7. Multimedia Operating System. Contents. 7.3 Resource Management. 7.4 Process Management. 7.2 Real Time Systems. 7.5 Prototype Systems. 7.

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1 Contents 7. Overview 7.2 Real Time Systems 7.3 Resource Management Dimensions in Resource Design Reservation Strategies 7.4 Process Management Classification of Real-Time Scheduling Strategies Schedulability Tests and Optimal Schedulers Preemptive vs. Non-Preemptive Task Scheduling 7.5 Prototype Systems Real-Time Mach YARTOS (Yet Another Real-Time Operating System) / 52

2 7. Overview 2 / 52 Operating System Aspects for Multimedia Processing: Most conventional operating systems do offer only little or no support for intime processing of continuous media. This concerns all functions of an operating system like process, memory, file or device management. Resource Management: How to achieve a coordinated processing of all operating system functions in order to achieve an end-to-end Quality of Service (delay, capacity, loss rate, jitter,...)? An abstract continuous resource model A common resource management procedure Process Management: How to schedule processes permitting each to terminate according to ist deadline? Rate-Monotonic Scheduling Earliest Deadline First

3 7. Overview / Resource Management Tasks. Admission control Is there enough remaining capacity to handle the additional data stream? 2. QoS calculation Which characteristics (e.g. in terms of throughput and delay) are available for the new stream? 3. Resource reservation Reserves the resources which are required to meet the deadlines. 4. QoS enforcement Provision of service guaranted by appropriate scheduling, e.g. by reordering (serving tasks with short deadline earlier to a task with less strict bounds). 3 / 52

4 7.2 Real Time Systems 4 / 52 Real Time: A real-time task is a process which delivers ist processing result in a given time or according to a given deadline. A deadline is given e.g. in the order of ms for interactive voice or video data or days for text documents. Deadline: A deadline represents the latest acceptable time for the presentation of the processing result of a task. Deadline are called hard if failures are mission-critical or threatening human beings. They are called soft deadlines if they cannot exactly be determined or a violation is less critical. Fields of application: Control systems for manufacturing processes, military systems, telecommunication systems, aircrafts, automobiles, nuclear power plants or interactive multimedia systems.

5 7.2 Real Time Systems 5 / 52 Processing Requirements: Predictable fast response to time-critical events, accurate timing information, high degree of schedulability, i.e. resource capacity is not wasted (clever scheduling; but finding optimal schedules is often a NP-complete task), and stability under transient overload, i.e. using buffering to cope with bursty systems. Aspects specific to Multimedia Systems: In-time processing, transmission and presentation of continuous media data like audio or video. Requirements are known as Quality of Service (QoS) parameters, e.g.: throughput local or global (i.e. end-to-end) delay, (delay) jitter, or reliability. These values are usually specified by giving an average value, a worst case value, peak rates, distribution function or moments of its distribution function.

6 7.2 Real Time Systems real-time systems soft real-time systems hard real-time systems high availabilty high integrity fail safe fail opeartion telephone switching on-line banking railway signaling flight control soft real-time system - consequences of a system failure are of the same magnitude as utility high availability - down-time is minimal high integrity - consitency of data must survive any system failure and malicious attempt to alter the data hard real-time system - consequences of failure are catastrophic fail-safe system - probability to detect any failure is close to - system can be stopped fail-operational system - minimal service even in case of failure - system cannot be stopped 6 / 52

7 Resource Requirements 7.3 Resource Management 7 / 52 Killer Application Interactive Video insufficient scarce High-Quality Audio Network File Access abundant Hardware Resources in Year x Even sophisticated compression techniques cannot compensate for the resource capacity necessary for audio and video transmission as well as processing in current (interactive) multimedia systems.

8 7.3 Resource Management / Dimensions in Resource Design Active vs. passive resources depending on its autonomous processing capabilities active resources: CPU, network adapter card, etc. passive resources: file system, main memory, etc. Shared vs. exclusive resource usage active resource are usually allocated exclusively whereas passive ones can be shared by multiple tasks Single vs. multiple resource occurences PCs usually contain only a single CPU whereas e.g. a SPARC 20 workstation contains two CPUs 8 / 52

9 7.3 Resource Management / Procedure Resources CPU 4. Reservation Resource Manager 2. Schedulability 3. QoS Calculation. Request by a new task.. I/O Assign Resources Queue 6. Add Task 5. Calculate Schedule Dispatcher 7. Schedule Task Steps to 5: preparing task processing Steps 6 to 8: task processing 9 / 52

10 7.3 Resource Management / Reservation Strategies 0 / 52 optimistic pessimistic principle detect and solve conflicts avoid conflicts base for schedulability test average test potentially high resource utilization overbooking possible maximum for peak rate load no overbooking timelines of processing no guarantee guarantee The airline example: Northwest risky, lots of overbooking, solve conflicts by finding a customers who leaves the aircraft ( by paying something in cash or -better- in voucher) Lufthansa very cautious airline (no overbooking, no-shows), low actual load, high prices

11 7.3 Resource Management / Abstract Continuous Media Modelling: Workload / 52 Resource Messages Messages Interfaces Data streams consist of periodically arriving Logical Data Units (called Messages) and are described by the Linear Bounded Arrival Process (LBAP) Model. A data stream is a tripel (M, R, B), where M is the maximum message size, R is the maximum message rate (i.e. the number of message per time unit), and B is the maximum burstiness or allowed workahead. The model is named linear bound arrival process because it assumes that the number of message arrivals N in a given time interval is bounded by N ( ) = R + B (R message arrivals in time units)

12 7.3 Resource Management / Abstract Continuous Media Modelling: Example Audio sampling: 44. khz 6 bit per sample (i.e. 6 x 4400 = bit/sec) R = 75 frames/sec a frame contains /75 = bit M = (here: frame message ) =ˆ Number N M of messages (of size M) in t time units is given by: N ( t ) = R R M M + B (if no variations due tu bursts) (if maximum burst size is B) 2 / 52

13 7.3 Resource Management / Abstract Continuous Media Modelling: Workahead w(t) a 3 a 4 a l(m ) a 2 l(m 2 ) l(m 3 ) l(m 4 ) t a i = actual arrival time The workahead w(t) of an LBAP at time t describes how many messages have arrived that are not processed yet. It is defined by w(t) = max {0, N( [t 0, t] ) - R t - t 0 }. The logical arrive time l(m i ) of message m i is the time at which a message is effectively being scheduled. It is defined by l(m i ) = a i + w(a i ) / R (= actual arrival time + delay due to workahead) l(m i+ ) = max {a i+, l(m i ) + / R} where a i is the actual arriving time of message m i. 3 / 52

14 7.3 Resource Management / Abstract Continuous Media Modelling: Resources minim. log. delay = U maxim. log. delay = D Resource Queue Server Messages Messages Interfaces The logical delay d(m) of messages m between two interfaces I and I2 is defined by d(m) = l2 (m) - l(m). The buffer requirements of resource for a given data stream are defined by buf = B + R(D-U) where: B = number of messages which arrive unexpectedly due to burstiness, D = maximum logical delay between input and output interfaces, U = minimum (unbuffered) actual delay between the same interfaces, and R(D-U) = number of msg. which may be build up due to the variation processing time 4 / 52

15 7.4 Process Management / Basics 5 / 52 Process Management deals with the assignment of the CPU to processes/tasks. A process may be in one of five basic states: initial, i.e. it is created, but not in schedule; process is idle ready, i.e. it is waiting for CPU assignment, running, i.e. it is running on the CPU, waiting, i.e. it is waiting for an external event, or in finished. The scheduler chooses the next process to become running according to a given schedule. The schedule determines the order of CPU assignment to processes. in re ru fi Goals of traditional scheduling optimal throughput, optimal resource utilization, or fair queueing Goals of real-time scheduling execute maximum number of processes in time, i.e. according to their deadlines minimize deadline violations

16 7.4 Process Management / Classification of Real-Time Scheduling Strategies Scheduling strategies can be distinguished by static vs. dynamic schedule calculation, (static = calculation of schedule in advance dynamic = re-calculation whenever a new task arrives) central vs. distributed schedule calculation, and preemptive vs. non-preemptive task processing (preemptive = a task may be interrupted by any task with higher priority) They schedule tasks with periodic or aperiodic processing requirements, and independent tasks or tasks with precedency constrains. They are applied to either uniprocessor systems, multiprocessor systems (neglecting communication delay), or multicomputer systems (taking communictaion delay into account). 6 / 52

17 7.4 Process Management / Schedulability Tests and Optimal Schedulers 7 / 52 The test to determine wether a schedule exists for a given task set is called a schedulability test. There are three kinds of test: sufficient, exact, and necessary ones: Sufficient tests, i.e. if the test is positive, the task set is schedulable. A negative result is possible even if the task set is schedulable ( cautious test). Necessary tests, i.e. if the test is negative, the task set is not schedulable. A positive result does not guarantee a task set s schedulability ( optimistic test). Exact tests return a positive result if the tasks set is schedulable. Most exact schedulability tests belong to the class of NP-complete problems. A scheduler is called optimal if it always finds a schedule for tasks sets satisfying an exact schedulability test.

18 7.4 Process Management / Model for Real-Time Tasks A task is characterized by ist timing constrains and its resource requirements. Most tasks in multimedia systems are periodic and have no precedence constraints. s i e i d i p i period period 2 period 3 period 4 Model for a tasks timing constraint: (s i, e i, d i, p i ) where: s i starting point, i.e. ready time for first period e i processing time for period p i d i deadline for period pi (relative to ist period s ready time) n e i p period of task T U = i = p i r rate of T (with p = r-) If a task set consisting of periodic tasks (T,..., T n ) is schedulable [T i = (s i, e i, d i, p i )] then the processor utilization is given by (where e i /p i = realtive processor utilization by task T i ) 8 / 52

19 7.4 Process Management / Preemptive vs. Non-Preemptive Task Scheduling There are tasks sets that have valid preemptive schedules but no non-preemptive ones. If the cost for preemption is neglected, preemptive scheduling is always better or equal than nonpreemptive scheduling. Deadlines d d 2 d a d b d c d d d e d f High-Rated Task T a b c d e f Low-Rated Task T 2 2 Non-Preemptive Schedule a b c d 2 e f Preemptive Schedule a b c d e f Deadline violation 9 / 52

20 7.4 Process Management / Rate Monotonic Algorithm (RM) - Basics 20 / 52 Classificaton static, preemptive algorithm for periodic tasks Assumptions all time-critical tasks have periodic computing demands tasks are mutually independent (i.e. no precedence constraints) a tasks deadline equals ist period (d i = p i ) a tasks maximum computing time is constant and a-priori known context switches are considered timeless preemption is assumed to come without Principles cost (at least without time cost) shortest period highest priority (i.e. tasks are ordered by decreasing period) priorities are recalculated if a new task is added to the task set or a task is deleted from the task set [schedule calculation only once for a given task set] RM is optimal among static scheduling algorithm, i.e. if a task set is schedulable by any static algorithm then there exists a feasible RM schedule.

21 7.4 Process Management / Rate Monotonic Algorithm - Example T receives priority over T 2 T preempts T 2 to meet ist deadlines Performance of RM depends on the arrival pattern: worst case ( critical instant ): every task with higher priority arrives at the same time instant Deadlines High-Rated Task T a b c d period of T 2 Low-Rated Task T 2 2 RM Schedule a b c 2 d preemption of T T is resumed (preemption would not have been necessary in this example in order to meet the deadlines) 2 / 52 period of T d a d b, d d c d d, d 2

22 7.4 Process Management / Earliest Deadline First (EDF) Classification dynamic (i.e. re-calculation whenever a new task arrives), preemptive algorithm for periodic tasks Principles earliest deadline <=> highest priority priorities are re-calculated each time a task becomes ready (even for an unchanged task set) calculation has worst case complexity of O(n 2 ) Deadlines d a d b, d d c d d, d 2 High-Rated Task T a b c d Low-Rated Task T 2 2 EDF Schedule a b c 2 d 22 / 52

23 7.4 Process Management / Comparison of EDF and RM RM schedules potentially require more context switches, i.e. more (and necessary) preemptions, than EDF. Deadlines d a d b, d d c d d, d 2 High-Rated Task T a b c d Low-Rated Task T 2 2 EDF Schedule a b c 2 d RM Schedule a b c 2 d The higher number of preemptions for RM has to be compared with the additional cost for EDF (due to recalculation of schedules). 23 / 52

24 7.4 Process Management / Comparison of EDF and RM 24 / 52 Deadlines Rate monotonic vs. EDF: processor utilization d A d B d d 2 d 3 d 4 d 5 d 6 d 7 d C High Rate Low Rate A B C EDF A 2 A 3 B 4 B 5 6 C 7 C 8 Rate Monotonic A 2 A 3 A B 4 B 6 C 7 B 5 C 8 C d A not met d C not met Deadline Violations EDF is better than RM: If RM can schedule a task set then the same is valid for EDF but not vice versa.

25 7.4 Process Management / Achievable Processor Utilization with RM 25 / 52 Minimum utilization for all sets of tasks (I,..., I n ) which are schedulable and which fully utilize the processor. A task set fully utilizes a processor if task set can be scheduled, and if a single task is increased in processing time by ε > 0 then the assignment becomes infeasible. Example: Given a task set (I, I 2, I 3 ) with period p i and processing time e i for each task: p = 3, e = ; p 2 = 4, e 2 = ; p 3 = 5, e 3 = It can be shown that in this case e p p 3 2 3,max = p 3 e 2 e = = 2 p p [x] = smallest integer x

26 7.4 Process Management / Achievable Processor Utilization with RM: Theorem A set of n independent and periodic tasks (T,..., T n ) can be scheduled if e p ( 2 ) 2 n n n e p 2 e p n For n this expression converges to ln 2 0,693 As a consequence: A lower bound for processor utilization is: ln 2 if RM is applied (lower and upper bound) if EDF is applied (and deadline = end of period) 26 / 52

27 7.4 Process Management / Achievable Processor Utilization with EDF 27 / 52 For EDF a much better (i.e. perfect) utilization is possible: e p e p 2 Mixed scheme: Suppose we have n periodic tasks. Priorities given according to RM are I, I 2,..., I n (I shortest period, I n = longest period), i.e. highest priority for task. e n p n Compromise: Schedule I, I 2,..., I k with RM ( k n) Schedule I k+, I k+2,..., I n with EDF when the processor is not occupied by I, I 2,..., I k

28 7.4 Process Management / Achievable Processor Utilization with EDF 28 / 52 A A 2 A 3 A 4 A 9 A 4 A B B 2 B 3 B 4 B 5 C C 2 C 3 A B C A 2 B 2 C 2 B 3 C 3 A 5 A 6 A 3 A 4 B 4 B 5 C 4 C A 7 8 B 6 A 8 C 5 3 A 9 4 B 7 5 occupation + + = C 6 A , i.e.3 out of B 8 A C 7 B 9 A 2 C 8 A 3 A B 4 C 9 A 3 6 A 8 B 5 B A 5 B 2 C 0 A 7 C B 4 A 9C 2 A 20 B 0 60 slots remain empty C C 2 C occupation + =, i.e.only out of 60 slots remain empty A C Á 2 A 3 A 4 B 3 A 5 B C B 2 C C C 3 B Task (p = 3, e = ) Task 2 (p 2 = 4, e 2 = ) Task 3 (p 3 = 5, e 3 = ) RM Task 3 with e 3 = 2 Mixed strategy

29 7.4 Process Management / Deadline Monotone Algorithm Shortest deadline first: static algorithm which gives higher priority to a task with shorter deadline, i.e. T > T 2 if d < d 2 If deadline = period RM algorithm. This is similar but not identical to EDF. 29 / 52

30 7.4 Process Management / Shortest (Remaining) Processing Time The task with shortest process time (if non-preemptive) shortest remaining time (if preemptive has highest priority. This strategy serves the maximum number of customers. In an overload situation and if all tasks have the same deadline then this strategy minimizes the number of deadline violations. 30 / 52

31 7.4 Process Management / Preemptive vs. Non-preemptive 3 / 52 Preemptive is more complicated but: increases the feasability of scheduling (in some cases a preemptive schedule exists but no non-preemptive schedule) reduces the amount of priority inversion (i.e. situations where lower priority jobs are executed while higher priority jobs are waiting). A task set with processing times e i and with request periods p i is schedulable if: e p e p i i i i ln 2 (if 0,693 (if priorities task priorities are fixed assigned) may be dynamically adjusted, e.g. by EDF strategy) In general the resulting schedule is a preemptive one.

32 7.4 Process Management / Preemptive vs. Non-preemptive 32 / 52 Schedulability for non-preemptive strategies is more complicated: Task I k can be scheduled (worst case consideration) if its deadline d k satisfies the following: n = + d j e j d k e k + max e i + e j + i k (*) j k p j here I n is the highest priority task I is the lowest priority task (*) = own execution time (**) = waiting time due to job found in service at arrival time (worst case: maximum execution time of all jobs of other priority classes) (***) = execution time of all jobs of higher priority which are present at arrival time or which arrive during the waiting time (**) (***) job of priority k arrives my service time higher prio jobs arrive and are served x Requiremen t : d k! x

33 7.4 Process Management / Example for Achievable Processor Utilization Given a task set (I, I 2, I 3 ) with period p i and processing time e i for each task: p = 3, e = ; p 2 = 4, e 2 = ; p 3 = 5, e 3,max = 2 Utilization (mixedscheme) Utilization (RM) Utilization(EDF) = = 2, ,983 0,783 = 98% 78% 00% when I scheduled with RM, I 2 and I 3 scheduled with EDF increasing e * 3 to e 3 = Conjecture: Mixed scheme is nearer to the better side (namely EDF) than to the poor side (namely RM). 33 / 52

34 7.4 Process Management / Achievable Processor Utilization (here RM) Task I I I I I Task I 2 I 2 I 2 I 2 Task I 3 I 3 I 3 RM schedule I I 2 I 3 I I 2 I 3 I I 2 t Utilization: U actual = e p = + + = 0,783 e p 2 e p % The actual utilization is slightly better than the theoretical minimum: U theorem = 3 (2 3 ) = 0,78 34 / 52

35 7.4 Process Management / Least Laxity First (LLF) 35 / 52 Task with shortest remaining Laxity is scheduled first. Definition: laxity l k (t) of task T in period k at time t The remaining time from t to the deadline d of task T in period k that is not used for processing task T. l k (t) = (s + (k-) p + d) - (t + e rem (t)) = (Deadline in period k) - (actual time + remaining processing time)

36 7.4 Process Management / Least Laxity First (LLF) l k (t ) e rem (t) l k (t 2 ) e Deadline in period k s + k p s s + (k-)p t t period k t 2 s + (k-) p+d LLF has no advantage over the EDF for uniprocessor systems. If tasks have similar laxity values context switches can occur frequently (calculation overhaead compared to EDF). LLf might be suitable only in multiprocessor systems, i.e. when several resources are scheduled simultaneously. 36 / 52

37 7.4 Process Management / Least Laxity First - Example for Two Tasks 37 / 52 Given a minimum CPU time granularity of 2 if no granularity assumed task preemption as often as possible processor sharing Task I Task I 2 LLF Schedule I I 2 I I 2 I I 2 I t L 2 (I ) = 20 L 2 (I 2 ) = 4 I 2 interrupts I L 8 (I ) = 4 L 8 (I 2 ) = 4 I interrupts I 2 L 0 (I ) = 4... L 0 (I 2 ) = 2 I 2 interrupts I I 2 finished I finished Laxities do not change during execution time of a task but become smaller when a task is not served. If the laxity of a waiting task becomes smaller than that of the running task then the running task is interrupted.

38 7.4 Process Management / Scheduling Aperiodic (but Independent) Tasks 38 / 52 Multimedia applications consists of both periodic and aperiodic tasks, like most real-time systems. Period tasks are used for transport and processing of continuous media data whereas control or management tasks are aperiodic. Often it is reasonable to give to periodic tasks priority over aperiodic tasks. Problem: How to schedule a task set which is composed of periodic as well as aperiodic tasks? Idea: One special periodic task (called server) is polling for ready aperiodic tasks to be processed.

39 7.4 Process Management / Scheduling Aperiodic (but Independent) Tasks Problem: Aperiodic tasks can only be processed when server task is scheduled, i.e. they miss their deadline if it is earlier than the next time the server is scheduled. Solution: So-called bandwidth preserving algorithm like Priority Exchange, Deferrable Server, or Sporadic Server. 39 / 52

40 7.4 Process Management / Bandwidth Preserving Algorithms Priority Exchange: One periodic task T s serves all aperiodic tasks. The server exchanges priority with a lower prior periodic task T i if no aperiodic task is ready. The server receive its initial priority for the next period. Case : T s serves > 0 aperiodic tasks T T s T... T i... T n T T s T... T i... T n period k period k+ Case 2: T s has no task to serve, changes priority with T i time T i was scheduled. and is reactivated at the T T i T... T s... T n T T s T... T i... T n period k period k+ 40 / 52

41 7.4 Process Management / Bandwidth Preserving Algorithms Deferrable Server: One periodic task (called deferrable server) serves all aperiodic tasks. It defers its processing time if no aperiodic task is ready but retains ist priority. As soon as an aperiodic task request occurs, it either (immediatly) preempts the running tasks (that has a lower priority), or resumes processing after the current task terminates. aperiodic task arrival T T s T 2 T 3 T 4... T n nothing to serve Interrup T 4 or wait until T 4 is finished 4 / 52

42 7.4 Process Management / Bandwidth Preserving Algorithms Sporadic server: It combines the advantages of the Priority Exchange and the Deferrable Server algorithm. A sporadic server exchanges its priority when no aperiodic task is ready. Any spare CPU capacity, i.e. time not used by periodic tasks, is transformed into a ticket, that is given to a sporadic server, which then replenishes its initial priority. This way, the sporadic server is allowed to use any idle time of the CPU. 42 / 52

43 7.4 Process Management / Scheduling Tasks with Blocking 43 / 52 The blocking of a task is caused by mutual exclusion when trying to access a critical section currently occupied by another task. Priority Inversion Effect: Assume a high-priority task T h wants to enter a critical section currently occupied by a low-priority task T l. T h is blocked until T l leaves the critical section. Until then, not only T l but all medium-priority tasks T m having higher priority than T l and lower priority than T h will be processed prior to T h. This phenomenon is called priority inversion. Task T l Task T m Task T h processing unit(s) signal(s) critical section proc. blocked until The rule: A low priority task must wait, i.e. cannot begin execution, if a higher priority task is running.

44 7.4 Process Management / Scheduling Tasks with Critical Sections 44 / 52 Several algorithms have been developed in order to avoid priority inversion, e.g. Priority Inheritance, or the Ceiling Protocol. Priority Inheritance: A low-priority task T l inherits the priority of a high-priority task T h if it causes the blocking of T h. T l retains ist initial priority when leaving the critical section. Thus: Ready jobs between low and high are blocked. Disadvantage: Lower utilization of server, possible deadline violations. Advantage: Sequence of schedules is preserved.

45 7.4 Process Management / Scheduling Tasks with Critical Sections More Problems with standard priority inheritance: Tasks can be blocked in each critical section, deadlocks can occur, as well as transitive blocking is possible, i.e. T 3 is blocked by T 2 that is blocked by T Principle (ceiling protocol): Each task has two priorities: a static one assigned by the scheduler, e.g. according to RM strategy a dynamic one, i.e. the maximum of the static priority and the highest priority inherited 45 / 52

46 7.4 Process Management / Scheduling Tasks with Critical Sections Each semaphore has a maximum or ceiling priority value of all tasks that actually use it. A semaphore only can be locked by a task with a higher dynamic priority than any currently locked semaphore. (comparable to hierarchy of resources approach to deadlock avoidance) Benefits: A task is only blocked once, no deadlocks and transitive blocking. Price: Restrictive locking policy (brute force method) longer blocking delays for other tasks. 46 / 52

47 7.5 Prototype Systems / Real-Time Mach 47 / 52 It is a distributed operating system for real-time applications based on the ARTS (Advanced Real-Time System), both developed at Carnegie Mellon University at Pittsburgh, PA, USA. RT Mach: supports multi-processor clusters n processors into a processor set assigns a separate run queue and scheduling strategies to each processor set enables an application to select the current scheduling strategy (at run-time) processor processor set RT Mach kernel scheduler strategy run queue

48 7.5 Prototype Systems / Task Scheduling in RT Mach 48 / 52 RT Mach offers three classes of tasks (called RT Threads): periodic or aperiodic (sporadic) tasks with hard deadlines, or tasks with soft deadlines. Tasks are scheduled by one of the following strategies (dynamically changable): rate monotonic (RM), with deferrable server (RM/DS), or sporadic server (RM/SS), fixed priority (FP), as well as round robin (RR). Thread dispatching management controls idle threads and aperiodic server (DS or SS). Processor set management performs context switching, thread preemption, or processor assignment. Generic Scheduler Thread Dispatching Mgmt. Processor Set Mgmt. RM RM/DS RM/DS FP RR

49 7.5 Prototype Systems / The QoS Ticket Model The QoS Ticket Model combines resource reservation and adaptation. It has been implemented on RT-Mach 3.0 supproting so-called Q-Threads. QoS Manager 2. Calculation of Resource Allocation. QoS Request 4. Issue Ticket Multimedia Session QoS Ticket 5. Consumption Information 3. Resource Reservation RT Mach kernel Qos Ticket allows users to specify tolerance ranges for period and computation time. Ticket is issued for each session comprising several threads. QoS parameters are adapted dynamically based on the current resource consumption. 49 / 52

50 7.5 Prototype Systems / Comparison of RT-Threads and Q-Threads invocation time periodic RT-Thread Q-Thread user-defined entry point is called periodically thread attributes fixed invocation period ranges for period and computation time invocation period fixed (can be re-specified) dynamic, within range, based on QoS control policy guarantee of execution purpose none (possible with CPU reservation) guaranteed, within the available computation time real-time processing continuous-media processing 50 / 52

51 7.5 Prototype Systems / YARTOS (Yet Another Real-Time Operating System) Micro kernel implemented in C on IBM PS/2 (Intel processor) at University of North Carolina at Chapel Hill, NC, USA. YARTOS supports.tasks, i.e. independent threads of control invoked by repeatedly occuring events, and 2. resources which synchronize concurrent access to shared data. Its scheduler guarantee that. each task invocation completes processing before ist deadline, and 2. no shared resource is accessed simultaneously by more than one task. 5 / 52

52 7.5 Prototype Systems / YARTOS (Yet Another Real-Time Operating System) 52 / 52 It offers two separate notions of deadlines for tasks,. One for initial aquisition of the processor, and 2. One for execution of operations on resources (to avoid priority inversion). It performs on-line scheduling of sporadic tasks extending the Earliest Deadline First algorithm with synchronized access to shared resources on a uniprocessor system. YARTOS e.g. serves for a workstation-based audio and video conferencing system (over a 6 Mbit Token Ring LAN).