1. Server Resources 6/9 - 2004 INF5070 – Media Storage and Distribution Systems:

2. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Overview Resources, real-time, … “Continuous” media streams (CPU) Scheduling Memory management for streaming

3. Resources and Real – Time

4. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Resources Resource: “A resource is a system entity required by a task for manipulating data” [Steimetz & Narhstedt 95] Characteristics:  active: provides a service, e.g., CPU, disk or network adapter  passive: system capabilities required by active resources, e.g., memory or bandwidth  exclusive: only one process at a time can use it, e.g., CPU  shared: can be used by several concurrent processed, e.g., memory  single: exists only once in the system, e.g., loudspeaker  multiple: several within a system, e.g., CPUs in a multi-processor system

5. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Real–Time Real-time process: “A process which delivers the results of the processing in a given time-span” Real-time system: “A system in which the correctness of a computation depends not only on obtaining the result, but also upon providing the result on time” Many real-time applications, e.g.:  temperature control in a nuclear/chemical plant driven by interrupts from an external device these interrupts occur irregularly  defense system on a navy boat driven by interrupts from an external device these interrupts occur irregularly  control of a flight simulator execution at periodic intervals scheduled by timer-services which the application requests from the OS ...

6. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Real–Time Deadline: “A deadline represents the latest acceptable time for the presentation of the processing result” Hard deadlines:  must never be violated  system failure  too late results have no value, e.g., processing weather forecasts means severe (catastrophic) system failure, e.g., processing of an incoming torpedo signal in a navy boat scenario Soft deadlines:  in some cases, the deadline might be missed not too frequently not by much time  result still may have some (but decreasing) value, e.g., a late I-frame in MPEG

7. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Real–Time and Multimedia Multimedia systems  typically have soft deadlines (may miss a frame)  are non-critical (user may be annoyed, but …)  have periodic processing requirements (e.g., each 33 ms in a 30 fps video)  require large bandwidths (e.g., average of 3.5 Mbps for DVD video only)  need predictability (guarantees)  adapt real-time mechanisms to continuous media  exploit resource-specific properties (like real-time resource allocation schemes, preemption,...)  priority-based schemes are of special importance

8. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Admission and Reservation To prevent overload, admission may be performed:  schedulability test: “are there enough resources available for a new stream?” “can we find a schedule for the new task without disturbing the existing workload?” a task is allowed if the utilization remains < 1  yes – allow new task, allocate/reserve resources  no – reject Resource reservation is analogous to booking (asking for resources)  pessimistic avoid resource conflicts making worst-case reservations potentially under-utilized resources guaranteed QoS  optimistic reserve according to average load high utilization overload may occur  perfect must have detailed knowledge about resource requirements of all processes too expensive to make/takes much time

9. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Real–Time and Operating Systems The operating system manages local resources (CPU, memory, disk, network card, busses,...) In a real-time, multimedia scenario, support is needed for:  real-time processing  efficient memory management This also means support for proper …  scheduling – high priorities for time-restrictive multimedia tasks  timer support – clock with fine granularity and event scheduling with high accuracy  kernel preemption – avoid long periods where low priority processes cannot be interrupted  memory replacement – prevent code for real-time programs from being paged out  fast switching – both interrupts and context switching should be fast ...

10. Continuous Media Streams

11. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Start playback at t 1 Consumed bytes (offset)  variable rate  constant rate Must start retrieving data earlier Data must arrive before consumption time Data must be sent before arrival time Data must be read from disk before sending time Streaming Data t1t1 time data offset consume function arrive function send function read function

12. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Need buffers to hold data between the functions, e.g., client B(t) = A(t) – C(t), i.e.,  t : A(t) ≥ C(t) Latest start of data arrival is given by min[B(t,t 0,t 1 ) ;  t B(t,t 0,t 1 ) ≥ 0], i.e., the buffer must at all times t have more data to consume Streaming Data time data offset t1t1 consume function arrive function t 0t 0

13. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems file system communication system application “Continuous Media” and “Streaming” are ILLUSIONS  retrieve data in blocks from disk  transfer blocks from file system to application  send packets to communication system  split packets into appropriate MTUs ... (intermediate nodes) ... (client)  different optimal sizes  pseudo-parallel processes (run in time slices)  need for scheduling (to have timing and appropriate resource allocation) Streaming Data

14. (CPU) Scheduling

15. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Scheduling A task is a schedulable entity (a process/thread executing a job, e.g., an packet through the communication system or a disk request through the file system) In a multi-tasking system, several tasks may wish to use a resource simultaneously A scheduler decides which task that may use the resource, i.e., determines order by which requests are serviced, using a scheduling algorithm Each active (CPU, disk, NIC) resources needs a scheduler (passive resources are also “scheduled”, but in a slightly different way) resource requests scheduler

16. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Scheduling Scheduling algorithm classification:  dynamic make scheduling decisions at run-time flexible to adapt considers only actual task requests and execution time parameters large run-time overhead finding a schedule  static make scheduling decisions at off-line (also called pre-run-time) generates a dispatching table for run-time dispatcher at compile time needs complete knowledge of task before compiling small run-time overhead  preemptive currently executing task may be interrupted (preempted) by higher priority processes preempted process continues later at the same state potential frequent contexts switching (almost!?) useless for disk and network cards  non-preemptive running tasks will be allowed to finish its time-slot (higher priority processes must wait) reasonable for short tasks like sending a packet (used by disk and network cards) less frequent switches

17. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Scheduling Preemption:  tasks waits for processing  scheduler assigns priorities  task with highest priority will be scheduled first  preempt current execution if a higher priority (more urgent) task arrives  real-time and best effort priorities (real-time processes have higher priority - if exists, they will run)  to kinds of preemption: preemption points o predictable overhead o simplified scheduler accounting immediate preemption o needed for hard real-time systems o needs special timers and fast interrupt and context switch handling resource requests scheduler preemption

18. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Scheduling Scheduling is difficult and takes time (both to find a schedule and to switch between threads/processes – not shown): process 1process 2process 3process 4process NRT process … request round-robin process 1 process 2process 3process 4process N … RT process request priority, non-preemtive delay RT process delay process 1process 2process 3process 4process N … request priority, preemtive p 1 process 2process 3process 4process N … RT process p 1process 2process 3process 4process N … only delay switching and interrupts NOTE: preemption may also be limited to preemption points (fixed points where the scheduler is allowed to interrupt a running process)  giving larger delays

19. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Priorities and Multimedia Multimedia streams need predictable access to resources – high priorities: Within each class one could have a second-level scheduler  1 and 2: real-time scheduling and fine grained priorities  3: may use traditional approaches as round-robin 1. multimedia traffic with guaranteed QoS 2. multimedia traffic with predictive QoS 3. other requests may not exist must not starve

20. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Scheduling in Windows 2000 Preemptive kernel Schedules threads individually Time slices given in quantums  3 quantums = 1 clock interval  different values used for different clock frequencies, e.g., x86 uniCPU: 10 ms x86 multiCPU: 15 ms  defaults: Win2000 server: 36 quantums Win2000 workstation: 6 quantums (professional)  may manually be increased between threads (1x, 2x, 4x, 6x)  foreground quantum boost (add 0x, 1x, 2x): active window can get longer time slices (assumed needs fast response)

21. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Scheduling in Windows 2000 32 priority levels: Round Robin (RR) within each level Interactive and throughput-oriented:  “Real time” – 16 system levels fixed priority may run forever  Variable – 15 user levels priority may change: thread priority = process priority ± 2 uses much  drops user interactions, I/O completions  increase  Idle/zero-page thread – 1 system level runs whenever there are no other processes to run clears memory pages for memory manager 31 30... 17 16 15 14... 2 1 0 Real Time (system thread) Variable (user thread) Idle (system thread)

22. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Scheduling in Linux Preemptive kernel Threads and processes used to be equal, but Linux uses (in 2.6) thread scheduling SHED_FIFO  may run forever, no timeslices  may use it’s own scheduling algorithm SHED_RR  each priority in RR  timeslices of 10 ms (quantums) SHED_OTHER  ordinary user processes  uses “nice”-values: 1≤ priority≤40  timeslices of 10 ms (quantums) Threads with highest goodness are selected first:  realtime (FIFO and RR): goodness = 1000 + priority  timesharing (OTHER): goodness = (quantum > 0 ? quantum + priority : 0) Quantums are reset when no ready process has quantums left: quantum = (quantum/2) + priority 1 2... 126 127 1 2... 126 127 default (20) -20 -19... 18 19 SHED_FIFO SHED_RR SHED_OTHER nice

23. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Scheduling in AIX Similar to Linux, but has always only used thread scheduling  SHED_FIFO  SHED_RR  SHED_OTHER BUT, SHED_OTHER may change “nice” values  running long (whole timeslices)  penalty – nice increase  interrupted (e.g., I/O) gives initial “nice” value back 1 2... 126 127 1 2... 126 127 default -20 -19... 18 19 SHED_FIFO SHED_RR SHED_OTHER nice

24. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Multimedia streams are usually periodic (fixed frame rates and audio sample frequencies) Time constraints for a periodic task:  s – starting point (first time the task require processing)  e – processing time  d – deadline  p – period (r – rate (r = 1/p))  0 ≤ e ≤ d (often d ≤ p: we’ll use d = p – end of period, but Σd ≤ Σp is enough)  the kth processing of the task is ready at time s + (k – 1) p must be finished at time s + (k – 1) p + d  the scheduling algorithm must account for these properties Real–Time Scheduling s time e d p

25. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Real–Time Scheduling Resource reservation  QoS can be guaranteed  relies on knowledge of tasks  no fairness  origin: time sharing operating systems  e.g., earliest deadline first (EDF) and rate monotonic (RM) (AQUA, HeiTS, RT Upcalls,...) Proportional share resource allocation  no guarantees  requirements are specified by a relative share  allocation in proportion to competing shares  size of a share depends on system state and time  origin: packet switched networks  e.g., Scheduler for Multimedia And Real-Time (SMART) (Lottery, Stride, Move-to-Rear List,...)

26. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Earliest Deadline First (EDF) Preemptive scheduling based on dynamic task priorities Task with closest deadline has highest priority  stream priorities vary with time Dispatcher selects the highest priority task Assumptions:  requests for all tasks with deadlines are periodic  the deadline of a task is equal to the end on its period (starting of next)  independent tasks (no precedence)  run-time for each task is known and constant  context switches can be ignored

27. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Earliest Deadline First (EDF) Example: Task A Task B time Dispatching deadlines priority A > priority B priority A < priority B

28. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Rate Monotonic (RM) Scheduling Classic algorithm for hard real-time systems with one CPU [Liu & Layland ‘73] Pre-emptive scheduling based on static task priorities Optimal: no other algorithms with static task priorities can schedule tasks that cannot be scheduled by RM Assumptions:  requests for all tasks with deadlines are periodic  the deadline of a task is equal to the end on its period (starting of next)  independent tasks (no precedence)  run-time for each task is known and constant  context switches can be ignored  any non-periodic task has no deadline

29. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Process priority based on task periods  task with shortest period gets highest static priority  task with longest period gets lowest static priority  dispatcher always selects task requests with highest priority Example: Rate Monotonic (RM) Scheduling priority period length shortest period, highest priority longest period, lowest priority Task 1 p1p1 Dispatching Task 2 p2p2 P 1 < P 2  P 1 highest priority preemption

30. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems EDF Versus RM It might be impossible to prevent deadline misses in a strict, fixed priority system: Task A Task B Fixed priorities, A has priority, no dropping Fixed priorities, B has priority, no dropping Fixed priorities, A has priority, dropping Fixed priorities, B has priority, dropping time deadline miss Earliest deadline first deadlines waste of time Rate monotonic (as the first) deadline miss RM may give some deadline violations which is avoided by EDF

31. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems NOTE: this means that EDF is usually more efficient than RM, i.e., if switches are free and EDF uses resources ≤ 1, then RM may need ≤ ln(2) resources to schedule the same workload EDF Versus RM EDF  dynamic priorities changing in time  overhead in priority switching  QoS calculation – maximal throughput:  R i x e i ≤ 1, R – rate, e – processing time RM  static priorities based on periods  may map priority onto fixed OS priorities (like Linux)  QoS calculation:  R i x e i ≤ ln(2), R – rate, e – processing time all streams i

32. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems SMART (Scheduler for Multimedia And Real–Time applications) Designed for multimedia and real-time applications Principles  priority – high priority tasks should not suffer degradation due to presence of low priority tasks  proportional sharing – allocate resources proportionally and distribute unused resources (work conserving)  tradeoff immediate fairness – real-time and less competitive processes (short-lived, interactive, I/O-bound,...) get instantaneous higher shares  graceful transitions – adapt smoothly to resource demand changes  notification – notify applications of resource changes No admission control

33. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Tasks have importance and urgency  urgency – an immediate real-time constraint, short deadline (determine when a task will get resources)  importance – determine the overall resource allocation expressed by a tuple: [ priority p, biased virtual finishing time bvft ] static priority: supplied by user or assigned a default value virtual finishing time: degree to which the share was consumed bias: bonus for interactive tasks Best effort schedule based on urgency and importance  find most important tasks – compare tuple: T 1 > T 2  (p 1 > p 2 )  (p 1 = p 2  bvft 1 > bvft 2 )  sort after urgency (EDF based sorting)  iteratively select task from candidate set as long as schedule is feasible SMART (Scheduler for Multimedia And Real–Time applications)

34. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Evaluation of a Real–Time Scheduling Tests performed  by IBM (1993)  executing tasks with and without EDF  on an 57 MHz, 32 MB RAM, AIX Power 1 Video playback program:  one real-time process  read compressed data  decompress data  present video frames via X server to user  process requires 15 timeslots of 28 ms each per second  42 % of the CPU time

35. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Evaluation of a Real–Time Scheduling task number laxity (remaining time to deadline) several deadline violations by the non-real-time scheduler the real-time scheduler reaches all its deadlines 3 load processes (competing with the video playback)

36. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Evaluation of a Real–Time Scheduling task number laxity (remaining time to deadline) Varied the number of load processes (competing with the video playback) NB! The EDF scheduler kept its deadlines 4 other processes 16 other processes Only video process

37. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Evaluation of a Real–Time Scheduling Tests again performed  by IBM (1993)  on an 57 MHz, 32 MB RAM, AIX Power 1 “Stupid” end system program:  3 real-time processes only requesting CPU cycles  each process requires 15 timeslots of 21 ms each per second  31.5 % of the CPU time each  94.5 % of the CPU time required for real-time tasks

38. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Evaluation of a Real–Time Scheduling 1 load process (competing with the real-time processes) task number laxity (remaining time to deadline) the real-time scheduler reaches all its deadlines

39. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Evaluation of a Real–Time Scheduling 16 load process (competing with the real-time processes) task number laxity (remaining time to deadline) Regardless of other load, the EDF-scheduler reach its deadlines (laxity almost equal as in 1 load process scenario) process 1 process 2 process 3 NOTE: Processes are scheduled in same order

40. Memory Management

41. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Pentium 4 Processor registers cache(s) I/O controller hub memory controller hub RDRAM PCI slots network card disk file system communication system application file system communication system application disknetwork card Copying on the Intel Hub Architecture

42. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Streaming Modes Using Copying Traditional applications: Streaming applications: device driver independent abstraction layer(s) HW device readwrite application-specific data modifications user kernel OS device driver independent abstraction layer(s) HW device readwrite user kernel OS

43. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Cost of Data Transfers – Example I First generation router built with 133 MHz Intel Pentium  mean packet size 500 B  interrupt time of 10 µs, word access 50 ns  per packet processing of 200 instructions (1.504 µs)  copy loop: 4 instructions 2 memory accesses 130.08 ns (per 4 byte)  per packet: processing + copy + interrupt = 1.504 µs + [(500/4) * 130 ns] + 10 µs = 27.754 µs  144 Mbps register  memory[read_ptr] memory[write_ptr]  register read_ptr  read_prt + 4 write_ptr  write_prt + 4 counter  counter – 1 if (counter not 0) goto top of loop

44. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Cost of Data Transfers – Example II Copying in NetBSD v1.5  by UniK/IFI (2000)  copyin(), copyout(), and memcpy()  933 MHz P3 CPU  theoretical max.: 25.6 Gbps  INTEL: larger is better  BUT: max at 2 – 8 KB decrease at larger sizes  caching effects

45. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Cost of Data Transfers – Example II (cont.) Assume sending 1 GB data wwhole operation, reading from disk and sending to network, takes about 10 s rreading 64 KB blocks from disk  137.10 µs per c opyout() ssending 4 KB packets  1.65 µs per c opyin() iin total:read + send = (16384 * 137.10 µs) + (262144 * 1.65 µs) = 2.679 s for copying only THUS; data movement costs should be kept small ccareful management of contiguous media data aavoid unnecessary physical copy operations aapply appropriate buffer management schemes rreduce overhead by removing physical in-memory copy operation, i.e., Z ZZ ZERO-COPY data paths

46. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems file system communication system application user space kernel space bus(es) mbufbuf b_datam_data Basic Idea of Zero–Copy Data Paths

47. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Application streaming using zero-copy:  read data into kernel buffer and send from there  application responsible for timing  send: explicit send automatic send Kernel streaming using zero-copy:  thread per stream  perform read and write operations  application specifies timing, but it is ensured by the tread  stream is only created – controlled by kernel user kernel OS device driver independent abstraction layer(s) HW device read & send Streaming Modes NOT Using Copying readwritecreate stream user kernel OS device driver independent abstraction layer(s) HW device thread read write

48. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Existing Zero – Copy Streaming Mechanisms Linux: sendfile()  between two descriptors (file and TCP-socket)  bi-directional: disk-network and network-disk  need TCP_CORK AIX: send_file()  only TCP  uni-directional: disk-network INSTANCE (MMBUF-based, in NetBSD v1.5 ):  by UniK/IFI (2000)  uni-directional: disk-network (network-disk ongoing work)  stream_read() and stream_send() (zero-copy 1)  stream_rdsnd() (zero-copy 2) splice(), stream(), IO-Lite, MMBUF, … Kernel streaming using zero-copy Application streaming using zero-copy

49. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems INSTANCE CPU Time Transfer 1 GB Used disk blocks of 64 KB Used UDP packets of 1–8 KB Results in seconds: Gain larger than expected:  removed other operations as well like buffer cache look-up (simplified the chain of functions)  some packet drop at server saved about 0.2 s time in seconds packet size in KB Removing copyMeasured 1 KB2.803.39 2 KB2.754.09 4 KB2.683.98 8 KB2.983.31

50. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems INSTANCE Zero–Copy Transfer Rate  Throughput increase of ~2.7 times per stream (can at least double the number of streams) Zero-copy transfer rate limited by network card and storage system  saturated a 1 Gbps NIC and 32-bit, 33 MHz PCI  reduced processing time by approximately 50 %  huge improvement in number of concurrent streams approx. 12 Mbps approx. 6 Mbps read, write, with copy read, write, no copy read, automatic write, no copy

51. The End: Summary

52. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Summary All (active) resources needs to be scheduled Scheduling algorithms for multimedia tasks have to…  … consider real-time requirements  … provide good resource utilization  (… be implementable) Memory management is an important issue  pinning frequently used data – or at least keep as long as possible (replacement algorithms later)  reservation of memory buffers  copying is expensive Rule of thumb: watch out for bottlenecks  copying  data touching operations  frequent context switches  scheduling of slow devices (disk) ...

53. 2004 Carsten Griwodz & Pål Halvorsen INF5070 – media storage and distribution systems Some References 1. Halvorsen, P.: “Improving I/O Performance of Multimedia Servers”, Thesis for the Dr. Scient. degree at University of Oslo, Unipub forlag, ISSN 1501-7710, No. 161, Oslo, Norway, August 2001 2. Liu, C.L., Layland, J.W.: "Scheduling Algorithms for Multi-Programming in a Hard Real-Time Environment“, Journal of the Association for Computing Machinery 20, 1 (January 1973): 40-61 3. Nieh, J., Lam, M.S.: “The Design, Implementation and Evaluation of SMART: A Scheduler for Multimedia Applications”, Proc. of 16th ACM Symp. on Operating System Principles (SOSP’97), St. Malo, France, October 1997, pp. 184-197 4. Plagemann, T., Goebel, V., Halvorsen, P., Anshus, O.: "Operating System Support for Multimedia Systems", The Computer Communications Journal, Elsevier, Vol. 23, No. 3, February 2000, pp. 267-289 5. Solomon, D.A., Russinovich, M.E.: “Inside Microsoft Windows2000”, 3rd edition, Microsoft Press, 2000 6. Steinmetz, R., Nahrstedt, C.: “Multimedia: Computing, Communications & Applications”, Prentice Hall, 1995 7. Tanenbaum, A.S.: “Modern Operating Systems” (2nd ed.), Prentice Hall, 2001 8. Wolf, L.C., Burke, W., Vogt, C.: “Evaluation of a CPU Scheduling Mechanism for Multimedia Systems”, Software - Practice and Experience, Vol. 26, No. 4, 1996, pp. 375 - 398