Missed Deadline Notification in Best-Effort Schedulers

1
Missed Deadline Notification in Best-Effort
Schedulers

• Balaji Raman

2
Outline

• Motivation
• Methodology
• Implementation Details
• Results
• Conclusion

3
Motivation What are Best Effort Schedulers?

• Best effort scheduling
• Does not provide facilities to specify or meet
  performance guarantees.
• Advantages
• Simple
• Applications do not require system interfaces
• Suitability
• Soft real time systems
• Disadvantages
• Not suitable for hard real time systems
• Under Utilization

4
Methodology-MDN

• Indicate deadline misses to scheduler.
• Problems in all existing general purpose system
  schedulers- Linux, BEST..
• Advantages of MDN
• Minimal interface

5
Implementation Details

• Implemented in three schedulers
• Linux Scheduler
• BEST Scheduler
• Be-Rate Scheduler
• We will look MDN implementation in Linux
  Scheduler in detail.

6
Linux Time-Share Scheduler

• Scheduling based on Quantum Counter Variable
• A process can block its task if it is periodic.

Scheduler
Q1 C5 TidA
Q2 C1 TidB
Q3 C1 TidC
Q4 C1 TidD
Scheduler
Q1 C1 TidB
Q2 C1 TidC
Q3 C1 TidD
Q4 C4 TidA
7
Linux Time-Share Scheduler

• When all counters in queue are zero
• CounterCounter/2nice
• Problem Deadlines could be missed for tasks with
  counter0.

Scheduler
Q1 C0 TidB
Q2 C0 TidC
Q3 C0 TidD
Q4 C1 TidA
8
MDN Linux

• Countercounter/2(nicemdn_nice)
• Initially mdn_nice0.
• If MDN received mdn_nice
• Priority boosted for not missing future
  deadlines.
• Anti-Cheat Policy

9
Best Scheduler

• Detect Periodic tasks
• Improves performance of soft real-time tasks
• Tasks entering Run Queue
• Run Q
• Period of the task Y-X

Scheduler (Task D Entering Times Rec_TimeX Pre
v_TimeY)
Q1 TidA
Q2 TidB
TidD Curr_TimeX
10
Best Scheduler Scheduling Policy

• Earliest Deadline First.
• Problems

BEST Detects Period
BEST Detects Period
BEST Detects Period
BEST Detects Period
BEST Detects Period
Deadline
11
BEST MDN

• MDN Detects Misses in deadline.
• Anti-cheat Policy
• Based on block ratio

MDN Deadline Missed
BEST Detects Period
BEST Detects Period
BEST Detects Period
Deadline
12
Be-Rate Scheduler

• Rate based algorithm.
• MDN Enhancement.
• Anti-Cheat Policy.

13
Experimental Evaluation

• Measure performance by observing the number of
  deadlines missed with without MDN.
• Benchmark 2 synthetic workload application
• CPU bound, Soft real-time process.
• Experiments done on Intel Pentium P3 and P4
  processors.

14
Results (Linux)
Progress (CPU seconds)
xxxxxxx xx xx xxx xx xxxxxxxx xxxx xxxx
xxxxxx
CPU-bound SRT-50 xx Deadline Missed
Time (seconds)
15
Results (Linux MDN)
Progress (CPU seconds)
xx x
CPU-bound SRT-50 xx Deadline Missed
Time (seconds)
16
Results (Linux)
Progress (CPU seconds)
xxxxxxx xx xx xxx xx xxxxxxxx xxxx xxxx
xxxxxx
CPU-bound SRT(25)-33 xx Deadline
Missed SRT(33)-33 xx Deadline Missed
xxxxxxx xx xx xxx xx xxxxxxxx xxxx xxxx
xxxxxx
Time (seconds)
17
Results (Anti Cheating Policy)
Progress (CPU seconds)
CPU-bound Cheating Process
Time (seconds)
18
Results (Linux MDN)
Progress (CPU seconds)
xx x
CPU-bound SRT(25)-33 xx Deadline
Missed SRT(33)-33 xx Deadline Missed
xx x
Time (seconds)
19
Conclusion

• Best-effort schedulers an attractive advantage.
• MDN makes Best-effort schedulers more efficient.
• Future work
• Tuned to workloads.
• Rate controlled MDN.

20
Back Up Slides

• BeRate
• Rate based algorithm
• Two Parameters
• Task should complete e units of computation over
  p interval of time.
• Schedules so that they can continue processing in
  the same rate as in the past.
• Uses EDF scheduler.

21
Back Up slides

• Berate Scheduler
• Given a fair share of processing Berate estimates
  period of the task so that task would meet its
  deadline using EDF scheduler
• MDN is added.
• If estmated deadline is earlier then the task
  will be serviced quicker.

22
Backup Slides

• Results of BEST, BeRate
• BeRate performance improvement slighter
• Best tasks misses many deadlines because of
  overload, Best cannot detect period but using MDN
  it is able to alleviate its effect.