<s>
Completely	B-Application
Fair	I-Application
Queuing	I-Application
(	O
CFQ	B-Application
)	O
is	O
an	O
I/O	B-General_Concept
scheduler	I-General_Concept
for	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
which	O
was	O
written	O
in	O
2003	O
by	O
Jens	O
Axboe	O
.	O
</s>
<s>
CFQ	B-Application
places	O
synchronous	O
requests	O
submitted	O
by	O
processes	B-Operating_System
into	O
a	O
number	O
of	O
per-process	O
queues	B-Application
and	O
then	O
allocates	O
timeslices	O
for	O
each	O
of	O
the	O
queues	B-Application
to	O
access	O
the	O
disk	B-Device
.	O
</s>
<s>
The	O
length	O
of	O
the	O
time	O
slice	O
and	O
the	O
number	O
of	O
requests	O
a	O
queue	B-Application
is	O
allowed	O
to	O
submit	O
depends	O
on	O
the	O
I/O	O
priority	O
of	O
the	O
given	O
process	O
.	O
</s>
<s>
Asynchronous	O
requests	O
for	O
all	O
processes	B-Operating_System
are	O
batched	O
together	O
in	O
fewer	O
queues	B-Application
,	O
one	O
per	O
priority	O
.	O
</s>
<s>
While	O
CFQ	B-Application
does	O
not	O
do	O
explicit	O
anticipatory	B-Application
I/O	B-General_Concept
scheduling	I-General_Concept
,	O
it	O
achieves	O
the	O
same	O
effect	O
of	O
having	O
good	O
aggregate	O
throughput	O
for	O
the	O
system	O
as	O
a	O
whole	O
,	O
by	O
allowing	O
a	O
process	O
queue	B-Application
to	O
idle	O
at	O
the	O
end	O
of	O
synchronous	O
I/O	O
thereby	O
"	O
anticipating	O
"	O
further	O
close	O
I/O	O
from	O
that	O
process	O
.	O
</s>
<s>
In	O
February	O
2003	O
Andrea	O
Arcangeli	O
put	O
forward	O
his	O
idea	O
for	O
a	O
Stochastic	O
Fair	O
Queueing	O
I/O	B-General_Concept
scheduler	I-General_Concept
to	O
Jens	O
Axboe	O
who	O
then	O
implemented	O
it	O
.	O
</s>
<s>
Jens	O
Axboe	O
made	O
improvements	O
to	O
his	O
first	O
implementation	O
,	O
calling	O
the	O
new	O
version	O
the	O
Completely	B-Application
Fair	I-Application
Queueing	I-Application
scheduler	O
,	O
and	O
produced	O
a	O
patch	O
to	O
apply	O
it	O
to	O
the	O
2.5.60	O
development	O
series	O
kernel	B-Operating_System
.	O
</s>
<s>
The	O
CFQ	B-Application
I/O	B-General_Concept
scheduler	I-General_Concept
was	O
first	O
integrated	O
into	O
the	O
mainline	O
kernel	B-Operating_System
as	O
an	O
optional	O
I/O	B-General_Concept
scheduler	I-General_Concept
.	O
</s>
<s>
It	O
was	O
possible	O
to	O
change	O
the	O
scheduler	O
at	O
boot	O
time	O
with	O
the	O
'	O
elevator	O
 '	O
parameter	O
to	O
kernel	B-Operating_System
.	O
</s>
<s>
Red	O
Hat	O
Enterprise	O
Linux	B-Operating_System
4	O
used	O
this	O
I/O	B-General_Concept
scheduler	I-General_Concept
as	O
the	O
default	O
even	O
though	O
it	O
used	O
a	O
kernel	B-Operating_System
based	O
on	O
a	O
2.6.9	O
.	O
</s>
<s>
The	O
second	O
release	O
of	O
the	O
CFQ	B-Application
scheduler	O
dubbed	O
CFQv2	O
is	O
included	O
in	O
the	O
2.6.10	O
,	O
improvements	O
include	O
better	O
responsiveness	O
and	O
the	O
elimination	O
of	O
some	O
starvation	O
issues	O
which	O
were	O
present	O
in	O
the	O
earlier	O
version	O
.	O
</s>
<s>
The	O
scheduler	O
now	O
is	O
also	O
switchable	O
at	O
run	O
time	O
by	O
modifying	O
the	O
/sys/block/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx1/queue/scheduler	O
variable	O
in	O
the	O
sysfs	B-Application
filesystem	O
.	O
</s>
<s>
CFQ	B-Application
scheduler	O
moved	O
to	O
a	O
new	O
time	O
sliced	O
design	O
dubbed	O
CFQv3	O
.	O
</s>
<s>
CFQ	B-Application
became	O
the	O
default	O
scheduler	O
,	O
replacing	O
the	O
anticipatory	B-Application
scheduler	I-Application
.	O
</s>
<s>
CFQ	B-Application
has	O
been	O
removed	O
.	O
</s>
<s>
CFQ	B-Application
evolved	O
into	O
Budget	O
Fair	O
Queueing	O
(	O
BFQ	O
)	O
.	O
</s>
