<s>
The	O
worst-case	B-General_Concept
execution	I-General_Concept
time	I-General_Concept
(	O
WCET	B-General_Concept
)	O
of	O
a	O
computational	O
task	O
is	O
the	O
maximum	O
length	O
of	O
time	O
the	O
task	O
could	O
take	O
to	O
execute	O
on	O
a	O
specific	O
hardware	B-Architecture
platform	O
.	O
</s>
<s>
Worst	B-General_Concept
case	I-General_Concept
execution	I-General_Concept
time	I-General_Concept
is	O
typically	O
used	O
in	O
reliable	O
real-time	B-General_Concept
systems	I-General_Concept
,	O
where	O
understanding	O
the	O
worst	B-General_Concept
case	I-General_Concept
timing	O
behaviour	O
of	O
software	O
is	O
important	O
for	O
reliability	O
or	O
correct	O
functional	O
behaviour	O
.	O
</s>
<s>
One	O
component	O
that	O
makes	O
up	O
the	O
response	O
time	O
is	O
the	O
time	O
spent	O
executing	O
the	O
software	O
–	O
hence	O
if	O
the	O
software	O
worst	B-General_Concept
case	I-General_Concept
execution	I-General_Concept
time	I-General_Concept
can	O
be	O
determined	O
,	O
then	O
the	O
designer	O
of	O
the	O
system	O
can	O
use	O
this	O
with	O
other	O
techniques	O
such	O
as	O
schedulability	B-Algorithm
analysis	I-Algorithm
to	O
ensure	O
that	O
the	O
system	O
responds	O
fast	O
enough	O
.	O
</s>
<s>
While	O
WCET	B-General_Concept
is	O
potentially	O
applicable	O
to	O
many	O
real-time	B-General_Concept
systems	I-General_Concept
,	O
in	O
practice	O
an	O
assurance	O
of	O
WCET	B-General_Concept
is	O
mainly	O
used	O
by	O
real-time	B-General_Concept
systems	I-General_Concept
that	O
are	O
related	O
to	O
high	O
reliability	O
or	O
safety	O
.	O
</s>
<s>
The	O
increasing	O
use	O
of	O
software	O
in	O
automotive	O
systems	O
is	O
also	O
driving	O
the	O
need	O
to	O
use	O
WCET	B-General_Concept
analysis	O
of	O
software	O
.	O
</s>
<s>
In	O
the	O
design	O
of	O
some	O
systems	O
,	O
WCET	B-General_Concept
is	O
often	O
used	O
as	O
an	O
input	O
to	O
schedulability	B-Algorithm
analysis	I-Algorithm
,	O
although	O
a	O
much	O
more	O
common	O
use	O
of	O
WCET	B-General_Concept
in	O
critical	O
systems	O
is	O
to	O
ensure	O
that	O
the	O
pre-allocated	O
timing	O
budgets	O
in	O
a	O
partition-scheduled	O
system	O
such	O
as	O
ARINC	O
653	O
are	O
not	O
violated	O
.	O
</s>
<s>
end-to-end	O
measurements	O
of	O
code	O
,	O
for	O
example	O
performed	O
by	O
setting	O
an	O
I/O	O
pin	O
on	O
the	O
device	O
to	O
high	O
at	O
the	O
start	O
of	O
the	O
task	O
,	O
and	O
to	O
low	O
at	O
the	O
end	O
of	O
the	O
task	O
and	O
using	O
a	O
logic	O
analyzer	O
to	O
measure	O
the	O
longest	O
pulse	O
width	O
,	O
or	O
by	O
measuring	O
within	O
the	O
software	O
itself	O
using	O
the	O
processor	B-General_Concept
clock	O
or	O
instruction	O
count	O
.	O
</s>
<s>
End	O
to	O
end	O
measurements	O
place	O
a	O
high	O
burden	O
on	O
software	O
testing	O
to	O
achieve	O
the	O
longest	O
path	O
;	O
counting	O
instructions	O
is	O
only	O
applicable	O
to	O
simple	O
software	O
and	O
hardware	B-Architecture
.	O
</s>
<s>
In	O
both	O
cases	O
,	O
a	O
margin	O
for	O
error	O
is	O
often	O
used	O
to	O
account	O
for	O
untested	O
code	O
,	O
hardware	B-Architecture
performance	O
approximations	O
or	O
mistakes	O
.	O
</s>
<s>
As	O
software	O
and	O
hardware	B-Architecture
have	O
increased	O
in	O
complexity	O
,	O
they	O
have	O
driven	O
the	O
need	O
for	O
tool	O
support	O
.	O
</s>
<s>
System	O
safety	O
arguments	O
based	O
on	O
a	O
high-water	O
mark	O
achieved	O
during	O
testing	O
are	O
widely	O
used	O
,	O
but	O
become	O
harder	O
to	O
justify	O
as	O
the	O
software	O
and	O
hardware	B-Architecture
become	O
less	O
predictable	O
.	O
</s>
<s>
The	O
problem	O
of	O
finding	O
WCET	B-General_Concept
by	O
analysis	O
is	O
equivalent	O
to	O
the	O
halting	O
problem	O
and	O
is	O
therefore	O
not	O
solvable	O
in	O
the	O
general	O
.	O
</s>
<s>
Fortunately	O
,	O
for	O
the	O
kind	O
of	O
systems	O
that	O
engineers	O
typically	O
want	O
to	O
find	O
WCET	B-General_Concept
for	O
,	O
the	O
software	O
is	O
typically	O
well	O
structured	O
,	O
will	O
always	O
terminate	O
and	O
is	O
analyzable	O
.	O
</s>
<s>
Most	O
methods	O
for	O
finding	O
a	O
WCET	B-General_Concept
involve	O
approximations	O
(	O
usually	O
a	O
rounding	O
upwards	O
when	O
there	O
are	O
uncertainties	O
)	O
and	O
hence	O
in	O
practice	O
the	O
exact	O
WCET	B-General_Concept
itself	O
is	O
often	O
regarded	O
as	O
unobtainable	O
.	O
</s>
<s>
Instead	O
,	O
different	O
techniques	O
for	O
finding	O
the	O
WCET	B-General_Concept
produce	O
estimates	O
for	O
the	O
WCET	B-General_Concept
.	O
</s>
<s>
Those	O
estimates	O
are	O
typically	O
pessimistic	O
,	O
meaning	O
that	O
the	O
estimated	O
WCET	B-General_Concept
is	O
known	O
to	O
be	O
higher	O
than	O
the	O
real	O
WCET	B-General_Concept
(	O
which	O
is	O
usually	O
what	O
is	O
desired	O
)	O
.	O
</s>
<s>
Much	O
work	O
on	O
WCET	B-General_Concept
analysis	O
is	O
on	O
reducing	O
the	O
pessimism	O
in	O
analysis	O
so	O
that	O
the	O
estimated	O
value	O
is	O
low	O
enough	O
to	O
be	O
valuable	O
to	O
the	O
system	O
designer	O
.	O
</s>
<s>
WCET	B-General_Concept
analysis	O
usually	O
refers	O
to	O
the	O
execution	O
time	O
of	O
single	O
thread	O
,	O
task	O
or	O
process	O
.	O
</s>
<s>
However	O
,	O
on	O
modern	O
hardware	B-Architecture
,	O
especially	O
multi-core	O
,	O
other	O
tasks	O
in	O
the	O
system	O
will	O
impact	O
the	O
WCET	B-General_Concept
of	O
a	O
given	O
task	O
if	O
they	O
share	O
cache	O
,	O
memory	O
lines	O
and	O
other	O
hardware	B-Architecture
features	O
.	O
</s>
<s>
Further	O
,	O
task	O
scheduling	O
events	O
such	O
as	O
blocking	B-Operating_System
or	O
to	O
be	O
interruptions	O
should	O
be	O
considered	O
in	O
WCET	B-General_Concept
analysis	O
if	O
they	O
can	O
occur	O
in	O
a	O
particular	O
system	O
.	O
</s>
<s>
Therefore	O
,	O
it	O
is	O
important	O
to	O
consider	O
the	O
context	O
in	O
which	O
WCET	B-General_Concept
analysis	O
is	O
applied	O
.	O
</s>
<s>
There	O
are	O
many	O
automated	O
approaches	O
to	O
calculating	O
WCET	B-General_Concept
beyond	O
the	O
manual	O
techniques	O
above	O
.	O
</s>
<s>
A	O
static	O
WCET	B-General_Concept
tool	O
attempts	O
to	O
estimate	O
WCET	B-General_Concept
by	O
examining	O
the	O
computer	O
software	O
without	O
executing	O
it	O
directly	O
on	O
the	O
hardware	B-Architecture
.	O
</s>
<s>
Static	O
analysis	O
tools	O
work	O
at	O
a	O
high-level	O
to	O
determine	O
the	O
structure	O
of	O
a	O
program	B-Application
's	O
task	O
,	O
working	O
either	O
on	O
a	O
piece	O
of	O
source	O
code	O
or	O
disassembled	O
binary	B-Application
executable	I-Application
.	O
</s>
<s>
They	O
also	O
work	O
at	O
a	O
low-level	O
,	O
using	O
timing	O
information	O
about	O
the	O
real	O
hardware	B-Architecture
that	O
the	O
task	O
will	O
execute	O
on	O
,	O
with	O
all	O
its	O
specific	O
features	O
.	O
</s>
<s>
By	O
combining	O
those	O
two	O
kinds	O
of	O
analysis	O
,	O
the	O
tool	O
attempts	O
to	O
give	O
an	O
upper	O
bound	O
on	O
the	O
time	O
required	O
to	O
execute	O
a	O
given	O
task	O
on	O
a	O
given	O
hardware	B-Architecture
platform	O
.	O
</s>
<s>
At	O
the	O
low-level	O
,	O
static	O
WCET	B-General_Concept
analysis	O
is	O
complicated	O
by	O
the	O
presence	O
of	O
architectural	O
features	O
that	O
improve	O
the	O
average-case	B-General_Concept
performance	O
of	O
the	O
processor	B-General_Concept
:	O
instruction/data	O
caches	O
,	O
branch	B-General_Concept
prediction	I-General_Concept
and	O
instruction	O
pipelines	B-General_Concept
,	O
for	O
example	O
.	O
</s>
<s>
It	O
is	O
possible	O
,	O
but	O
increasingly	O
difficult	O
,	O
to	O
determine	O
tight	O
WCET	B-General_Concept
bounds	O
if	O
these	O
modern	O
architectural	O
features	O
are	O
taken	O
into	O
account	O
in	O
the	O
timing	O
model	O
used	O
by	O
the	O
analysis	O
.	O
</s>
<s>
Static	O
analysis	O
has	O
resulted	O
in	O
good	O
results	O
for	O
simpler	O
hardware	B-Architecture
,	O
however	O
a	O
possible	O
limitation	O
of	O
static	O
analysis	O
is	O
that	O
the	O
hardware	B-Architecture
(	O
the	O
CPU	O
in	O
particular	O
)	O
has	O
reached	O
a	O
complexity	O
which	O
is	O
extremely	O
hard	O
to	O
model	O
.	O
</s>
<s>
In	O
particular	O
,	O
the	O
modelling	O
process	O
can	O
introduce	O
errors	O
from	O
several	O
sources	O
:	O
errors	O
in	O
chip	O
design	O
,	O
lack	O
of	O
documentation	O
,	O
errors	O
in	O
documentation	O
,	O
errors	O
in	O
model	O
creation	O
;	O
all	O
leading	O
to	O
cases	O
where	O
the	O
model	O
predicts	O
a	O
different	O
behavior	O
to	O
that	O
observed	O
on	O
real	O
hardware	B-Architecture
.	O
</s>
<s>
Typically	O
,	O
where	O
it	O
is	O
not	O
possible	O
to	O
accurately	O
predict	O
a	O
behavior	O
,	O
a	O
pessimistic	O
result	O
is	O
used	O
,	O
which	O
can	O
lead	O
to	O
the	O
WCET	B-General_Concept
estimate	O
being	O
much	O
larger	O
than	O
anything	O
achieved	O
at	O
run-time	O
.	O
</s>
<s>
Obtaining	O
tight	O
static	O
WCET	B-General_Concept
estimation	O
is	O
particularly	O
difficult	O
on	O
multi-core	O
processors	O
.	O
</s>
<s>
Measurement-based	O
and	O
hybrid	O
approaches	O
usually	O
try	O
to	O
measure	O
the	O
execution	O
times	O
of	O
short	O
code	O
segments	O
on	O
the	O
real	O
hardware	B-Architecture
,	O
which	O
are	O
then	O
combined	O
in	O
a	O
higher	O
level	O
analysis	O
.	O
</s>
<s>
loops	O
,	O
branches	O
)	O
,	O
to	O
produce	O
an	O
estimate	O
of	O
the	O
WCET	B-General_Concept
of	O
the	O
larger	O
program	B-Application
.	O
</s>
<s>
A	O
worst	B-General_Concept
case	I-General_Concept
effect	O
needs	O
only	O
to	O
be	O
seen	O
once	O
during	O
testing	O
for	O
the	O
analysis	O
to	O
be	O
able	O
to	O
combine	O
it	O
with	O
other	O
worst	B-General_Concept
case	I-General_Concept
events	O
in	O
its	O
analysis	O
.	O
</s>
<s>
Typically	O
,	O
the	O
small	O
sections	O
of	O
software	O
can	O
be	O
measured	O
automatically	O
using	O
techniques	O
such	O
as	O
instrumentation	O
(	O
adding	O
markers	O
to	O
the	O
software	O
)	O
or	O
with	O
hardware	B-Architecture
support	O
such	O
as	O
debuggers	O
,	O
and	O
CPU	O
hardware	B-Architecture
tracing	O
modules	O
.	O
</s>
<s>
These	O
markers	O
result	O
in	O
a	O
trace	O
of	O
execution	O
,	O
which	O
includes	O
both	O
the	O
path	O
taken	O
through	O
the	O
program	B-Application
and	O
the	O
time	O
at	O
which	O
different	O
points	O
were	O
executed	O
.	O
</s>
<s>
The	O
trace	O
is	O
then	O
analyzed	O
to	O
determine	O
the	O
maximum	O
time	O
that	O
each	O
part	O
of	O
the	O
program	B-Application
has	O
ever	O
taken	O
to	O
execute	O
,	O
what	O
the	O
maximum	O
observed	O
iteration	O
time	O
of	O
each	O
loop	O
is	O
and	O
whether	O
there	O
are	O
any	O
parts	O
of	O
the	O
software	O
that	O
are	O
untested	O
(	O
Code	B-Application
coverage	I-Application
)	O
.	O
</s>
<s>
Measurement-based	O
WCET	B-General_Concept
analysis	O
has	O
resulted	O
in	O
good	O
results	O
for	O
both	O
simple	O
and	O
complex	O
hardware	B-Architecture
,	O
although	O
like	O
static	O
analysis	O
it	O
can	O
suffer	O
excessive	O
pessimism	O
in	O
multi-core	O
situations	O
,	O
where	O
the	O
impact	O
of	O
one	O
core	O
on	O
another	O
is	O
hard	O
to	O
define	O
.	O
</s>
<s>
A	O
limitation	O
of	O
measurement	O
is	O
that	O
it	O
relies	O
on	O
observing	O
the	O
worst-case	B-General_Concept
effects	O
during	O
testing	O
(	O
although	O
not	O
necessarily	O
at	O
the	O
same	O
time	O
)	O
.	O
</s>
<s>
It	O
can	O
be	O
hard	O
to	O
determine	O
if	O
the	O
worst	B-General_Concept
case	I-General_Concept
effects	O
have	O
necessarily	O
been	O
tested	O
.	O
</s>
<s>
The	O
first	O
international	O
WCET	B-General_Concept
Tool	O
Challenge	O
took	O
place	O
during	O
the	O
autumn	O
of	O
2006	O
.	O
</s>
<s>
The	O
aim	O
of	O
the	O
Challenge	O
was	O
to	O
inspect	O
and	O
to	O
compare	O
different	O
approaches	O
in	O
analyzing	O
the	O
worst-case	B-General_Concept
execution	I-General_Concept
time	I-General_Concept
.	O
</s>
<s>
All	O
available	O
tools	O
and	O
prototypes	O
able	O
to	O
determine	O
safe	O
upper	O
bounds	O
for	O
the	O
WCET	B-General_Concept
of	O
tasks	O
have	O
participated	O
.	O
</s>
