<s>
Earliest	O
deadline	O
first	O
(	O
EDF	O
)	O
or	O
least	O
time	O
to	O
go	O
is	O
a	O
dynamic	B-Algorithm
priority	I-Algorithm
scheduling	I-Algorithm
algorithm	O
used	O
in	O
real-time	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
to	O
place	O
processes	O
in	O
a	O
priority	B-Application
queue	I-Application
.	O
</s>
<s>
That	O
is	O
,	O
EDF	O
can	O
guarantee	O
that	O
all	O
deadlines	O
are	O
met	O
provided	O
that	O
the	O
total	O
CPU	B-General_Concept
utilization	O
is	O
not	O
more	O
than	O
100%	O
.	O
</s>
<s>
Compared	O
to	O
fixed	B-Operating_System
priority	I-Operating_System
scheduling	I-Operating_System
techniques	O
like	O
rate-monotonic	B-Operating_System
scheduling	I-Operating_System
,	O
EDF	O
can	O
guarantee	O
all	O
the	O
deadlines	O
in	O
the	O
system	O
at	O
higher	O
loading	O
.	O
</s>
<s>
The	O
algorithm	O
is	O
also	O
difficult	O
to	O
implement	O
in	O
hardware	B-Architecture
and	O
there	O
is	O
a	O
tricky	O
issue	O
of	O
representing	O
deadlines	O
in	O
different	O
ranges	O
(	O
deadlines	O
can	O
not	O
be	O
more	O
precise	O
than	O
the	O
granularity	O
of	O
the	O
clock	O
used	O
for	O
the	O
scheduling	O
)	O
.	O
</s>
<s>
Instead	O
,	O
most	O
real-time	O
computer	O
systems	O
use	O
fixed	B-Operating_System
priority	I-Operating_System
scheduling	I-Operating_System
(	O
usually	O
rate-monotonic	B-Operating_System
scheduling	I-Operating_System
)	O
.	O
</s>
<s>
There	O
is	O
a	O
significant	O
body	O
of	O
research	O
dealing	O
with	O
EDF	O
scheduling	O
in	O
real-time	B-General_Concept
computing	I-General_Concept
;	O
it	O
is	O
possible	O
to	O
calculate	O
worst	O
case	O
response	O
times	O
of	O
processes	O
in	O
EDF	O
,	O
to	O
deal	O
with	O
other	O
types	O
of	O
processes	O
than	O
periodic	O
processes	O
and	O
to	O
use	O
servers	O
to	O
regulate	O
overloads	O
.	O
</s>
<s>
A	O
process	O
may	O
use	O
a	O
shared	O
resource	O
inside	O
a	O
critical	B-Operating_System
section	I-Operating_System
,	O
to	O
prevent	O
it	O
from	O
being	O
pre-emptively	O
descheduled	O
in	O
favour	O
of	O
another	O
process	O
with	O
an	O
earlier	O
deadline	O
.	O
</s>
<s>
This	O
is	O
especially	O
important	O
if	O
the	O
process	O
running	O
the	O
critical	B-Operating_System
section	I-Operating_System
has	O
a	O
much	O
longer	O
time	O
to	O
complete	O
and	O
exit	O
from	O
its	O
critical	B-Operating_System
section	I-Operating_System
,	O
which	O
will	O
delay	O
releasing	O
the	O
shared	O
resource	O
.	O
</s>
<s>
This	O
hazard	O
of	O
deadline	O
interchange	O
is	O
analogous	O
to	O
priority	B-Operating_System
inversion	I-Operating_System
when	O
using	O
fixed	B-Operating_System
priority	I-Operating_System
pre-emptive	I-Operating_System
scheduling	I-Operating_System
.	O
</s>
<s>
It	O
is	O
commonly	O
accepted	O
that	O
an	O
implementation	O
of	O
fixed-priority	B-Operating_System
pre-emptive	I-Operating_System
scheduling	I-Operating_System
(	O
FPS	O
)	O
is	O
simpler	O
than	O
a	O
dynamic	O
priority	O
scheduler	O
,	O
like	O
the	O
EDF	O
.	O
</s>
<s>
However	O
,	O
when	O
comparing	O
the	O
maximum	O
usage	O
of	O
an	O
optimal	O
scheduling	O
under	O
fixed	O
priority	O
(	O
with	O
the	O
priority	O
of	O
each	O
thread	O
given	O
by	O
the	O
rate-monotonic	B-Operating_System
scheduling	I-Operating_System
)	O
,	O
the	O
EDF	O
can	O
reach	O
100%	O
while	O
the	O
theoretical	O
maximum	O
value	O
for	O
rate-monotonic	B-Operating_System
scheduling	I-Operating_System
is	O
around	O
69%	O
.	O
</s>
<s>
Although	O
EDF	O
implementations	O
are	O
not	O
common	O
in	O
commercial	O
real-time	O
kernels	B-Operating_System
,	O
here	O
are	O
a	O
few	O
links	O
of	O
open-source	O
and	O
real-time	O
kernels	B-Operating_System
implementing	O
EDF	O
:	O
</s>
<s>
ERIKA	O
Enterprise	O
,	O
which	O
provides	O
an	O
implementation	O
of	O
EDF	O
optimized	O
for	O
small	O
microcontrollers	O
with	O
an	O
API	O
similar	O
to	O
the	O
OSEK	B-Application
API	O
.	O
</s>
<s>
The	O
Everyman	O
Kernel	B-Operating_System
implements	O
either	O
EDF	O
or	O
Deadline	O
Monotonic	O
scheduling	O
depending	O
on	O
the	O
user	O
's	O
configuration	O
.	O
</s>
<s>
The	O
AQuoSA	B-Application
project	O
constitutes	O
a	O
modification	O
to	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
enriching	O
the	O
process	O
scheduler	O
with	O
EDF	O
scheduling	O
capabilities	O
.	O
</s>
<s>
The	O
timing	O
of	O
the	O
scheduling	O
cannot	O
be	O
as	O
precise	O
as	O
in	O
the	O
case	O
of	O
the	O
above	O
hard	O
real-time	B-Operating_System
Operating	I-Operating_System
Systems	I-Operating_System
,	O
yet	O
it	O
is	O
sufficiently	O
precise	O
so	O
as	O
to	O
greatly	O
enhance	O
predictability	O
,	O
and	O
thus	O
fulfill	O
the	O
real-time	O
requirements	O
,	O
of	O
multimedia	O
applications	O
.	O
</s>
<s>
AQuoSA	B-Application
is	O
one	O
of	O
a	O
few	O
projects	O
that	O
provides	O
real-time	O
scheduling	O
capabilities	O
to	O
unprivileged	O
users	O
on	O
a	O
system	O
in	O
a	O
controlled	O
way	O
,	O
by	O
means	O
of	O
a	O
properly	O
designed	O
access-control	O
model	O
.	O
</s>
<s>
The	O
Linux	B-Operating_System
kernel	I-Operating_System
has	O
an	O
earliest	O
deadline	O
first	O
implementation	O
named	O
SCHED	B-Application
DEADLINE	I-Application
which	O
is	O
available	O
since	O
the	O
release	O
3.14	O
.	O
</s>
<s>
The	O
developed	O
in	O
the	O
context	O
of	O
the	O
European	O
Project	O
is	O
a	O
multi-processor	O
real-time	O
scheduler	O
for	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
,	O
particularly	O
suitable	O
for	O
temporal	O
isolation	O
and	O
provisioning	O
of	O
QoS	O
guarantees	O
to	O
complex	O
multi-threaded	O
software	O
components	O
and	O
also	O
entire	O
virtual	B-Architecture
machines	I-Architecture
.	O
</s>
<s>
For	O
example	O
,	O
when	O
using	O
Linux	B-Operating_System
as	O
host	O
OS	O
and	O
KVM	B-Application
as	O
hypervisor	O
,	O
IRMOS	O
can	O
be	O
used	O
to	O
provide	O
scheduling	O
guarantees	O
to	O
individual	O
VMs	O
and	O
at	O
the	O
same	O
time	O
isolate	B-General_Concept
their	I-General_Concept
performance	I-General_Concept
so	O
as	O
to	O
avoid	O
undesired	O
temporal	O
interferences	O
.	O
</s>
<s>
However	O
,	O
reservations	O
are	O
multi-CPU	O
,	O
and	O
global	O
FP	O
over	O
multi-processors	O
is	O
used	O
at	O
the	O
inner	O
level	O
in	O
order	O
to	O
schedule	O
the	O
threads	O
(	O
and/or	O
processes	O
)	O
attached	O
to	O
each	O
outer	O
EDF	O
reservation	O
.	O
</s>
<s>
is	O
a	O
real-time	O
extension	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
with	O
a	O
focus	O
on	O
multiprocessor	O
real-time	O
scheduling	O
and	O
synchronization	O
.	O
</s>
