<s>
A	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
is	O
system	O
software	O
over	O
a	O
collection	O
of	O
independent	O
software	O
,	O
networked	B-Architecture
,	O
communicating	B-Operating_System
,	O
and	O
physically	O
separate	O
computational	O
nodes	O
.	O
</s>
<s>
The	O
first	O
is	O
a	O
ubiquitous	O
minimal	O
kernel	B-Operating_System
,	O
or	O
microkernel	B-Operating_System
,	O
that	O
directly	O
controls	O
that	O
node	O
's	O
hardware	B-Architecture
.	O
</s>
<s>
These	O
components	O
abstract	O
microkernel	B-Operating_System
functions	O
and	O
support	O
user	B-Application
applications	O
.	O
</s>
<s>
The	O
microkernel	B-Operating_System
and	O
the	O
management	O
components	O
collection	O
work	O
together	O
.	O
</s>
<s>
This	O
seamless	O
integration	O
of	O
individual	O
nodes	O
into	O
a	O
global	O
system	O
is	O
referred	O
to	O
as	O
transparency	O
,	O
or	O
single	B-Operating_System
system	I-Operating_System
image	I-Operating_System
;	O
describing	O
the	O
illusion	O
provided	O
to	O
users	O
of	O
the	O
global	O
system	O
's	O
appearance	O
as	O
a	O
single	O
computational	O
entity	O
.	O
</s>
<s>
A	O
distributed	B-Operating_System
OS	I-Operating_System
provides	O
the	O
essential	O
services	O
and	O
functionality	O
required	O
of	O
an	O
OS	O
but	O
adds	O
attributes	O
and	O
particular	O
configurations	B-General_Concept
to	O
allow	O
it	O
to	O
support	O
additional	O
requirements	O
such	O
as	O
increased	O
scale	O
and	O
availability	O
.	O
</s>
<s>
To	O
a	O
user	B-Application
,	O
a	O
distributed	B-Operating_System
OS	I-Operating_System
works	O
in	O
a	O
manner	O
similar	O
to	O
a	O
single-node	O
,	O
monolithic	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
.	O
</s>
<s>
Separating	O
minimal	O
system-level	O
functionality	O
from	O
additional	O
user-level	O
modular	O
services	O
provides	O
a	O
"	O
separation	O
of	O
mechanism	O
and	O
policy	O
"	O
.	O
</s>
<s>
This	O
separation	O
increases	O
flexibility	B-Application
and	O
scalability	O
.	O
</s>
<s>
At	O
each	O
locale	B-Operating_System
(	O
typically	O
a	O
node	O
)	O
,	O
the	O
kernel	B-Operating_System
provides	O
a	O
minimally	O
complete	O
set	O
of	O
node-level	O
utilities	O
necessary	O
for	O
operating	O
a	O
node	O
's	O
underlying	O
hardware	B-Architecture
and	O
resources	O
.	O
</s>
<s>
These	O
mechanisms	O
include	O
allocation	O
,	O
management	O
,	O
and	O
disposition	O
of	O
a	O
node	O
's	O
resources	O
,	O
processes	B-Operating_System
,	O
communication	O
,	O
and	O
input/output	B-General_Concept
management	O
support	O
functions	O
.	O
</s>
<s>
Within	O
the	O
kernel	B-Operating_System
,	O
the	O
communications	O
sub-system	O
is	O
of	O
foremost	O
importance	O
for	O
a	O
distributed	B-Operating_System
OS	I-Operating_System
.	O
</s>
<s>
In	O
a	O
distributed	B-Operating_System
OS	I-Operating_System
,	O
the	O
kernel	B-Operating_System
often	O
supports	O
a	O
minimal	O
set	O
of	O
functions	O
,	O
including	O
low-level	O
address	B-General_Concept
space	I-General_Concept
management	O
,	O
thread	B-Operating_System
management	I-Operating_System
,	O
and	O
inter-process	B-Operating_System
communication	I-Operating_System
(	O
IPC	O
)	O
.	O
</s>
<s>
A	O
kernel	B-Operating_System
of	O
this	O
design	O
is	O
referred	O
to	O
as	O
a	O
microkernel	B-Operating_System
.	O
</s>
<s>
Its	O
modular	O
nature	O
enhances	O
reliability	O
and	O
security	O
,	O
essential	O
features	O
for	O
a	O
distributed	B-Operating_System
OS	I-Operating_System
.	O
</s>
<s>
System	O
management	O
components	O
are	O
software	O
processes	B-Operating_System
that	O
define	O
the	O
node	O
's	O
policies	O
.	O
</s>
<s>
These	O
components	O
are	O
the	O
part	O
of	O
the	O
OS	O
outside	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
The	O
architecture	O
and	O
design	O
of	O
a	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
must	O
realize	O
both	O
individual	O
node	O
and	O
global	O
system	O
goals	O
.	O
</s>
<s>
In	O
doing	O
so	O
,	O
a	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
attempts	O
to	O
provide	O
an	O
efficient	O
and	O
reliable	O
distributed	B-Architecture
computing	I-Architecture
framework	O
allowing	O
for	O
an	O
absolute	O
minimal	O
user	B-Application
awareness	O
of	O
the	O
underlying	O
command	O
and	O
control	O
efforts	O
.	O
</s>
<s>
The	O
multi-level	O
collaboration	O
between	O
a	O
kernel	B-Operating_System
and	O
the	O
system	O
management	O
components	O
,	O
and	O
in	O
turn	O
between	O
the	O
distinct	O
nodes	O
in	O
a	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
is	O
the	O
functional	O
challenge	O
of	O
the	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
.	O
</s>
<s>
This	O
challenge	O
is	O
the	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
's	O
opportunity	O
to	O
produce	O
the	O
foundation	O
and	O
framework	O
for	O
a	O
reliable	O
,	O
efficient	O
,	O
available	O
,	O
robust	O
,	O
extensible	O
,	O
and	O
scalable	O
system	O
.	O
</s>
<s>
In	O
a	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
,	O
the	O
exceptional	O
degree	O
of	O
inherent	O
complexity	O
could	O
easily	O
render	O
the	O
entire	O
system	O
an	O
anathema	O
to	O
any	O
user	B-Application
.	O
</s>
<s>
For	O
instance	O
,	O
a	O
deep	O
understanding	O
of	O
a	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
's	O
overall	O
architectural	O
and	O
design	O
detail	O
is	O
required	O
at	O
an	O
exceptionally	O
early	O
point	O
.	O
</s>
<s>
An	O
exhausting	O
array	O
of	O
design	O
considerations	O
are	O
inherent	O
in	O
the	O
development	O
of	O
a	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
.	O
</s>
<s>
As	O
an	O
aid	O
in	O
this	O
effort	O
,	O
most	O
rely	O
on	O
documented	O
experience	O
and	O
research	O
in	O
distributed	B-Architecture
computing	I-Architecture
power	O
.	O
</s>
<s>
A	O
number	O
of	O
distributed	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
were	O
introduced	O
during	O
this	O
period	O
;	O
however	O
,	O
very	O
few	O
of	O
these	O
implementations	O
achieved	O
even	O
modest	O
commercial	O
success	O
.	O
</s>
<s>
Fundamental	O
and	O
pioneering	O
implementations	O
of	O
primitive	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
component	O
concepts	O
date	O
to	O
the	O
early	O
1950s	O
.	O
</s>
<s>
Some	O
of	O
these	O
individual	O
steps	O
were	O
not	O
focused	O
directly	O
on	O
distributed	B-Architecture
computing	I-Architecture
,	O
and	O
at	O
the	O
time	O
,	O
many	O
may	O
not	O
have	O
realized	O
their	O
important	O
impact	O
.	O
</s>
<s>
These	O
pioneering	O
efforts	O
laid	O
important	O
groundwork	O
,	O
and	O
inspired	O
continued	O
research	O
in	O
areas	O
related	O
to	O
distributed	B-Architecture
computing	I-Architecture
.	O
</s>
<s>
In	O
the	O
mid-1970s	O
,	O
research	O
produced	O
important	O
advances	O
in	O
distributed	B-Architecture
computing	I-Architecture
.	O
</s>
<s>
The	O
accelerating	O
proliferation	O
of	O
multi-processor	B-Operating_System
and	O
multi-core	B-Architecture
processor	I-Architecture
systems	O
research	O
led	O
to	O
a	O
resurgence	O
of	O
the	O
distributed	B-Operating_System
OS	I-Operating_System
concept	O
.	O
</s>
<s>
One	O
of	O
the	O
first	O
efforts	O
was	O
the	O
DYSEAC	B-Device
,	O
a	O
general-purpose	O
synchronous	O
computer	O
.	O
</s>
<s>
In	O
one	O
of	O
the	O
earliest	O
publications	O
of	O
the	O
Association	O
for	O
Computing	O
Machinery	O
,	O
in	O
April	O
1954	O
,	O
a	O
researcher	O
at	O
the	O
National	O
Bureau	O
of	O
Standards	O
now	O
the	O
National	O
Institute	O
of	O
Standards	O
and	O
Technology	O
(	O
NIST	O
)	O
presented	O
a	O
detailed	O
specification	O
of	O
the	O
DYSEAC	B-Device
.	O
</s>
<s>
This	O
was	O
a	O
"	O
portable	B-Device
computer	I-Device
"	O
,	O
housed	O
in	O
a	O
tractor-trailer	O
,	O
with	O
2	O
attendant	O
vehicles	O
and	O
6	O
tons	O
of	O
refrigeration	O
capacity	O
.	O
</s>
<s>
Described	O
as	O
an	O
experimental	O
input-output	B-General_Concept
system	O
,	O
the	O
Lincoln	B-Device
TX-2	I-Device
emphasized	O
flexible	O
,	O
simultaneously	O
operational	O
input-output	B-General_Concept
devices	I-General_Concept
,	O
i.e.	O
,	O
multiprogramming	O
.	O
</s>
<s>
The	O
design	O
of	O
the	O
TX-2	B-Device
was	O
modular	O
,	O
supporting	O
a	O
high	O
degree	O
of	O
modification	O
and	O
expansion	O
.	O
</s>
<s>
This	O
technique	O
allowed	O
multiple	O
program	B-General_Concept
counters	I-General_Concept
to	O
each	O
associate	O
with	O
one	O
of	O
32	O
possible	O
sequences	O
of	O
program	O
code	O
.	O
</s>
<s>
Similar	O
to	O
DYSEAC	B-Device
the	O
TX-2	B-Device
separately	O
programmed	O
devices	O
can	O
operate	O
simultaneously	O
,	O
increasing	O
throughput	O
.	O
</s>
<s>
The	O
TX-2	B-Device
was	O
another	O
example	O
of	O
a	O
system	O
exhibiting	O
distributed	O
control	O
,	O
its	O
central	O
unit	O
not	O
having	O
dedicated	O
control	O
.	O
</s>
<s>
One	O
early	O
effort	O
at	O
abstracting	O
memory	B-General_Concept
access	O
was	O
Intercommunicating	O
Cells	O
,	O
where	O
a	O
cell	O
was	O
composed	O
of	O
a	O
collection	O
of	O
memory	B-General_Concept
elements	O
.	O
</s>
<s>
A	O
memory	B-General_Concept
element	O
was	O
basically	O
a	O
binary	O
electronic	O
flip-flop	B-General_Concept
or	O
relay	O
.	O
</s>
<s>
Information	O
was	O
accessed	O
in	O
two	O
ways	O
,	O
direct	O
and	O
cross-retrieval	O
.	O
</s>
<s>
Direct	O
retrieval	B-Library
accepts	O
a	O
name	O
and	O
returns	O
a	O
parameter	O
set	O
.	O
</s>
<s>
Cross-retrieval	O
projects	O
through	O
parameter	O
sets	O
and	O
returns	O
a	O
set	O
of	O
names	O
containing	O
the	O
given	O
subset	O
of	O
parameters	O
.	O
</s>
<s>
This	O
was	O
similar	O
to	O
a	O
modified	O
hash	B-Algorithm
table	I-Algorithm
data	B-General_Concept
structure	I-General_Concept
that	O
allowed	O
multiple	O
values	O
(	O
parameters	O
)	O
for	O
each	O
key	B-Application
(	O
name	O
)	O
.	O
</s>
<s>
This	O
configuration	B-General_Concept
was	O
ideal	O
for	O
distributed	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
The	O
constant-time	O
projection	O
through	O
memory	B-General_Concept
for	O
storing	O
and	O
retrieval	B-Library
was	O
inherently	O
atomic	B-General_Concept
and	O
exclusive	B-Operating_System
.	O
</s>
<s>
The	O
cellular	O
memory	B-General_Concept
's	O
intrinsic	O
distributed	O
characteristics	O
would	O
be	O
invaluable	O
.	O
</s>
<s>
The	O
impact	O
on	O
the	O
user	B-Application
,	O
hardware/device	O
,	O
or	O
Application	B-Application
programming	I-Application
interfaces	I-Application
was	O
indirect	O
.	O
</s>
<s>
The	O
authors	O
were	O
considering	O
distributed	B-Architecture
systems	I-Architecture
,	O
stating	O
:	O
</s>
<s>
To	O
better	O
illustrate	O
this	O
point	O
,	O
examine	O
three	O
system	O
architectures	B-Architecture
;	O
centralized	O
,	O
decentralized	O
,	O
and	O
distributed	O
.	O
</s>
<s>
A	O
centralized	B-General_Concept
system	I-General_Concept
has	O
one	O
level	O
of	O
structure	O
,	O
where	O
all	O
constituent	O
elements	O
directly	O
depend	O
upon	O
a	O
single	O
control	O
element	O
.	O
</s>
<s>
A	O
distributed	B-Architecture
system	I-Architecture
is	O
a	O
collection	O
of	O
autonomous	O
elements	O
with	O
no	O
concept	O
of	O
levels	O
.	O
</s>
<s>
A	O
decentralized	O
system	O
(	O
aka	O
network	B-Protocol
system	I-Protocol
)	O
incorporates	O
direct	O
and	O
indirect	O
paths	O
between	O
constituent	O
elements	O
and	O
the	O
central	O
entity	O
.	O
</s>
<s>
Finally	O
,	O
the	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
requires	O
no	O
pattern	O
;	O
direct	O
and	O
indirect	O
connections	O
are	O
possible	O
between	O
any	O
two	O
elements	O
.	O
</s>
<s>
Centralized	O
and	O
decentralized	O
systems	O
have	O
directed	O
flows	O
of	O
connection	O
to	O
and	O
from	O
the	O
central	O
entity	O
,	O
while	O
distributed	B-Architecture
systems	I-Architecture
communicate	O
along	O
arbitrary	O
paths	O
.	O
</s>
<s>
Distributed	B-Architecture
systems	I-Architecture
are	O
more	O
difficult	O
to	O
explicitly	O
control	O
,	O
but	O
scale	O
better	O
horizontally	O
and	O
offer	O
fewer	O
points	O
of	O
system-wide	O
failure	O
.	O
</s>
<s>
Transparency	O
or	O
single-system	B-Operating_System
image	I-Operating_System
refers	O
to	O
the	O
ability	O
of	O
an	O
application	O
to	O
treat	O
the	O
system	O
on	O
which	O
it	O
operates	O
without	O
regard	O
to	O
whether	O
it	O
is	O
distributed	O
and	O
without	O
regard	O
to	O
hardware	B-Architecture
or	O
other	O
implementation	O
details	O
.	O
</s>
<s>
The	O
consideration	O
of	O
transparency	O
directly	O
affects	O
decision	O
making	O
in	O
every	O
aspect	O
of	O
design	O
of	O
a	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
.	O
</s>
<s>
For	O
example	O
,	O
a	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
may	O
present	O
a	O
hard	O
drive	O
on	O
one	O
computer	O
as	O
"	O
C	O
:	O
"	O
and	O
a	O
drive	O
on	O
another	O
computer	O
as	O
"	O
G	O
:	O
"	O
.	O
</s>
<s>
The	O
user	B-Application
does	O
not	O
require	O
any	O
knowledge	O
of	O
device	O
drivers	O
or	O
the	O
drive	O
's	O
location	O
;	O
both	O
devices	O
work	O
the	O
same	O
way	O
,	O
from	O
the	O
application	O
's	O
perspective	O
.	O
</s>
<s>
Location	O
transparency	O
–	O
Location	O
transparency	O
comprises	O
two	O
distinct	O
aspects	O
of	O
transparency	O
,	O
naming	O
transparency	O
and	O
user	B-Application
mobility	O
.	O
</s>
<s>
Naming	O
transparency	O
requires	O
that	O
nothing	O
in	O
the	O
physical	O
or	O
logical	O
references	O
to	O
any	O
system	O
entity	O
should	O
expose	O
any	O
indication	O
of	O
the	O
entity	O
's	O
location	O
,	O
or	O
its	O
local	O
or	O
remote	O
relationship	O
to	O
the	O
user	B-Application
or	O
application	O
.	O
</s>
<s>
User	B-Application
mobility	O
requires	O
the	O
consistent	O
referencing	O
of	O
system	O
entities	O
,	O
regardless	O
of	O
the	O
system	O
location	O
from	O
which	O
the	O
reference	O
originates	O
.	O
</s>
<s>
Access	O
transparency	O
–	O
Local	O
and	O
remote	O
system	O
entities	O
must	O
remain	O
indistinguishable	O
when	O
viewed	O
through	O
the	O
user	B-Application
interface	I-Application
.	O
</s>
<s>
The	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
maintains	O
this	O
perception	O
through	O
the	O
exposure	O
of	O
a	O
single	O
access	O
mechanism	O
for	O
a	O
system	O
entity	O
,	O
regardless	O
of	O
that	O
entity	O
being	O
local	O
or	O
remote	O
to	O
the	O
user	B-Application
.	O
</s>
<s>
Transparency	O
dictates	O
that	O
any	O
differences	O
in	O
methods	O
of	O
accessing	O
any	O
particular	O
system	O
entity	O
—	O
either	O
local	O
or	O
remote	O
—	O
must	O
be	O
both	O
invisible	O
to	O
,	O
and	O
undetectable	O
by	O
the	O
user	B-Application
.	O
</s>
<s>
Migration	O
transparency	O
–	O
Resources	O
and	O
activities	O
migrate	O
from	O
one	O
element	O
to	O
another	O
controlled	O
solely	O
by	O
the	O
system	O
and	O
without	O
user/application	O
knowledge	O
or	O
action	O
.	O
</s>
<s>
Replication	B-Architecture
transparency	O
–	O
The	O
process	O
or	O
fact	O
that	O
a	O
resource	O
has	O
been	O
duplicated	O
on	O
another	O
element	O
occurs	O
under	O
system	O
control	O
and	O
without	O
user/application	O
knowledge	O
or	O
intervention	O
.	O
</s>
<s>
Concurrency	B-Architecture
transparency	O
–	O
Users/applications	O
are	O
unaware	O
of	O
and	O
unaffected	O
by	O
the	O
presence/activities	O
of	O
other	O
users	O
.	O
</s>
<s>
No	O
user	B-Application
knowledge/action	O
is	O
involved	O
other	O
than	O
waiting	O
for	O
the	O
system	O
to	O
resolve	O
the	O
problem	O
.	O
</s>
<s>
Note	O
that	O
system	O
policies	O
may	O
prefer	O
some	O
users/user	O
classes/tasks	O
over	O
others	O
.	O
</s>
<s>
No	O
user	B-Application
knowledge	O
or	O
interaction	O
.	O
</s>
<s>
Size/Scale	O
transparency	O
–	O
The	O
system	O
is	O
responsible	O
for	O
managing	O
its	O
geographic	O
reach	O
,	O
number	O
of	O
nodes	O
,	O
level	O
of	O
node	O
capability	O
without	O
any	O
required	O
user	B-Application
knowledge	O
or	O
interaction	O
.	O
</s>
<s>
Revision	O
transparency	O
–	O
The	O
system	O
is	O
responsible	O
for	O
upgrades	O
and	O
revisions	O
and	O
changes	O
to	O
system	O
infrastructure	O
without	O
user	B-Application
knowledge	O
or	O
action	O
.	O
</s>
<s>
Control	O
transparency	O
–	O
The	O
system	O
is	O
responsible	O
for	O
providing	O
all	O
system	O
information	O
,	O
constants	O
,	O
properties	O
,	O
configuration	B-General_Concept
settings	O
,	O
etc	O
.	O
</s>
<s>
Data	O
transparency	O
–	O
The	O
system	O
is	O
responsible	O
for	O
providing	O
data	O
to	O
applications	O
without	O
user	B-Application
knowledge	O
or	O
action	O
relating	O
to	O
where	O
the	O
system	O
stores	O
it	O
.	O
</s>
<s>
Parallelism	B-Operating_System
transparency	O
–	O
The	O
system	O
is	O
responsible	O
for	O
exploiting	O
any	O
ability	O
to	O
parallelize	O
task	O
execution	O
without	O
user	B-Application
knowledge	O
or	O
interaction	O
.	O
</s>
<s>
Arguably	O
the	O
most	O
difficult	O
aspect	O
of	O
transparency	O
,	O
and	O
described	O
by	O
Tanenbaum	O
as	O
the	O
"	O
Holy	O
grail	O
"	O
for	O
distributed	B-Architecture
system	I-Architecture
designers	O
.	O
</s>
<s>
Inter-Process	B-Operating_System
Communication	I-Operating_System
(	O
IPC	O
)	O
is	O
the	O
implementation	O
of	O
general	O
communication	O
,	O
process	O
interaction	O
,	O
and	O
dataflow	B-Application
between	O
threads	B-Operating_System
and/or	O
processes	B-Operating_System
both	O
within	O
a	O
node	O
,	O
and	O
between	O
nodes	O
in	O
a	O
distributed	B-Operating_System
OS	I-Operating_System
.	O
</s>
<s>
In	O
this	O
sense	O
,	O
Interprocess	B-Operating_System
communication	I-Operating_System
is	O
the	O
greatest	O
underlying	O
concept	O
in	O
the	O
low-level	O
design	O
considerations	O
of	O
a	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
.	O
</s>
<s>
Process	B-Operating_System
management	I-Operating_System
provides	O
policies	O
and	O
mechanisms	O
for	O
effective	O
and	O
efficient	O
sharing	O
of	O
resources	O
between	O
distributed	O
processes	B-Operating_System
.	O
</s>
<s>
These	O
policies	O
and	O
mechanisms	O
support	O
operations	O
involving	O
the	O
allocation	O
and	O
de-allocation	O
of	O
processes	B-Operating_System
and	O
ports	O
to	O
processors	O
,	O
as	O
well	O
as	O
mechanisms	O
to	O
run	O
,	O
suspend	O
,	O
migrate	O
,	O
halt	O
,	O
or	O
resume	O
process	O
execution	O
.	O
</s>
<s>
While	O
these	O
resources	O
and	O
operations	O
can	O
be	O
either	O
local	O
or	O
remote	O
with	O
respect	O
to	O
each	O
other	O
,	O
the	O
distributed	B-Operating_System
OS	I-Operating_System
maintains	O
state	O
and	O
synchronization	O
over	O
all	O
processes	B-Operating_System
in	O
the	O
system	O
.	O
</s>
<s>
As	O
an	O
example	O
,	O
load	B-Application
balancing	I-Application
is	O
a	O
common	O
process	B-Operating_System
management	I-Operating_System
function	O
.	O
</s>
<s>
Load	B-Application
balancing	I-Application
monitors	O
node	O
performance	O
and	O
is	O
responsible	O
for	O
shifting	O
activity	O
across	O
nodes	O
when	O
the	O
system	O
is	O
out	O
of	O
balance	O
.	O
</s>
<s>
One	O
load	B-Application
balancing	I-Application
function	O
is	O
picking	O
a	O
process	O
to	O
move	O
.	O
</s>
<s>
The	O
kernel	B-Operating_System
may	O
employ	O
several	O
selection	O
mechanisms	O
,	O
including	O
priority-based	O
choice	O
.	O
</s>
<s>
Systems	B-General_Concept
resources	I-General_Concept
such	O
as	O
memory	B-General_Concept
,	O
files	O
,	O
devices	O
,	O
etc	O
.	O
</s>
<s>
Load	B-Application
sharing	I-Application
and	O
load	B-Application
balancing	I-Application
require	O
many	O
policy-oriented	O
decisions	O
,	O
ranging	O
from	O
finding	O
idle	O
CPUs	O
,	O
when	O
to	O
move	O
,	O
and	O
which	O
to	O
move	O
.	O
</s>
<s>
Distributed	B-Operating_System
OS	I-Operating_System
can	O
provide	O
the	O
necessary	O
resources	O
and	O
services	O
to	O
achieve	O
high	O
levels	O
of	O
reliability	O
,	O
or	O
the	O
ability	O
to	O
prevent	O
and/or	O
recover	O
from	O
errors	O
.	O
</s>
<s>
Faults	B-General_Concept
are	O
physical	O
or	O
logical	O
defects	O
that	O
can	O
cause	O
errors	O
in	O
the	O
system	O
.	O
</s>
<s>
For	O
a	O
system	O
to	O
be	O
reliable	O
,	O
it	O
must	O
somehow	O
overcome	O
the	O
adverse	O
effects	O
of	O
faults	B-General_Concept
.	O
</s>
<s>
The	O
primary	O
methods	O
for	O
dealing	O
with	O
faults	B-General_Concept
include	O
fault	O
avoidance	O
,	O
fault	B-General_Concept
tolerance	I-General_Concept
,	O
and	O
fault	O
detection	O
and	O
recovery	O
.	O
</s>
<s>
Fault	O
avoidance	O
covers	O
proactive	O
measures	O
taken	O
to	O
minimize	O
the	O
occurrence	O
of	O
faults	B-General_Concept
.	O
</s>
<s>
These	O
proactive	O
measures	O
can	O
be	O
in	O
the	O
form	O
of	O
transactions	B-General_Concept
,	O
replication	B-Architecture
and	O
backups	O
.	O
</s>
<s>
Fault	B-General_Concept
tolerance	I-General_Concept
is	O
the	O
ability	O
of	O
a	O
system	O
to	O
continue	O
operation	O
in	O
the	O
presence	O
of	O
a	O
fault	O
.	O
</s>
<s>
In	O
any	O
event	O
,	O
any	O
actions	O
taken	O
should	O
make	O
every	O
effort	O
to	O
preserve	O
the	O
single	B-Operating_System
system	I-Operating_System
image	I-Operating_System
.	O
</s>
<s>
With	O
respect	O
to	O
a	O
distributed	B-Operating_System
OS	I-Operating_System
,	O
performance	O
most	O
often	O
distills	O
to	O
a	O
balance	O
between	O
process	B-Operating_System
parallelism	I-Operating_System
and	O
IPC	O
.	O
</s>
<s>
Managing	O
the	O
task	O
granularity	O
of	O
parallelism	B-Operating_System
in	O
a	O
sensible	O
relation	O
to	O
the	O
messages	O
required	O
for	O
support	O
is	O
extremely	O
effective	O
.	O
</s>
<s>
Also	O
,	O
identifying	O
when	O
it	O
is	O
more	O
beneficial	O
to	O
migrate	B-Operating_System
a	I-Operating_System
process	I-Operating_System
to	O
its	O
data	O
,	O
rather	O
than	O
copy	O
the	O
data	O
,	O
is	O
effective	O
as	O
well	O
.	O
</s>
<s>
Cooperating	O
concurrent	B-Architecture
processes	I-Architecture
have	O
an	O
inherent	O
need	O
for	O
synchronization	O
,	O
which	O
ensures	O
that	O
changes	O
happen	O
in	O
a	O
correct	O
and	O
predictable	O
fashion	O
.	O
</s>
<s>
one	O
or	O
more	O
processes	B-Operating_System
must	O
synchronize	O
at	O
a	O
given	O
point	O
for	O
one	O
or	O
more	O
other	O
processes	B-Operating_System
to	O
continue	O
,	O
</s>
<s>
one	O
or	O
more	O
processes	B-Operating_System
must	O
wait	O
for	O
an	O
asynchronous	O
condition	O
in	O
order	O
to	O
continue	O
,	O
</s>
<s>
or	O
a	O
process	O
must	O
establish	O
exclusive	B-Operating_System
access	O
to	O
a	O
shared	O
resource	O
.	O
</s>
<s>
Improper	O
synchronization	O
can	O
lead	O
to	O
multiple	O
failure	O
modes	O
including	O
loss	O
of	O
atomicity	B-General_Concept
,	O
consistency	O
,	O
isolation	O
and	O
durability	O
,	O
deadlock	B-Operating_System
,	O
livelock	B-Application
and	O
loss	O
of	O
serializability	B-Operating_System
.	O
</s>
<s>
Flexibility	B-Application
in	O
a	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
is	O
enhanced	O
through	O
the	O
modular	O
characteristics	O
of	O
the	O
distributed	B-Operating_System
OS	I-Operating_System
,	O
and	O
by	O
providing	O
a	O
richer	O
set	O
of	O
higher-level	O
services	O
.	O
</s>
<s>
The	O
completeness	O
and	O
quality	O
of	O
the	O
kernel/microkernel	O
simplifies	O
implementation	O
of	O
such	O
services	O
,	O
and	O
potentially	O
enables	O
service	O
providers	O
greater	O
choice	O
of	O
providers	O
for	O
such	O
services	O
.	O
</s>
<s>
Scale	O
and	O
performance	O
in	O
the	O
Denali	O
isolation	O
kernel	B-Operating_System
.	O
</s>
<s>
The	O
multikernel	B-Operating_System
:	O
a	O
new	O
OS	O
architecture	O
for	O
scalable	O
multicore	B-Architecture
systems	O
.	O
</s>
<s>
Helios	O
:	O
heterogeneous	O
multiprocessing	B-Operating_System
with	O
satellite	O
kernels	B-Operating_System
.	O
</s>
