<s>
Elapsed	B-General_Concept
real	I-General_Concept
time	I-General_Concept
,	O
real	O
time	O
,	O
wall-clock	B-General_Concept
time	I-General_Concept
,	O
wall	B-General_Concept
time	I-General_Concept
,	O
or	O
walltime	B-General_Concept
is	O
the	O
actual	O
time	O
taken	O
from	O
the	O
start	O
of	O
a	O
computer	B-Application
program	I-Application
to	O
the	O
end	O
.	O
</s>
<s>
Wall	B-General_Concept
time	I-General_Concept
is	O
thus	O
different	O
from	O
CPU	O
time	O
,	O
which	O
measures	O
only	O
the	O
time	O
during	O
which	O
the	O
processor	O
is	O
actively	O
working	O
on	O
a	O
certain	O
task	O
.	O
</s>
<s>
The	O
difference	O
between	O
the	O
two	O
can	O
arise	O
from	O
architecture	B-General_Concept
and	O
run-time	B-Device
dependent	O
factors	O
,	O
e.g.	O
</s>
<s>
programmed	B-Operating_System
delays	I-Operating_System
or	O
waiting	O
for	O
system	B-General_Concept
resources	I-General_Concept
to	O
become	O
available	O
.	O
</s>
<s>
Consider	O
the	O
example	O
of	O
a	O
mathematical	O
program	O
that	O
reports	O
that	O
it	O
has	O
used	O
"	O
CPU	O
time	O
0m0.04s	O
,	O
Wall	B-General_Concept
time	I-General_Concept
6m6.01s	O
"	O
.	O
</s>
<s>
Conversely	O
,	O
programs	O
running	B-Operating_System
in	I-Operating_System
parallel	I-Operating_System
on	O
more	O
than	O
one	O
processing	O
unit	O
can	O
spend	O
CPU	O
time	O
many	O
times	O
beyond	O
their	O
elapsed	O
time	O
.	O
</s>
<s>
Since	O
in	O
concurrent	B-Architecture
computing	I-Architecture
the	O
definition	O
of	O
elapsed	O
time	O
is	O
non-trivial	O
,	O
the	O
conceptualization	O
of	O
the	O
elapsed	O
time	O
as	O
measured	O
on	O
a	O
separate	O
,	O
independent	O
wall	O
clock	O
is	O
convenient	O
.	O
</s>
<s>
Another	O
definition	O
of	O
"	O
wall	B-General_Concept
time	I-General_Concept
"	O
is	O
the	O
measurement	O
of	O
time	O
via	O
a	O
separate	O
,	O
independent	O
clock	O
as	O
opposed	O
to	O
the	O
local	O
system	O
's	O
internal	O
time	O
,	O
i.e.	O
</s>
<s>
"	O
Real	O
time	O
"	O
in	O
the	O
meaning	O
of	O
"	O
wall	B-General_Concept
time	I-General_Concept
"	O
should	O
not	O
be	O
confused	O
with	O
real-time	B-General_Concept
computing	I-General_Concept
.	O
</s>
<s>
The	O
term	O
wall-clock	B-General_Concept
time	I-General_Concept
has	O
also	O
found	O
widespread	O
adoption	O
in	O
computer	O
simulation	O
,	O
to	O
distinguish	O
between	O
(	O
1	O
)	O
the	O
(	O
often	O
compressed	O
or	O
expanded	O
)	O
simulation	O
time	O
,	O
and	O
(	O
2	O
)	O
the	O
time	O
as	O
it	O
passes	O
for	O
the	O
user	O
of	O
the	O
simulation	O
tool	O
.	O
</s>
