Schedulers for BGW Tasks to Guarantee Quality of Service

of Embedded Real-time Systems

Mohamed Ould Sass and Maryline Chetto

IRCCyN Laboratory, University of Nantes, Nantes, France

Keywords:

Embedded, Real-time, Processor Overload, Fault-tolerance, Uniprocessor, Scheduling.

Abstract:

We present a new task model called BGW for preemptable, periodic task sets, scheduled on a uniprocessor

embedded platform. The tasks may be subject to faults and the processor may be overloaded. According to

BGW, any Black job has to execute a primary algorithm before deadline, any Grey job may execute either the

primary or the back-up algorithm and any White job may be discarded. We describe several Earliest Deadline

First (EDF) based scheduling frameworks suitable for this model. We also present and discuss the results of

experiments that compare the EDF scheduler applied to conventional Liu and Layland task sets to various

schedulers applied to BGW task sets. The Quality of Service is observed through metrics including ratio of

deadline success, preemption rate, etc.

1 INTRODUCTION

We consider the problem of scheduling preemptable,

periodic real-time task systems with arbitrary rela-

tive deadlines, scheduled on a single processor by

an online scheduler. We focus our attention on ﬁrm

real-time systems for which producing no result be-

fore deadline can be accepted only under some pre-

speciﬁed conditions that depend on the application.

The problem of real-time scheduling has been studied

extensively from about forty years starting with the

famous research paper of Liu and Layland in 1973

(Liu and Layland, 1973). Most of these works (see

a survey in (Liu, 2000)) have focussed on hard real-

time systems and resulted in a number of ﬁxed and

dynamic priority driven scheduling algorithms with

associated off-line schedulability tests.

In this paper, we propose to describe and evaluate

a new task model for answering requirements of ﬁrm

real-time systems that accept deadline violations due

to either occurrence of faults or/and processing over-

load. Overload conditions can be caused by a bad sys-

tem design, not anticipated simultaneous arrivals of

interrupts, hardware defects in data acquisition from

sensors, under-estimatedcomputational demands, op-

erating system exceptions, etc. Fault-tolerance tech-

niques intend to keep the system operational in the

presence of faults, even with producing degraded re-

sults. We will show how the BGW task model permits

to guarantee online graceful and controlled degrada-

tion of the Quality of Service in embedded real-time

systems.

2 BACKGROUND AND RELATED

WORK

2.1 Traditional Task Scheduling

Traditionally, a periodic task τ

is characterized by

two parameters at least: C

, worst case execution time

and T

, activation period. Every task periodically gen-

erates an inﬁnite set of jobs for execution. The uti-

lization factor of a periodic task gives the ratio of ex-

ecution requirement per period: u

. As a conse-

quence, the total utilization factor of a task set com-

posed of n tasks is :

∑

i=1

. The following EDF

schedulability test was established (Liu and Layland,

1973):

∑

i=1

≤ 1 i.e. total utilization is at most one.

A task set is said to be feasible if there exists at least

one schedule where all jobs of all tasks complete by

their deadline at run time. Earliest Deadline First was

proved optimal and Deadline Monotonic was proved

the best one in the class of ﬁxed priority schedulers

(Liu and Layland, 1973). However, EDF as well as

DM are suitable for under-loaded processing systems

where the processing demand is lower than the pro-

cessing capacity at every time. Such schedulers are

particularly adapted to a hard real-time context that

Ould Sass M. and Chetto M..

Schedulers for BGW Tasks to Guarantee Quality of Service of Embedded Real-time Systems.

DOI: 10.5220/0005181700530058

In Proceedings of the 5th International Conference on Pervasive and Embedded Computing and Communication Systems (PECCS-2015), pages 53-58

ISBN: 978-989-758-084-0

 2015 SCITEPRESS (Science and Technology Publications, Lda.)

imposes underload conditions.

2.2 Fault-tolerant Scheduling

Redundancy is the foundation of fault tolerance tech-

niques. There are three types of redundancy: hard-

ware, software and temporal. Permanent faults are

generally dealt through hardware redundancy while

temporal redundancy techniques serve for transient

or intermittent faults. They consist in re-executing

a task which has failed with either the same cod-

ing version (pure temporal redundancy) or a differ-

ent version, currently a shorter one. We are interested

with the Deadline Mechanism (DL) model. Each task

has two independent software versions (Liestman and

Campbell, 1986). Firstly, a major version called pri-

mary produces results with high precision when it is

completely executedbefore deadline. Secondly, a ver-

sion called alternate or back-up with shorter execution

time has to run for producing a just acceptable result

whenever the primary fails in executing timely due to

a fault or processor overload.

Two distinct scheduling frameworks may be im-

plemented for the DL mechanism (Chetto and Chetto,

1989) (Chetto and Chetto, 1991). Firstly, according

to the First Chance (FC) technique the alternate ver-

sion of any job executes completely ﬁrst before the

primary version of the same job starts execution. If

the primary version ﬁnishes before deadline, its re-

sults are used in preference to those of the alternate.

Secondly, the Last Chance (LC) technique attempts to

execute ﬁrst the primary version. Nevertheless sufﬁ-

cient processing time intervals have to be reserved to

guarantee feasible execution of the alternate version

if the primary fails. Consequently, success of any pri-

mary leads to discard the corresponding alternate and

recover processing time since the result of the alter-

nate becomes no longer necessary. In this strategy, the

scheduler has to suspend any running primary when-

ever an alternate requires to be executed so as to meet

its deadline. Theoretical and simulation studies estab-

lished that the LC strategy outperforms the FC strat-

egy (Chetto and Chetto, 1989) (Chetto and Chetto,

1991).

2.3 Scheduling with Overload

Conditions

Any scheduling algorithm should aim at minimizing

the overall damage to the system performance when-

ever a processing overload occurs. This can be per-

formed by dynamically changing some timing param-

eters of the tasks (e.g. execution time or period), using

importance values attached to the tasks or skipping

some jobs of recurring tasks. In that work, we opt for

the Skip-Over (SO) model (Koren and Shasha, 1995).

Each periodic task is characterized by a value called

skip factor denoted by s

, (2 ≤ s

≤ ∞) signifying that

among s

successive jobs, at most one can be skipped.

Every job of a task has one of the two colours: red or

blue. A red job has to complete before deadline while

a blue one can be aborted at any time. Moreover, af-

ter a deadline missing, at least (s

−1) jobs are red and

must be executed timely. Several scheduling schemes

have been proposed and analysed for the SO model

such as RTO (Red Tasks Only) or BWP (Blue When

Possible). Red jobs are scheduled as soon as possible

according to the EDF (Earliest Deadline First) rule

while blue ones are always rejected (that is never exe-

cuted). The BWP scheme is an improvement of RTO.

BWP schedules blue jobs whenever their execution

does not prevent the red ones from completing within

their deadlines. In other words, blue jobs are served

in background relatively to red jobs.

3 THE BGW TASK MODEL

BGW (Black GreyWhite) is a noveltask model which

uses time redundancy to cope with both transient pro-

cessor overload and faults (Sass et al., 2013). The

BGW model derives from:

• the DL Model where each periodic task has two

independent versions for fault-tolerance and over-

load management.

• and the SO model where each periodic task has a

skip parameter for overload management.

Every job generated by a periodic task takes at every

time instant one of the three following colours :

• Black if the job has to imperatively produce a re-

sult through the primary version,

• Grey if the job has to produce a result through at

least one version, but in preference the primary

one,

• White if the job may be dropped i.e. the job has

no execution requirement even if it is preferable

to execute one of the two versions.

Formally, a BGW task set τ = {τ

,τ

,...,τ

} is com-

posed of n periodic tasks where each task τ

is charac-

terized by its conventional timing parameters and two

additional QoS parameters (n

). Let us deﬁne the

term ”distance” as a number of requests. n

expresses

the maximum distance allowed between two consecu-

tive successful executions of the primary version. l

the maximum distance allowed between two consecu-

tive successful executions of a job (whatever primary

or alternate).

PECCS2015-5thInternationalConferenceonPervasiveandEmbeddedComputingandCommunicationSystems

We have introduced this model to guarantee the

stability of computer-based control systems. As the

processor may sometimes be overloaded, this model

will permit us to determine online the algorithm (pri-

mary or alternate) that has to be executed for every

task. Control may be realized through two kinds of

algorithm: one which returns very precise results but

require high processor utilization (primary executed

by a black job or a grey job) and one which returns

just acceptable results with very short execution time.

As a consequence, at a given time instant for a given

task, the scheduler has to execute a job which may be

either a black job, or a grey job or it may execute no

job at all. The choice depends ﬁrst on the algorithms

(primary or alternate) that have been executed in the

past and second on the parameters n

and l

of the task.

Any scheduling scheme for BGW-tasks should :

• guarantee that the requirements of the BGW tasks

are satisﬁed. At least one primary version over

successive jobs has to be executed timely, and

one alternate version over l

successive jobs has to

be executed timely. In other words, the scheduler

should timely execute the primary version of each

Black job and either the primary version or the

alternate version of each Grey job.

• maximize the number of successful primary exe-

cutions,

• minimize the number of unsuccessful jobs i.e jobs

which are either discarded or not completed be-

fore deadline.

In this paper, we report the results of a simulation

study where the performance of three EDF based

schedulers is analyzed.

4 SCHEDULERS UNDER STUDY

We apply the FC technique where every alternate ex-

ecutes entirely and systematically for producing a re-

sult with just acceptable precision. After the corre-

sponding primary is authorized to start execution for

producing a result with a better precision but never-

theless with a longer execution time. By deﬁnition of

the Black colour, only the primary version is executed

for black jobs. Either the alternate version or the pri-

mary one has to be executed timely for every grey job.

The white job can be aborted.

The FC technique applied to the the BGW model

can be implemented through different scheduling

variants. We analyse here three EDF based schedul-

ing strategies, each one deﬁned by a speciﬁc ordering

of versions. Let “X > Y” express that job with type

X should be executed with a higher priority than any

job with type Y. Any scheduling framework uses at

most ﬁve ordered lists which are respectively BP (BA

does not exist by deﬁnition of black colour), GA, GP,

WA and WP. This can be easily implemented in any

real-time operating system with only one list of jobs

that contains ﬁve ordered sub-lists. We assume in that

work that all the lists are ordered according to the ear-

liest deadline ﬁrst rule. Our simulation results will

concern scheduling frameworks based on the follow-

ing priority ordering:

1. BP > GA > GP > WA > WP: Both the alternate

and the primary versions of any grey job should

be executed with a higher priority than the white

jobs. As a consequence this schedulingpolicy will

be denoted by GbWA (Grey before White Alter-

nate),

2. BP > GA > GP > WP: Both the alternate and

the primary versions of any grey job should be

executed with a higher priority than the primary

versions of the white jobs. In that policy, alter-

nate versions of white jobs are never executed. As

a consequence this scheduling policy will be de-

noted by GbWP (Grey before White Primary),

3. BP > GA > WA > GP > WP: The alternate ver-

sions of all grey jobs and white jobs should be

executed with a higher priority than the primary

versions of all grey jobs and white jobs. As a con-

sequence this scheduling policy will be denoted

by AbP (Alternate before Primary)

5 EXPERIMENTS

5.1 Simulation Environment

We developed a task set generator that outputs BGW-

schedulable task sets. The task generation proce-

dure was parameterized so thattask sets exhibitdiffer-

ent degrees of scheduling difﬁculty and consequently

allows us to provide an objective evaluation of the

BGW mechanism. The generator has the following

input parameters: number of tasks (n), Least Com-

mon Multiple of the periods (P), worst case alternate

load (U

), worst case primary load (U

), (n

) and (l

)

parameters.

In all simulations reported in this paper, parame-

ters n and P take constant values 22 and 3360 respec-

tively. We considered 14 values for Up which uni-

formly vary from 0.8 to 2.2. U

is a linear function of

with U

= 0.2∗U

and n

= 7,l

= 4.

Outputs are the timing parameters of tasks (i.e. pe-

riod, relative deadline, worst case alternate execution

time and worst case primary execution time. More

SchedulersforBGWTaskstoGuaranteeQualityofServiceofEmbeddedReal-timeSystems

precisely, for a given alternate load U

, C

and C

are proportional to T

≤ C

) with a minimal value

equal to 1. The task sets which result from the dif-

ferent combinations of U

and U

were scheduled ac-

cording to BGW and EDF successively.

5.2 Metrics

We measure the resulting Quality of Service of the

system under the BGW mechanism by the ratio of

primary versions which are executed timely over the

total number of jobs (NPJ) and the ratio of jobs (pri-

maries and alternates) which are executed timely over

the total number of jobs (NSJ). Scheduling tasks in

overloaded conditions implies to discard some un-

completed jobs. Consequently, as processor time can

be wasted, we measure the wasted time ratio (WTR)

i.e. the percentage of time used by the processor for

producing no result or useless results.

The EDF scheduling strategy wastes time because

of uncompleted primaries. Under the BGW strategy,

time is wasted when both primaries and alternates are

uncompleted and white jobs do not execute.There are

also some processing time lost in preemptions and

context switches. So we measured the relative pre-

emption cost (RPC) i.e. the number of preemptions

per the total number of jobs within a time reference

window.

5.3 NSJ Analysis

We analyse NSJ which gives the number of jobs

which are executed timely (either by primary or alter-

nate versions) over the total number of jobs. We com-

pare it for the three different BGW strategies in addi-

tion to the classical EDF scheduling algorithm (where

every job has only one version i.e. the primary one).

Fig.1 shows variation of NSJ by making vary the

primary load,U

, (and consequently the alternate load

). As shown by the four graphs, for high load, AbP

and GbWA strategies outperform GbWP and EDF.

The AbP strategy exhibits a higher NSJ in com-

parison to the GbWA strategy, which in turn, is bet-

ter than GbWP. Indeed, under low alternate load (U

)

compared to primary load (U

), there are additional

chances of executing alternate versions of grey and

white jobs, when they have a higher priority in the

job execution order.

The basic EDF scheduler causes more jobs to fail

compared to the three scheduling strategies applied to

BGW-tasksets. Only primary versions are executed

under classical EDF, thus requiring large processing

time. These observations conﬁrm the effectiveness of

our speciﬁc task model for overload control. For all

Figure 1: Percentage of successful jobs as a function of

load.

BGW strategies, the number of deadline misses in-

creases asU

increases. High value ofU

causes more

processor time consumption by alternate version of

grey jobs, thus leading to higher deadline misses for

grey primaries. Clearly, the priority of alternate ver-

sions in the job execution order signiﬁcantly impacts

the BGW performance in terms of global success.

This conﬁrms usefulness of AbP and GbWA strate-

gies.

From U

= 100%, NSJ highly decreases until

reaching 50% for U

= 200% under the EDF algo-

rithm, and 40% under the GbWP strategy. For AbP

and GbWA, the decreasing is insigniﬁcant and in-

dependent from the primary load, until U

= 210%

where NSJ visibly starts decreasing. AbP and GbWA

have similar NSJ until U

reaches 170% where a

small difference between the two strategies can be ob-

served.

Under higher load, the ratio of deadline misses

for EDF algorithm is approximately ten times higher

than for the AbP strategy. This observation outlines

why both the BGW model and speciﬁc scheduling

strategies improve signiﬁcantly the resulting Quality

of Service of embedded systems under transient pro-

cessor overload. NSJ appears to have the highest val-

ues for the AbP and GbWA strategies, when the GA

list respectively WA list has higher priority than the

GP list respectively the WP list.

The GbWP policy behaves like EDF when the

primary load (U

) is increasing. Therefore, execut-

ing WP jobs before WA jobs leads to a signiﬁcant

decrease in global success for GbWP relatively to

GbWA because primaries have a higher execution

time in comparison to alternates.

Let us note that 1/(n

+ 1) = 14.28% and 1/(l

1) = 71.72% that respectively represent the ratio of

black jobs and grey jobs which have to meet dead-

lines. When the primary load Up is high, all the

BGW policies execute the lowest number of black

jobs which is permitted by distance parameters. In

that situation, the BGW and EDF policies behave

identically in terms of success ratio. Nevertheless,

there is no control on which job fails under classical

PECCS2015-5thInternationalConferenceonPervasiveandEmbeddedComputingandCommunicationSystems

EDF in contrast to BGW policies.

5.4 NPJ Analysis

Fig.2 depicts variation of NPJ i.e. the ratio of suc-

cessful primaries by making vary U

(primary load).

For all values of U

, GbWA outperforms EDF. GbWP

and EDF strategies offer the best performance since,

for the former strategy, the highest priorities are af-

fected to the GP and WP jobs. Whereas, for the latter

strategy, there is no execution of the two versions of

grey and white jobs by re-executing the primary ver-

sion after executing the alternate one. NPJ is greater

for the GbWP strategy, compared to the GbWA strat-

egy which is in turn, greater than the AbP algorithm.

With the small difference observed between the

EDF algorithm and the GbWP strategy in term of

successful primaries, we can state that, the BGW

strategies have a comparable performance if priorities

given to GP and WP jobs are greater compared to pri-

orities given to the WA jobs. It is very interesting to

note that NPJ, the percentage of successful primaries

remains greater than or equal to 14.28%, that is the

smallest ratio of primary versions to be executed in

accordance with the requirement of the BGW model.

At least 1/(n

) jobs have to execute their primary

version. In this study we have (n

= 7). As a con-

sequence, at least 1/7 = 14.28% primaries must be

executed timely. This explains why we can continue

the simulation experiment until U

= 700% and we

observe that NPJ is decreasing under 14.28%. It is

also observed that NPJ for EDF decrease rapidly as

grows, until it stands comparison to the AbP and

GbWA policies.

In fact, at higher values of the utilization factorU

in particular between U

= 210% and U

= 220%, we

observe that all schedulers behave identically when

observing NPJ i.e. the ratio of successful primaries.

Figure 2: Percentage of successful primaries as a function

of load.

Considering the jobs execution order under BGW

scheduling strategies, we notice that when the compu-

tation times of GA jobs and WA jobs tend to zero, all

BGW scheduling policies tend to behave as the EDF

scheduler.

We note also that, the lower the priority is given to

WA jobs, the more similar are the behaviours of BGW

and EDF policies in terms of successful primaries ra-

tio. When the primary load U

becomes very high

(more than 210%), all the BGW policies execute only

primaries of the black jobs and so BGW and EDF

policies have similar NPJ. We can draw the same con-

clusion when the alternate load is roughly equal to the

primary load.

5.5 WTR Analysis

The wasted time ratio (WTR) is the percentage of

time used by the processor for producing useless re-

sults (notably jobs which are aborted). The EDF

scheduling strategy leads to waste processing time

when primaries are aborted before completion be-

cause of time starvation. Generally, wasted time

comes from the execution of primary grey jobs and

all versions of white jobs in the BGW policies.

Fig.3 shows the variation of WTR.

The best performance regarding WTR is given by

the GbWA algorithm for all load conditions. In under-

load conditions, WTR is equal to zero for EDF be-

cause only the primary versions are executed, and

EDF is optimal if there is no overload. For GbWP,

WTR is also equal to zero until U

= 110% and in-

creases slowly untilU

= 130%. Then, it continues to

increase with large values until 21% at U

= 220%.

Figure 3: Percentage of wasted time as a function of load.

Both the increase in the number of lost jobs and

the increase in computation time of alternate and pri-

mary versions lead to the increase in wasted time.

Indeed, abortion of any primary or alternate version

will create large unusable processing time. As U

in-

creases, the ratio of lost jobs under EDF increases

more than the AbP and GbWA strategies. Hence,

WTR under EDF is greater than under AbP and

GbWA strategies when U

is very high.

5.6 RPC Analysis

It is interesting to compare the number of preemptions

SchedulersforBGWTaskstoGuaranteeQualityofServiceofEmbeddedReal-timeSystems

generated by the different scheduling strategies, so as

to evaluate correctly the relative overhead. In fact, the

previous performance evaluation may have no signif-

icance if some schedulers exhibit unacceptable over-

head at runtime due to context switches.Fig.4 shows

RPC i.e. the preemption ratio.

In under-load conditions, we notice a light dif-

ference between the different algorithms. Globally,

RPC is approximately constant at 19% for the AbP

and GbWA strategies. We also observe similarity be-

tween GbWP and EDF with different values of al-

ternate loads. RPC for AbP and GbWA strategies is

greater than for GbWP and EDF algorithms. Con-

sequently, the number of selected WA, GP and WP

jobs to be executed for AbP and GbWA strategies in-

creases, which increases the preemption ratios.

Figure 4: Preemption Rate as a function of load.

Under all load conditions, the GbWP strategy of-

fers the best performance in terms of RPC compared

to the other BGW strategies. It is always less than

10% aboveU

= 110%. We observe small differences

between GbWP and EDF policies. EDF gives high

RPC in under-load conditions and decreases in over-

load conditions between 100% and 150%. RPC starts

to increase from U

= 150% for EDF until it reaches

the same level as AbP and GbWA.

Globally, RPC is stable for AbP and GbWA poli-

cies unlike GbWP and EDF strategies. RPC for BGW

policies, particularly for AbP and GbWA is higher

than for the EDF scheduler applied to jobs which are

all primary versions. When the primary version load

is high, the probability to execute timely the GP jobs

decreases. Thus, neither the execution of WA nor WP

jobs is achieved, hence, the decrease in RPC for the

GbWA strategy. According to jobs execution order

under BGW strategies, for example for GbWA when

the computation time of GA and WA jobs tends to be

close to ones of the GP and WP jobs, the BGW poli-

cies tends to behave as EDF in terms of RPC.

6 CONCLUDING REMARKS

The contribution of this paper was twofold.

• We described a new approach for modelling Qual-

ity of Service (QoS) requirements of periodic task

systems which may be the object of both tran-

sient faults and processor overloads. This ap-

proach consists in integrating the Deadline Mech-

anism and the Skip-Over model in a uniﬁed task

model, namely BGW. We have shown how BGW

permits to provide an acceptable quality of service

through adequate task scheduling.

• We evaluated several scheduling schemes for

BGW-task sets with experiments. The simula-

tion demonstrates the merits of our proposed task

model compared to the classical Liu and Layland

task model. Simulations show the improvement

of speciﬁc scheduling frameworks for the BGW

model achievedin terms of ratio of successful jobs

and efﬁciency of processor usage which alleviates

the performance degradation under overload con-

ditions.

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