Systems

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  1. Achieving (hard) Real-Time with General Purpose Operating Systems

  2. Motivation • There is a conflict between the requirements of real-time systems and the need to run common day applications. • Can we use a general purpose OS for real-time applications?

  3. Outline • Basic definitions • The challenges • Can we use existing systems • Proposed solutions • Case studies of real-time oss.

  4. Basic definitions • A task is the basic unit of execution. Each task has three important properties: 1. release time: the point in time from which the task can be executed. 2. deadline: the point in time by which the task must complete. 3. execution time: the time the task takes to execute • E.g. brakes controller

  5. 0 1 2 3 4 5 6 7 8 9 10 11 example • If we have the following set of tasks: task1: release=0 deadline=3 exec time=2 task2: release=2 deadline=5 exec time=1 task3: release=6 deadline=10 exec time=3 Then they must be scheduled during these periods:

  6. Basic definitions • A processor is a concept used to denote a resource that is active in the execution of a task • Usually refers to the CPU.

  7. Classification of task timing constraints • There are two types of timing constraints hard and soft. • Definitions: • if missing a deadline is a fatal error or has disastrous consequences it is considered hard (missing an occasional deadline is not acceptable) • otherwise (if some percentage of missed deadlines is acceptable) it is soft. • Classic examples: braking systems controller => hard multimedia streaming => soft

  8. Soft real time systems • Contain only tasks with no hard timing constraints. • Also known as “best effort” systems • Most modern operating systems can serve as the base for a soft real time systems. • Examples: multimedia transmission and reception, networking, telecom (cellular) networks, web sites and services, computer games.

  9. Hard real time systems • Contains tasks with hard timing constraints. • Requires formal verification/guarantees of being to always meet its hard deadlines (except for fatal errors). • Examples: air traffic control , vehicle subsystems control, medical systems

  10. Classifications of real time systems • The type of system can be either a uni-processor , mp or distributed system. • Moving to an mp or distributed real time systems adds lots of difficulty by requiring real time communication and synchronization mechanisms between the processors.

  11. Classifications of real time systems • There are two different execution models: • In a preemptive model of execution a task may be interrupted (preempted) during its execution and another task run in its place. • In a non-preemptive model of execution after a task that starts executing no other task may execute until this task concludes or yields the CPU.

  12. Classifications of real time systems • The task model for a real time system has two main types: 1. purely periodic: every task is cyclic and executes periodically, even IO is polled. The nature of tasks doesn’t vary much between cycles. 2. aperiodic/asynchronous: most tasks in the system aren’t periodic.

  13. Classifications of real time systems • A static system is a system where all tasks are knows at design time including their release times (full apriory knowledge). • A dynamic systems is a system that can dynamically create and destroy tasks at runtime (No full apriory knowledge).

  14. Classifications of real time systems • In many real time systems the tasks have different priorities. There are two possible models : • static-priorities : the priorities of tasks don’t change during execution • dynamic-priorities the priority of tasks may change during execution

  15. Classifications of real time systems • Sets of tasks are classified according to the way they share resources: • We will call a task set dependent if the tasks in it share resources. • We will call a task set independent if they do not.

  16. Outline • Basic definitions • The challenges • Can we use existing systems • Proposed solutions • Case studies of real-time oss.

  17. The main challenges • Processor sharing – scheduling • Resource sharing • Handling external input / communication

  18. Scheduling • a scheduler needs to decide when to run each task in order to meet the deadlines imposed on the tasks. • In order to do this the scheduler needs to base its operation upon the properties of the tasks (release times, deadlines ,etc..).

  19. Scheduling • If scheduling is done online, the overhead of scheduling needs to be reasonable (this can be hard because most of the optimal scheduling algorithms are np-hard, even for a single processor). • If the task model is dynamic we need the scheduler to decide if adding some new task would prevent it from filling obligations to currently running tasks (admissibility testing).

  20. Resource sharing • When tasks share resources they become dependant on one another and this limits our flexibility when scheduling, requiring us to use synchronization mechanisms to protect critical sections in the tasks. • This raises the issues of deadlocks, process starvation and priority inversion.

  21. Handling external input / communication • In real time system there will often be constraints on the minimal time until the system processes input/communication. • These constraints must be reflected in the scheduling of tasks, the handling of external interrupts and the communication infrastructure and protocols chosen.

  22. Outline • Basic definitions • The challenges • Deficiencies in modern operating systems • Proposed solutions • Case studies of real-time oss.

  23. Why most modern OSs are unsuitable • The basic guiding concept for most operating systems and schedulers is to strive for good average performance while in real time systems meeting the deadlines is far more important than average performance. • many modern operating features are problematic in a hard real time setting.

  24. Problematic modern features • Protected memory: while important for the stability of the system it carries a high overhead in a real time environment because of the added cost to a context switch and the caches that need to be flushed each context switch ( the tlb for example). • User and kernel modes: while protecting the systems from user processes adds significant overhead because each mode switch incurs a performance penalty because of the trap instructions and cache flushes. • memory paging: can increase the cost of a context switch by several magnitudes if the memory pages we need were switched out. This causes a lot of unpredictability in the timing.

  25. Problematic modern features • Dynamic memory allocation: this is a critical feature used by most modern application, however it creates memory fragmentation and as a result adds to timing unpredictability.

  26. Linux • Linux Is based on UNIX which is a time sharing system designed to optimize average performance (as weighted by priorities) and Linux preserves this nature. This means that as the load increases the real time processes will suffer the most. • Linux currently doesn’t support an interface for telling the OS that a task is real time or informing it of any timing constraints on a task.

  27. Linux • The basic Linux scheduler is a time slice based on priorities, however it is a “fair” scheduler and favors interactive (IO bound) programs rewarding programs who haven’t used up all of their CPU share. • This presents several problems: 1. the time slice resolution may be higher than requires for the timing and so it may be impossible to meet the timing demands. 2. raising the priority of real time tasks is insufficient because enough lower priority tasks will cause us to miss deadlines anyway. 3.fairness is not a desirable quality in real-time systems.

  28. Linux • Linux does not provide a reliable mechanism to wake a task up at a certain time. The sleep timer can only promise to wake a task up after a certain time. • The kernel can’t be preempted so long system calls executed in the kernel can interfere with timing constraints. • External interrupts are disabled at several sections in the kernel to protect OS data from corruption (saves the effort of making them reentrant). This adds unpredictability to the amount of time it takes to respond to IO.

  29. Linux – kernel 2.6 changes • These has been some progress on some of these issues in the 2.6 Linux kernel: 1.the kernel is now preemptable although not all of it is. 2.improvements in IO handling. • despite these changes 2.6 is still not suitable for hard real time performance.

  30. Windows • Windows (NT based) suffers from most of the same problems Linux does. • Windows doesn’t support the specification of timing constraints for tasks.

  31. Windows • Interrupts may be delayed for an arbitrarily long time in critical sections of the kernel. • Handling of interrupts is done in two parts the second part the deferred procedure call is executed later in a FIFO manner and this may cause delays if a lot of DPCs are awaiting dispatch. • Requests for synchronization objects such as semaphores are also done in FIFO, leading to similar unpredictable delays.

  32. Windows • The windows scheduler supports threads on an OS level. Each thread is given a priority (0-31). • The scheduler has compensation for input bound threads and anti-starvation mechanisms. • Windows has a “real time” priority for threads but other real-time threads and the kernel can preempt a task with real time priority (15-26). • A “real time” priority is higher than some parts of the OS, causing most non-real-time apps to become unusable

  33. Fixing the problems • Fixing these problems in either OS would require either some form of “hack” into the operating systems design or a massive rewrite of the kernel. • This has been tried using both of the operating systems as we will see in later case studies (rt-linux, rialto).

  34. Outline • Basic definitions • The challenges • Deficiencies in modern operating systems • Proposed solutions • Case studies of real-time OSs.

  35. Scheduling • Scheduling algorithm properties: • An algorithm is said to be optimal if for each task set for which a feasible scheduling is possible the algorithm will find one (assumes tasks are independent). • Schedulable utilization is the maximal utilization of a task set such that a task load under this utilization will be scheduled successfully by the algorithm.

  36. Cyclic executive • This is a static table driven approach to scheduling. It assumes full apriori knowledge meaning tasks are static and periodic. • The schedule is created offline, so scheduling doesn’t load the system during runtime. • performance guarantees can be given before system is built. • The “algorithm”: analyze and schedule tasks offline using some algorithm which creates a static schedule for running all tasks during runtime.

  37. Cyclic executive • The cyclic executive approach can incorporate tasks of which there is no foreknowledge by scheduling them in empty spaces in the cycle, however it can’t make any guarantees about their timing. This approach works for the other algorithms as well.

  38. Priority driven algorithms • This is a family of algorithms which gives the tasks priorities according to some criterion and schedules at each point the task with the highest priority. They are efficient in that if there are tasks ready to execute the CPU is always busy. • This approach is attractive because in case of performance degradation the most important tasks will be affected the least.

  39. Rate monotonic • This algorithm assumes a periodic task model with preemption. • The Algorithm: is a priority driven algorithm where the tasks are assigned priorities proportional to their cycle length, the shorter the cycle the higher the priority.

  40. Rate monotonic • The algorithm is optimal in a way: if there is a feasible schedule for a periodic task load where Pi<=Di on a single processor with fixed priorities then RM will find a feasible schedule. • Liu and Leyland show that the schedulable utilization of RM is bounded by : where N is the number of real time tasks

  41. 4 1 3 2 1 1 2 2 0 1 2 3 4 5 6 7 8 9 10 example • If we have the following set of tasks: task1: period=deadline=3 execution time=1 task2: period=deadline=5 execution time=3 Then the schedule will look like this:

  42. Task 1 must have higher priority 0 1 2 3 4 5 6 7 8 9 10 The problem with static priority driven algorithms • This simple example shows that a schedulable set of tasks can’t be scheduled based on static priorities task1: period=deadline=4 execution time=2 task2: period=deadline=10 execution time=5 1 3 2 1 1 Task 2 must have higher priority

  43. EDF algorithm • The Earliest Deadline First algorithm is a priority driven algorithm in which the tasks get priorities according to how close their nearest deadline is. The closer the deadline the higher the priority.

  44. 0 1 2 3 4 5 6 7 8 9 10 example • If we have the following set of tasks: task1: period=deadline=3 execution time=1 task2: period=deadline=5 execution time= 3 Then the schedule will look like this: 4 1 3 2 1 2 2

  45. EDF properties • The algorithm is optimal. • The schedulable utilization of the algorithm is 1 . • Priorities must only be recalculated when a new task arrives, as time advances equally for all tasks. • the algorithm behaves unpredictably at overloads, and suffers more from overloads than RM.

  46. LST algorithm • The slack time of a task is the amount of time until the task must be scheduled or miss it’s deadline. • The least slack time algorithm is a priority driven algorithm in which the tasks receive priorities according the their slack times. The shorter the slack time the higher the priority. • The LST algorithm is optimal

  47. Priority inversion • The priority inversion problem exists in the preemptive model when tasks are dependant on one another (share resources). it causes a high priority task to wait for a low priority task to conclude because it is holding a critical resource. • This problem can be at least partially solved by priority inheritance, or the priority ceiling protocol based on a similar concept.

  48. Outline • Basic definitions • The challenges • Deficiencies in modern operating systems • Proposed solutions • Case studies of real-time OSs

  49. Case study • We will go over how some actual implementations of real time operating systems tackled these problems. • Rt-Linux • Rialto • VxWorks

  50. Rt-linux • RT-Linux is a system developed as a way to adapt Linux to hard real time performance and still give the user most of the advantages of the Linux environment. • The main idea: add a hard real time scheduler into (under) the kernel which will schedule the real time tasks and will run the rest of Linux as the lowest priority task.