<s>
A	O
unikernel	B-Operating_System
is	O
a	O
specialised	O
,	O
single	B-Operating_System
address	I-Operating_System
space	I-Operating_System
machine	O
image	O
constructed	O
by	O
using	O
library	O
operating	O
systems	O
.	O
</s>
<s>
These	O
libraries	O
are	O
then	O
compiled	O
with	O
the	O
application	O
and	O
configuration	O
code	O
to	O
build	O
sealed	O
,	O
fixed-purpose	O
images	O
(	O
unikernels	B-Operating_System
)	O
which	O
run	O
directly	O
on	O
a	O
hypervisor	B-Operating_System
or	O
hardware	B-Architecture
without	O
an	O
intervening	O
OS	O
such	O
as	O
Linux	O
or	O
Windows	O
.	O
</s>
<s>
The	O
first	O
such	O
systems	O
were	O
Exokernel	B-Operating_System
and	O
Nemesis	O
in	O
the	O
late	O
1990s	O
.	O
</s>
<s>
In	O
a	O
library	O
operating	O
system	O
,	O
protection	O
boundaries	O
are	O
pushed	O
to	O
the	O
lowest	O
hardware	B-Architecture
layers	O
,	O
resulting	O
in	O
:	O
</s>
<s>
a	O
set	O
of	O
libraries	O
that	O
implement	O
mechanisms	O
such	O
as	O
those	O
needed	O
to	O
drive	O
hardware	B-Architecture
or	O
talk	O
network	O
protocols	O
;	O
</s>
<s>
One	O
of	O
the	O
advantages	O
is	O
that	O
since	O
there	O
is	O
only	O
a	O
single	B-Operating_System
address	I-Operating_System
space	I-Operating_System
,	O
there	O
is	O
no	O
need	O
for	O
repeated	O
privilege	O
transitions	O
to	O
move	O
data	O
between	O
user	O
space	O
and	O
kernel	O
space	O
.	O
</s>
<s>
Therefore	O
,	O
a	O
library	O
OS	O
can	O
provide	O
improved	O
performance	O
by	O
allowing	O
direct	O
access	O
to	O
hardware	B-Architecture
without	O
having	O
to	O
transition	O
between	O
user	O
mode	O
and	O
kernel	O
mode	O
(	O
on	O
a	O
traditional	O
kernel	O
this	O
transition	O
consists	O
of	O
a	O
single	O
TRAP	O
instruction	O
and	O
is	O
not	O
the	O
same	O
as	O
a	O
context	O
switch	O
)	O
.	O
</s>
<s>
In	O
addition	O
,	O
device	O
drivers	O
are	O
required	O
for	O
the	O
specific	O
hardware	B-Architecture
the	O
library	O
OS	O
runs	O
on	O
.	O
</s>
<s>
Since	O
hardware	B-Architecture
is	O
rapidly	O
changing	O
this	O
creates	O
the	O
burden	O
of	O
regularly	O
rewriting	O
drivers	O
to	O
remain	O
up	O
to	O
date	O
.	O
</s>
<s>
OS	O
virtualization	B-General_Concept
can	O
overcome	O
some	O
of	O
these	O
drawbacks	O
on	O
commodity	O
hardware	B-Architecture
.	O
</s>
<s>
A	O
modern	O
hypervisor	B-Operating_System
provides	O
virtual	O
machines	O
with	O
CPU	O
time	O
and	O
strongly	O
isolated	O
virtual	O
devices	O
.	O
</s>
<s>
A	O
library	O
OS	O
running	O
as	O
a	O
virtual	O
machine	O
only	O
needs	O
to	O
implement	O
drivers	O
for	O
these	O
stable	O
virtual	O
hardware	B-Architecture
devices	O
and	O
can	O
depend	O
on	O
the	O
hypervisor	B-Operating_System
to	O
drive	O
the	O
real	O
physical	O
hardware	B-Architecture
.	O
</s>
<s>
Additionally	O
,	O
reliance	O
on	O
a	O
hypervisor	B-Operating_System
may	O
reintroduce	O
performance	O
overheads	O
when	O
switching	O
between	O
the	O
unikernel	B-Operating_System
and	O
hypervisor	B-Operating_System
,	O
and	O
when	O
passing	O
data	O
to	O
and	O
from	O
hypervisor	B-Operating_System
virtual	O
devices	O
.	O
</s>
<s>
By	O
reducing	O
the	O
amount	O
of	O
code	O
deployed	O
,	O
unikernels	B-Operating_System
necessarily	O
reduce	O
the	O
likely	O
attack	O
surface	O
and	O
therefore	O
have	O
improved	O
security	O
properties	O
.	O
</s>
<s>
An	O
example	O
unikernel-based	O
messaging	O
client	O
has	O
around	O
4%	O
the	O
size	O
of	O
the	O
equivalent	O
code	O
bases	O
using	O
Linux	O
.	O
</s>
<s>
For	O
example	O
,	O
off-the-shelf	O
applications	O
such	O
as	O
nginx	O
,	O
SQLite	O
,	O
and	O
Redis	O
running	O
over	O
a	O
unikernel	B-Operating_System
have	O
shown	O
a	O
1.7x-2.7x	O
performance	O
improvement	O
.	O
</s>
<s>
Unikernels	B-Operating_System
have	O
been	O
regularly	O
shown	O
to	O
boot	O
extremely	O
quickly	O
,	O
in	O
time	O
to	O
respond	O
to	O
incoming	O
requests	O
before	O
the	O
requests	O
time-out	O
.	O
</s>
<s>
Unikernels	B-Operating_System
lend	O
themselves	O
to	O
creating	O
systems	O
that	O
follow	O
the	O
service-oriented	O
or	O
microservices	O
software	O
architectures	O
.	O
</s>
<s>
The	O
high	O
degree	O
of	O
specialisation	O
means	O
that	O
unikernels	B-Operating_System
are	O
unsuitable	O
for	O
the	O
kind	O
of	O
general	O
purpose	O
,	O
multi-user	O
computing	O
that	O
traditional	O
operating	O
systems	O
are	O
used	O
for	O
.	O
</s>
<s>
Adding	O
additional	O
functionality	O
or	O
altering	O
a	O
compiled	O
unikernel	B-Operating_System
is	O
generally	O
not	O
possible	O
and	O
instead	O
the	O
approach	O
is	O
to	O
compile	O
and	O
deploy	O
a	O
new	O
unikernel	B-Operating_System
with	O
the	O
desired	O
changes	O
.	O
</s>
