<s>
In	O
computing	O
,	O
working	B-General_Concept
set	I-General_Concept
size	I-General_Concept
is	O
the	O
amount	O
of	O
memory	O
needed	O
to	O
compute	O
the	O
answer	O
to	O
a	O
problem	O
.	O
</s>
<s>
In	O
any	O
computing	O
scenario	O
,	O
but	O
especially	O
high	B-Architecture
performance	I-Architecture
computing	I-Architecture
where	O
mistakes	O
can	O
be	O
costly	O
,	O
this	O
is	O
a	O
significant	O
design-criteria	O
for	O
a	O
given	O
super	B-Architecture
computer	I-Architecture
system	O
in	O
order	O
to	O
ensure	O
that	O
the	O
system	O
performs	O
as	O
expected	O
.	O
</s>
<s>
For	O
any	O
given	O
instance	O
of	O
the	O
problem	O
,	O
the	O
program	O
has	O
one	O
such	O
data	O
set	O
,	O
which	O
is	O
called	O
the	O
working	B-General_Concept
set	I-General_Concept
.	O
</s>
<s>
The	O
Working	B-General_Concept
Set	I-General_Concept
Size	I-General_Concept
(	O
WSS	O
)	O
is	O
the	O
size	O
of	O
this	O
data	O
set	O
.	O
</s>
<s>
The	O
significance	O
of	O
this	O
is	O
that	O
if	O
the	O
Working	B-General_Concept
Set	I-General_Concept
Size	I-General_Concept
is	O
larger	O
than	O
the	O
available	O
memory	O
in	O
a	O
virtual	B-Architecture
memory	I-Architecture
system	O
then	O
the	O
memory	O
manager	O
must	O
refer	O
to	O
the	O
next	O
level	O
in	O
the	O
memory	O
hierarchy	O
(	O
usually	O
hard	B-Device
disk	I-Device
)	O
to	O
perform	O
a	O
swap	O
operation	O
swapping	O
some	O
memory	O
contents	O
from	O
RAM	B-Architecture
to	O
hard	B-Device
disk	I-Device
to	O
enable	O
the	O
program	O
to	O
continue	O
working	O
on	O
the	O
problem	O
.	O
</s>
<s>
This	O
phenomenon	O
is	O
known	O
as	O
thrashing	B-General_Concept
.	O
</s>
