preemption handling and scalability of feedback dvs-edf

Preemption Handling and Scalability of

Feedback DVS-EDF

Yifan Zhu , Frank [email protected]

Center for Embedded Systems Research /

Department of Computer ScienceNorth Carolina State University

9/22/2002 NC State University 2

Background

Dynamic voltage scalingReduce energy consumption by lowering the supply voltage and operating frequency

Energy consumption scales lineally with frequencyquadratically with voltage

E ~ f V²

Hard real-time systemActual exec. time Worst case exec. time (WCET)Most practical system: utilization<1


DVS-EDF

Original EDF:

DVS-EDF: , =scaling level

Greedily scale current task

Capitalize on early completion

1}{\},...,1{

1

kni i

i

k

k

P

C

P

C

1},...,1{

ni i

i

P

C


Slack Generation

Two opportunities for slack generation:1. Early completion of tasks

2. Idle slots

T1

c1j

WCET

slack

T1 T2 T3


Idle Slots --> Slack

Introducing idle task into the task set, to convert idle time slots into slack

We choose

so that there is at least one idle time slot lying between any actual task’s invocation

UP

C

idle

idle 1

minmin )1(, PUCPP idleidle

(actual exec. time=0)


Slack Passing

Slack generated by one task will usually not be exhausted when the task completes

Initial slack capable to be passed on = C1-c1j

T1 T2

t2 t4 t5 t8 t9

C1 (WCET)

c1j slack d1 d2

S=S+C1-c1j,

Note: it’s possible that

c1j > C1, as long as the

total slack is not over-

exploited


Deadline Restriction

Initial slack can’t be passed in full to the next task. Slack beyond the next task’s release time and deadline cannot be used.

Assume: T1’s deadline is d1, T2 releases at r2

T2 can use:all the slack part of the slack none of the slack

r2<t1 t1 r2<d1 d1 t2

T1 T2 T1 T2 T1 T2

d1 d1d1

t1 t1t1

r2r2r2


Adding Idle Slots

Total slack can be further increased if idle task exists between T1 and T2’s deadline

slack = slack + idle(d1…d2)

T1 T2

t2 t4 t5 t8 t9

C1 (WCET)

c1j slack d1 d2

idle


Preemption Handling

Preempted task relinquishes its remaining slack, passing it on to the next task

Two differences herePreempted task itself cannot generate any slack

Remaining exec. time of the preempted task must be reserved in advance


idle

idle

Forward Sweep Scheme

Remaining exec. time is reserved by stealing slack from the next task

Assume: = 3 slots

Forward sweep: zero slack for T2 and T3 (in the worst case, if P2>>P1, T2 is likely to get no slack always)

1Tleft

T1 preempted T2 T3

T1cont.

idle

idle

idle

idle

idle

d1d3d2t2t1


idle

idle

Backward Sweep SchemeRemaining exec. time is reserved as late as possible ( but no later than the preempted task’s deadline ).

Assume: = 3 slots

Backward sweep: 2 idle slots for T2, 1 idle slots for T3

More greedy than forward sweep scheme

1Tleft

T1 preempted T2 T3

T1cont.

idle

idle

idle

idle

idle

d1d3d2t2t1


Feedback Scheme

Split task k: Ck = Ca + Cb

The average actual exec. time of Ck over past is used to anticipate future Ca portions.

Ca Cb

100%

2,1

)2(

,2

1

11

1

ii

ciCaCa

WCETCa

iii

Ck

•the 1st instance of Ck:

•later instances:


Experiments

Parameters: 3-task sets and 10-tasks sets

Vary: utilization and cc/WCET

Compare

Look-ahead DVS [Pillai et al.]

Our feedback DVS

Optimal: ideal scaling, disregard deadline

All energy values are relative to Look-ahead DVS :

Look-ahead DVS : 0%

Ours: additional savings over Look-ahead DVS


Scalability ( lowest workload)

cc=25%WCET

0%

10%

20%

30%

40%

50%

60%

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Utilization

Pe

rce

nta

ge

Our DVS on 3-task set


Optimal of 3-task set


Look-ahead DVS


Scalability ( medium workload)

cc=50% WCET

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Utilization

Pe

rce

nta

ge





Look ahead DVS


Scalability ( higher workload)

cc= 75% WCET

-5%

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Utilization

Pe

rce

nta

ge


Our DVS on 10-taks set



Look ahead DVS


Scalability ( maximum workload)

cc= 100% WCET

-10%

-5%

0%

5%

10%

15%

20%

25%

30%

35%

40%

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Utilization

Pe

rce

nta

ge



Optimal of 3-task setl


Look ahead DVS

majorsaving

s

minorlosses


Feedback Effect

Varying actual exec. time of task set1. Actual exec. time (cc) always equals to 50% WCET

2. Actual exec. Time = sin(t), in [0.1WCET, 0.9 WCET]

t

WCET

50% cc

WCET

cc50%

t


Feedback Effect

Not much difference good! Feedback scheme compensates for fluctuations

200000

300000

400000

500000

600000

0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

Utilization

En

erg

y

cc=50% WCET

cc = sin(t) in [0.1WCET, 0.9 WCET]


Conclusion

Feedback-EDF DVS for hard real-time systems

Slack generation and passing

Slots reservation at preemption points

Up to 37% additional energy savings over the best prior work

Scalability: 3-tasks and 10-tasks show savings.


Future Work

Time penalty for voltage changesCost depends on architecture (getting smaller)

Experiments with embedded device

Handling of dependent tasks


Related Work

Pillai et al. [SOSP’01]Assuming same scaling factor for all real-time tasks

Look ahead DVS

F. Gruian [ISLPED’01]Deriving dual-voltage schedule from the ideal case with stochastic information

D. Mosse et al. [RTSS’01]Dynamically exploiting early completion of tasks’ future executions


Slack Passing (Over-Scaling)

Over-scaling: actual exec. time > WCET

is feasible if the total slack is not exceeded as well

T1

T2

t2 t4 t5 d1 d2

C1 (WCET)

c1j

Slack=C1-c1j<0T2 can still be scaled without any deadline misses as long as S=S+C1-c1j >0

preemption handling and scalability of feedback dvs-edf

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