Abstract RTOS Modelling for Multi-Processor SoC using SystemC

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Abstract RTOS Modelling for Multi-Processor SoC using

SystemCProf. Jan Madsen

Informatics and Mathematical ModellingTechnical University of Denmark

Richard Petersens Plads, Building 321DK2800 Lyngby, Denmark

Jan Madsen 2SoC-MOBINET courseware

Motivation

1

34

2

a b c

1 2 os

3

4

mappinga b c


Principles of mapping

12

3

Partitioning/clusteringBreak processes toincrease parallelism

Cluster processes toreduce communication

Allocation

a b

MappingSchedulingCommunication

1

3 2

a

b 1 2 & 3

deadline


Motivation

rtosa

rtosb

rtosc

rtosd

network


Uni-processor ...

rtosa

rtosb

rtosc

rtosd

network


Uni-processor ...

rtosa

Framework to experiment with different RTOS strategiesFocus on analysis of timing and resource sharing

Abstract software model, i.e. no behavior/functionality

Easy to create tasks and implement RTOS models

Based on SystemC


System model

rtosa


System model

rtos


System model

Task messages: ready finished

RTOS commands: run preemept Resume

rtos

task

schedulerlinks


System model - SystemC

pa = new task("task_a",1,50,3,12,0,ready);

registerTask(pa); pb = new

task("task_b",2,40,2,10,0,ready); registerTask(pb);

pc = new task("task_c",3,30,1,10,0,ready);

registerTask(pc);rtos

identifierperiod

priority

offsetWCET


Link model

Aim: Adding tasks without having to create seperate communication links

Uses the SystemC master-slave library

If two tasks send a message at the same time – they are executed in sequence, but in undefined order

Global ”clock” is used to keep track of time

rtos

clock


Task model

r1

r1 = time at which task becomes released (or active)

e1

e1 = worst case execution time (WCET)

d1 = deadline, task should complete before this!

d1s1

s1 = time at which task starts its execution

T1

T1 = period, minimum time between task releases

1

o1 = offset (or phase) for first released

o1


Task model

idle

running preempt

readyCp>0 !run

!resumerun

Cp==0 ready

preempt

resume

Cr==0finish

& Cr>0!preempt


SC_MODULE(perTask){ sc_outmaster<message_type> out_message; sc_inslave<message_type> in_command; sc_in_clk clock;

sc_event newStateEvent; void next_state(); void update_state(); void get_comm_from_scheduler(); SC_HAS_PROCESS(perTask); perTask(sc_module_name name_, int id_, ...) : sc_module(name_),id(id_), ...) { SC_METHOD(next_state); sensitive << newStateEvent; SC_METHOD(update_state); sensitive << clock; SC_SLAVE(get_comm_from_scheduler,in_command); } private: t_state state, new_state;

Task model - SystemC

new_state

state


Scheduling model

Aim: Simple way to describe the scheduling algorithm

Scheduler should only handle one message at a time

Rate-monotonic scheduling preemptive WCET d=T Fixed priority

rtosscheduler


void RM_scheduler::doSchedule() { ti = in_message; if (ti.comm==ready) { Q.push(ti); } else { tk.id = 0; } tj = Q.top(); if (tj.id != 0) { if (tk.id != 0) { if (tk < tj) { out_command = *(preemptTask(&tk, &Q)); tk = tj; out_command = *(runTask(&tj, &Q)); } else { } } else { tk = tj; out_command = *(runTask(&tj, &Q)); } } else { }}

Scheduling model - SystemC

ti: message received from from task i

tj: message from task j with highest priority from ready list

tk: message of the currently running task

Rate monotonic scheduling


An example

p3

p2

p1

50

40

30

0 20 30 40 50 6010

1

3

2

1

2

3

priorityeiTi

1030

1040

1250

2

3

1


An example

p3

p2

p1

50

40

30

0 20 30 40 50 6010

1

3

2


Extending the task model

Varying execution times [emin:emax]

r1 emin d1s1 T1

1

emax


Extending the task model

Varying execution times [emin:emax] Context switching Data dependencies

r1 d1s1 T1

1


Data dependencies

synchronizer

scheduler


Resource sharing

synchronizer

scheduler

allocator


Multi-processors

rtosa

rtosb

rtosc

rtosd

network


Multi-processors

rtosb

rtosc

rtosa

rtosd

network


Multi-processors

rtos rtos

rtosb

rtosc


Multi-processors

synchronizer

schedulerscheduler


Example

1

34

2

oi Ti ei

0 4 2

0 6 2+2

4 6 3

1

2

4

3

bos

4

aos

1

2

3


Dynamic scheduling

bos

4

aos

1

2

3

0 4 6 8 102 14 1612

1

2a

0 4 6 8 102 14 1612

3

4b

RM RM


Changing synchronization protocol

0 4 6 8 102

0 4 6 8 102

14 1612

14 1612

bos

4

aos

1

2

31

3

2

4

a

b

RM EDF


Multi-processing anomalies

Assume a set of tasks optimally scheduled on a multiprocessor system with: fixed number of processors fixed execution times (ei) precedence constraints

Then changing the priority list increasing the number of processor reducing execution times weakening the precedence constraints

May increase the scheduling length!


Example of anomalies

1

3

2

4 5

1

3

2

4 5Reduce e1 of task 1

a

b

a

b

Task 2 and 4 are sharing a resource, i.e. mutually exclusion

1 3

42

5


Consequences of anomalies

Tasks may complete before their WCETs So most on-line scheduling algorithms are

subject to experience anomalies Simple but inefficient solution:

Have tasks completing early idle


Network-on-Chip extension

bos

aos

cos

L1 L2

L3

S1 S2 S3

1

3

4

23

2 5

415mx

mz

my

mx: S1,L3,S3,L2,S2


Network-on-Chip model


Design space exploration

bus

4,5c

3b

1,2a

b

a

c

bus

1

4

3

2

5

xy z

b

a

c

bus

1

4

3

2

5

xyz

L1

L2

b

a

c

L3

2

4

3

1

5

x

y

z

zL1

L2 b

a

c

L3

1

4 5

3

2

x

x

z

y

QoS AwareAny traffic from a has higher priority

1,2a

3b

4,5c

L1 L2

L3

L1

L2

b

a

c

L3

2

4 5

3

1

x

x

z

yTiming Aware

L3 L4 y xz

b

a

c

bus

1

4

32

5

zyx

L1

L2

b

a

c 1

4

32

5

z

L1

L3

b

a

c 1

4

32

5

z

xx

xyAllocation AwareSwap task on PEs to: 2,3 | 4,5 | 1

3b

4,5cL1 L2

L3L4

L1

L2

b

a

c

L3

2

4

3

1

5

x

y

z

z

1,2a


Summary

Simple SystemC based framework to study the dynamic behavior of a task set running under the supervision of an abstract RTOS

Synchronizer to handle data dependencies Extension to multi-processor/RTOS systems Network-on-Chip extension (new) Not covered,

Allocator to handle resource sharing Power estimation/profile Multi-processing anomalies (in your slides)


References

Scheduling in Real-Time SystemsF. Cottet, J. Delacriox, C. Kaiser, Z. MammeriJohn Wiley & Sons, 2002

Real-Time SystemsJ. W. S. LiuPrintice Hall, 2000

Real-Time Systems and Programming LanguagesA. Burns, A. WellingsAddison-Wesley, 2001 (third edition)

System Design with SystemCT. Grotker, S. Liao, G. Martin, S. SwanKluwer, 2002

Abstract RTOS Modelling for Multi-Processor SoC using SystemC

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