<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
a	O
multilevel	B-Operating_System
feedback	I-Operating_System
queue	I-Operating_System
is	O
a	O
scheduling	O
algorithm	O
.	O
</s>
<s>
Scheduling	O
algorithms	O
are	O
designed	O
to	O
have	O
some	O
process	O
running	O
at	O
all	O
times	O
to	O
keep	O
the	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	B-General_Concept
)	O
busy	O
.	O
</s>
<s>
The	O
multilevel	B-Operating_System
feedback	I-Operating_System
queue	I-Operating_System
extends	O
standard	O
algorithms	O
with	O
the	O
following	O
design	O
requirements	O
:	O
</s>
<s>
Give	O
preference	O
to	O
processes	O
with	O
short	O
CPU	B-General_Concept
bursts	O
.	O
</s>
<s>
Give	O
preference	O
to	O
processes	O
with	O
high	O
I/O	B-General_Concept
bursts	O
.	O
</s>
<s>
(	O
I/O	B-General_Concept
bound	I-General_Concept
processes	O
will	O
sleep	O
in	O
the	O
wait	O
queue	O
to	O
give	O
other	O
processes	O
CPU	B-General_Concept
time	O
.	O
)	O
</s>
<s>
The	O
multilevel	B-Operating_System
feedback	I-Operating_System
queue	I-Operating_System
was	O
first	O
developed	O
by	O
Fernando	O
J	O
.	O
Corbató	O
(	O
1962	O
)	O
.	O
</s>
<s>
Whereas	O
the	O
multilevel	B-Algorithm
queue	I-Algorithm
algorithm	O
keeps	O
processes	O
permanently	O
assigned	O
to	O
their	O
initial	O
queue	O
assignments	O
,	O
the	O
multilevel	B-Operating_System
feedback	I-Operating_System
queue	I-Operating_System
shifts	O
processes	O
between	O
queues	O
.	O
</s>
<s>
The	O
shift	O
is	O
dependent	O
upon	O
the	O
CPU	B-General_Concept
bursts	O
of	O
prior	O
time-slices	O
.	O
</s>
<s>
If	O
a	O
process	O
uses	O
too	O
much	O
CPU	B-General_Concept
time	O
,	O
it	O
will	O
be	O
moved	O
to	O
a	O
lower-priority	O
queue	O
.	O
</s>
<s>
If	O
a	O
process	O
is	O
I/O	B-General_Concept
-bound	I-General_Concept
or	O
an	O
interactive	O
process	O
,	O
it	O
will	O
be	O
moved	O
to	O
a	O
higher-priority	O
queue	O
.	O
</s>
<s>
If	O
a	O
process	O
is	O
waiting	O
too	O
long	O
in	O
a	O
low-priority	O
queue	O
and	O
starving	B-Operating_System
,	O
it	O
will	O
be	O
aged	B-Operating_System
to	O
a	O
higher-priority	O
queue	O
.	O
</s>
<s>
Multiple	O
FIFO	B-Operating_System
queues	O
are	O
used	O
and	O
the	O
operation	O
is	O
as	O
follows	O
:	O
</s>
<s>
A	O
new	O
process	O
is	O
inserted	O
at	O
the	O
end	O
(	O
tail	O
)	O
of	O
the	O
top-level	O
FIFO	B-Operating_System
queue	O
.	O
</s>
<s>
At	O
some	O
stage	O
the	O
process	O
reaches	O
the	O
head	O
of	O
the	O
queue	O
and	O
is	O
assigned	O
the	O
CPU	B-General_Concept
.	O
</s>
<s>
If	O
the	O
process	O
voluntarily	O
relinquishes	O
control	O
of	O
the	O
CPU	B-General_Concept
,	O
it	O
leaves	O
the	O
queuing	O
network	O
,	O
and	O
when	O
the	O
process	O
becomes	O
ready	O
again	O
it	O
is	O
inserted	O
at	O
the	O
tail	O
of	O
the	O
same	O
queue	O
which	O
it	O
relinquished	O
earlier	O
.	O
</s>
<s>
If	O
the	O
process	O
uses	O
all	O
the	O
quantum	O
time	O
,	O
it	O
is	O
pre-empted	B-Operating_System
and	O
inserted	O
at	O
the	O
end	O
of	O
the	O
next	O
lower	O
level	O
queue	O
.	O
</s>
<s>
At	O
the	O
base	O
level	O
queue	O
the	O
processes	O
circulate	O
in	O
round	B-Algorithm
robin	I-Algorithm
fashion	O
until	O
they	O
complete	O
and	O
leave	O
the	O
system	O
.	O
</s>
<s>
Optionally	O
,	O
if	O
a	O
process	O
blocks	O
for	O
I/O	B-General_Concept
,	O
it	O
is	O
'	O
promoted	O
 '	O
one	O
level	O
,	O
and	O
placed	O
at	O
the	O
end	O
of	O
the	O
next-higher	O
queue	O
.	O
</s>
<s>
This	O
allows	O
I/O	B-General_Concept
bound	I-General_Concept
processes	O
to	O
be	O
favored	O
by	O
the	O
scheduler	O
and	O
allows	O
processes	O
to	O
'	O
escape	O
 '	O
the	O
base	O
level	O
queue	O
.	O
</s>
<s>
In	O
the	O
multilevel	B-Operating_System
feedback	I-Operating_System
queue	I-Operating_System
a	O
process	O
is	O
given	O
just	O
one	O
chance	O
to	O
complete	O
at	O
a	O
given	O
queue	O
level	O
before	O
it	O
is	O
forced	O
down	O
to	O
a	O
lower	O
level	O
queue	O
.	O
</s>
<s>
In	O
general	O
,	O
a	O
multilevel	B-Operating_System
feedback	I-Operating_System
queue	I-Operating_System
scheduler	O
is	O
defined	O
by	O
the	O
following	O
parameters	O
:	O
</s>
<s>
The	O
scheduling	O
algorithm	O
for	O
each	O
queue	O
which	O
can	O
be	O
different	O
from	O
FIFO	B-Operating_System
.	O
</s>
