<s>
Spring	O
is	O
a	O
discontinued	O
project	O
in	O
building	O
an	O
experimental	O
microkernel-based	O
object-oriented	B-Language
operating	B-General_Concept
system	I-General_Concept
(	O
OS	O
)	O
developed	O
at	O
Sun	O
Microsystems	O
in	O
the	O
early	O
1990s	O
.	O
</s>
<s>
Using	O
technology	O
substantially	O
similar	O
to	O
concepts	O
developed	O
in	O
the	O
Mach	B-Operating_System
kernel	I-Operating_System
,	O
Spring	O
concentrated	O
on	O
providing	O
a	O
richer	O
programming	O
environment	O
supporting	O
multiple	B-Application
inheritance	I-Application
and	O
other	O
features	O
.	O
</s>
<s>
Spring	O
was	O
also	O
more	O
cleanly	O
separated	O
from	O
the	O
operating	B-General_Concept
systems	I-General_Concept
it	O
would	O
host	O
,	O
divorcing	O
it	O
from	O
its	O
Unix	B-Application
roots	O
and	O
even	O
allowing	O
several	O
OSes	O
to	O
be	O
run	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
Development	O
faded	O
out	O
in	O
the	O
mid-1990s	O
,	O
but	O
several	O
ideas	O
and	O
some	O
code	O
from	O
the	O
project	O
was	O
later	O
re-used	O
in	O
the	O
Java	B-Language
programming	I-Language
language	I-Language
libraries	O
and	O
the	O
Solaris	B-Application
operating	I-Application
system	I-Application
.	O
</s>
<s>
Spring	O
started	O
in	O
a	O
roundabout	O
fashion	O
in	O
1987	O
,	O
as	O
part	O
of	O
Sun	O
and	O
T	O
's	O
collaboration	O
to	O
create	O
a	O
merged	O
UNIX	B-Application
.	O
</s>
<s>
Both	O
companies	O
decided	O
it	O
was	O
also	O
a	O
good	O
opportunity	O
to	O
"	O
reimplement	O
UNIX	B-Application
in	O
an	O
object-oriented	B-Language
fashion	O
"	O
.	O
</s>
<s>
Along	O
with	O
combining	O
Unix	B-Application
flavours	O
,	O
the	O
new	O
system	O
would	O
also	O
be	O
able	O
to	O
run	O
almost	O
any	O
other	O
system	O
,	O
and	O
in	O
a	O
distributed	B-Architecture
fashion	O
.	O
</s>
<s>
Described	O
as	O
a	O
"	O
clean	O
slate	O
"	O
intended	O
to	O
help	O
Sun	O
improve	O
its	O
existing	O
Unix	B-Application
products	O
,	O
the	O
software	O
was	O
made	O
available	O
at	O
a	O
cost	O
of	O
$75	O
,	O
with	O
Sun	O
targeting	O
universities	O
and	O
computer	O
scientists	O
.	O
</s>
<s>
The	O
Spring	O
project	O
began	O
soon	O
after	O
the	O
release	O
of	O
Mach	B-Operating_System
3	O
.	O
</s>
<s>
In	O
earlier	O
versions	O
Mach	B-Operating_System
was	O
simply	O
a	O
modified	O
version	O
of	O
existing	O
BSD	B-Operating_System
kernels	O
,	O
but	O
in	O
Mach	B-Operating_System
3	O
the	O
Unix	B-Application
services	O
were	O
separated	O
out	O
and	O
run	O
as	O
a	O
user-space	O
program	O
like	O
any	O
other	O
,	O
a	O
concept	O
Mach	B-Operating_System
referred	O
to	O
as	O
a	O
server	O
.	O
</s>
<s>
Data	O
which	O
would	O
normally	O
be	O
private	O
in	O
the	O
kernel	O
under	O
a	O
traditional	B-Application
Unix	I-Application
system	O
was	O
now	O
passed	O
between	O
the	O
servers	O
and	O
user	O
programs	O
using	O
an	O
inter-process	B-Operating_System
communication	I-Operating_System
(	O
IPC	O
)	O
system	O
,	O
ending	O
in	O
ports	O
which	O
both	O
programs	O
held	O
.	O
</s>
<s>
Mach	B-Operating_System
implemented	O
these	O
ports	O
in	O
the	O
kernel	O
,	O
using	O
virtual	B-Architecture
memory	I-Architecture
to	O
move	O
data	O
from	O
program	O
to	O
program	O
,	O
relying	O
on	O
the	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
(	O
MMU	O
)	O
and	O
the	O
copy	B-Language
on	I-Language
write	I-Language
algorithm	O
to	O
do	O
so	O
with	O
reasonable	O
performance	O
.	O
</s>
<s>
In	O
its	O
ultimate	O
development	O
,	O
an	O
OS	O
on	O
Mach	B-Operating_System
would	O
consist	O
of	O
a	O
number	O
of	O
such	O
servers	O
,	O
each	O
handling	O
a	O
specific	O
task	O
.	O
</s>
<s>
Examples	O
would	O
include	O
the	O
file	B-Application
system	I-Application
or	O
network	B-Protocol
stack	I-Protocol
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
server	O
in	O
such	O
a	O
system	O
would	O
be	O
quite	O
small	O
,	O
providing	O
services	O
unique	O
to	O
that	O
OS	O
,	O
and	O
forwarding	O
most	O
other	O
calls	O
to	O
other	O
servers	O
.	O
</s>
<s>
Since	O
the	O
OS	O
was	O
running	O
on	O
top	O
of	O
single	O
set	O
of	O
common	O
servers	O
,	O
several	O
OS	O
servers	O
could	O
be	O
run	O
at	O
the	O
same	O
time	O
,	O
allowing	O
a	O
single	O
system	O
to	O
"	O
natively	O
"	O
support	O
DOS	B-Device
,	O
Unix	B-Application
and	O
other	O
operating	B-General_Concept
systems	I-General_Concept
at	O
the	O
same	O
time	O
.	O
</s>
<s>
This	O
capability	O
was	O
particularly	O
exciting	O
to	O
companies	O
like	O
IBM	O
,	O
who	O
were	O
already	O
supporting	O
several	O
different	O
systems	O
,	O
and	O
saw	O
Mach	B-Operating_System
as	O
a	O
way	O
to	O
combine	O
these	O
with	O
common	O
underlying	O
code	O
.	O
</s>
<s>
Mach	B-Operating_System
made	O
several	O
decisions	O
at	O
a	O
low-level	O
which	O
made	O
any	O
system	O
running	O
on	O
it	O
Unix-like	B-Operating_System
to	O
some	O
degree	O
.	O
</s>
<s>
Most	O
notable	O
was	O
a	O
security	O
system	O
which	O
was	O
modelled	O
on	O
fairly	O
inflexible	O
inherited	O
model	O
of	O
Unix	B-Application
programs	O
.	O
</s>
<s>
The	O
performance	O
was	O
so	O
poor	O
that	O
many	O
commercial	O
projects	O
to	O
port	O
existing	O
operating	B-General_Concept
systems	I-General_Concept
to	O
Mach	B-Operating_System
,	O
notably	O
IBM	O
's	O
Workplace	B-Operating_System
OS	I-Operating_System
,	O
were	O
eventually	O
abandoned	O
.	O
</s>
<s>
Although	O
Sun	O
was	O
also	O
interested	O
in	O
supporting	O
multiple	O
operating	B-General_Concept
systems	I-General_Concept
,	O
their	O
needs	O
were	O
nowhere	O
as	O
pressing	O
as	O
IBM	O
or	O
Apple	O
.	O
</s>
<s>
By	O
this	O
point	O
in	O
time	O
they	O
had	O
already	O
moved	O
platforms	O
from	O
their	O
early	O
68k-based	O
machines	O
to	O
their	O
SPARC-based	O
lineup	O
,	O
and	O
their	O
UNIX	B-Application
System	I-Application
V-based	O
Solaris	B-Application
operating	I-Application
system	I-Application
was	O
taking	O
over	O
from	O
their	O
BSD-based	O
SunOS	O
.	O
</s>
<s>
Sun	O
's	O
concerns	O
were	O
somewhat	O
more	O
subtle	O
:	O
keeping	O
developers	O
interested	O
in	O
Sun	O
's	O
version	O
of	O
Unix	B-Application
;	O
and	O
,	O
allowing	O
their	O
system	O
to	O
scale	O
downwards	O
onto	O
smaller	O
devices	O
such	O
as	O
set-top	O
boxes	O
.	O
</s>
<s>
A	O
microkernel-based	O
system	O
would	O
be	O
particularly	O
useful	O
in	O
this	O
latter	O
role	O
.	O
</s>
<s>
The	O
primary	O
addition	O
in	O
this	O
respect	O
was	O
the	O
development	O
of	O
a	O
rich	O
interface	B-General_Concept
definition	I-General_Concept
language	I-General_Concept
(	O
IDL	O
)	O
,	O
which	O
exported	O
interfaces	O
with	O
considerably	O
more	O
information	O
than	O
the	O
one	O
used	O
in	O
Mach	B-Operating_System
.	O
</s>
<s>
Given	O
a	O
proper	O
language	O
,	O
programs	O
,	O
including	O
operating	B-General_Concept
system	I-General_Concept
servers	O
,	O
could	O
import	O
multiple	O
interfaces	O
and	O
combine	O
them	O
as	O
if	O
they	O
were	O
objects	O
native	O
to	O
that	O
language	O
notably	O
C++	B-Language
.	O
</s>
<s>
Some	O
time	O
later	O
the	O
Spring	O
IDL	O
was	O
adopted	O
with	O
minor	O
changes	O
as	O
the	O
CORBA	B-Architecture
IDL	I-Architecture
.	O
</s>
<s>
Spring	O
also	O
explored	O
a	O
number	O
of	O
specific	O
software	O
advances	O
in	O
file	B-Application
systems	I-Application
,	O
virtual	B-Architecture
memory	I-Architecture
and	O
IPC	O
performance	O
.	O
</s>
<s>
The	O
result	O
was	O
a	O
single	O
Unix-like	B-Operating_System
system	I-Operating_System
with	O
much	O
better	O
performance	O
than	O
Mach	B-Operating_System
.	O
</s>
<s>
For	O
instance	O
,	O
Mach	B-Operating_System
tasks	O
are	O
referred	O
to	O
as	O
domains	O
,	O
ports	O
as	O
doors	B-General_Concept
,	O
and	O
the	O
kernel	O
as	O
the	O
nucleus	O
.	O
</s>
<s>
The	O
Spring	O
kernel	O
was	O
divided	O
into	O
two	O
parts	O
:	O
a	O
virtual	B-Architecture
memory	I-Architecture
system	O
and	O
the	O
nucleus	O
.	O
</s>
<s>
Although	O
the	O
nucleus	O
is	O
equivalent	O
to	O
only	O
one	O
portion	O
of	O
the	O
Mach	B-Operating_System
kernel	I-Operating_System
,	O
the	O
kernels	O
of	O
each	O
OS	O
are	O
analogous	O
enough	O
to	O
be	O
considered	O
to	O
perform	O
the	O
same	O
function	O
.	O
</s>
<s>
Primarily	O
this	O
includes	O
state	O
to	O
maintain	O
lists	O
of	O
running	O
programs	O
(	O
domains	O
)	O
and	O
their	O
threads	O
,	O
as	O
well	O
as	O
the	O
communications	O
links	O
between	O
them	O
(	O
doors	B-General_Concept
)	O
.	O
</s>
<s>
Normally	O
this	O
would	O
preclude	O
it	O
from	O
use	O
in	O
realtime	B-General_Concept
settings	O
,	O
but	O
it	O
is	O
not	O
clear	O
that	O
is	O
the	O
case	O
.	O
</s>
<s>
Normally	O
kernels	O
need	O
to	O
be	O
threaded	O
in	O
order	O
to	O
ensure	O
a	O
long-running	O
task	O
such	O
as	O
disk	O
I/O	B-General_Concept
wo	O
n't	O
tie	O
up	O
the	O
system	O
and	O
prevent	O
a	O
subsequent	O
call	O
from	O
being	O
serviced	O
in	O
time	O
;	O
under	O
Spring	O
the	O
kernel	O
almost	O
immediately	O
hands	O
off	O
the	O
vast	O
majority	O
of	O
requests	O
to	O
the	O
servers	O
,	O
so	O
under	O
this	O
model	O
it	O
is	O
only	O
the	O
servers	O
which	O
,	O
in	O
theory	O
,	O
need	O
to	O
be	O
threaded	O
.	O
</s>
<s>
One	O
major	O
difference	O
between	O
Mach	B-Operating_System
and	O
Spring	O
was	O
the	O
IPC	O
system	O
.	O
</s>
<s>
In	O
Mach	B-Operating_System
,	O
the	O
system	O
was	O
arranged	O
as	O
a	O
set	O
of	O
one-way	O
asynchronous	O
pipes	B-Operating_System
(	O
ports	O
)	O
between	O
programs	O
,	O
a	O
concept	O
derived	O
from	O
Unix	B-Operating_System
pipes	I-Operating_System
.	O
</s>
<s>
In	O
programming	O
,	O
however	O
,	O
the	O
most	O
common	O
method	O
of	O
communications	O
is	O
the	O
procedure	O
call	O
,	O
or	O
call/return	O
,	O
which	O
Mach	B-Operating_System
did	O
not	O
support	O
directly	O
.	O
</s>
<s>
This	O
resulted	O
in	O
a	O
change	O
of	O
terminology	O
from	O
ports	O
in	O
Mach	B-Operating_System
,	O
to	O
doors	B-General_Concept
in	O
Spring	O
.	O
</s>
<s>
Doors	B-General_Concept
were	O
known	O
to	O
the	O
kernel	O
only	O
;	O
programs	O
were	O
handed	O
a	O
"	O
handle	O
"	O
to	O
the	O
door	O
with	O
an	O
identifier	O
which	O
was	O
unique	O
to	O
that	O
program	O
.	O
</s>
<s>
Additionally	O
,	O
the	O
Mach	B-Operating_System
model	O
was	O
asynchronous	O
the	O
call	O
would	O
return	O
if	O
and	O
when	O
the	O
server	O
had	O
data	O
.	O
</s>
<s>
This	O
followed	O
the	O
original	O
Unix	B-Application
model	O
of	O
pipes	B-Operating_System
,	O
which	O
allowed	O
other	O
programs	O
to	O
run	O
if	O
the	O
server	O
was	O
busy	O
.	O
</s>
<s>
Under	O
Mach	B-Operating_System
,	O
the	O
virtual	B-Architecture
memory	I-Architecture
system	O
,	O
supported	O
by	O
the	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
(	O
MMU	O
)	O
,	O
was	O
expected	O
to	O
provide	O
a	O
lightweight	O
solution	O
to	O
copying	O
data	O
,	O
by	O
simply	O
mapping	O
the	O
same	O
data	O
in	O
memory	O
into	O
the	O
two	O
programs	O
.	O
</s>
<s>
Unlike	O
Mach	B-Operating_System
's	O
one-size-fits-all	O
solution	O
to	O
IPC	O
,	O
Spring	O
used	O
a	O
variety	O
of	O
methods	O
to	O
physically	O
pass	O
data	O
between	O
programs	O
.	O
</s>
<s>
One	O
of	O
these	O
,	O
the	O
bulk-path	O
,	O
was	O
basically	O
identical	O
to	O
Mach	B-Operating_System
's	O
ports	O
and	O
messages	O
,	O
but	O
in	O
practice	O
the	O
bulk-path	O
was	O
the	O
least	O
common	O
message	O
type	O
.	O
</s>
<s>
The	O
fast-path	O
allowed	O
for	O
extremely	O
fast	O
invocations	O
at	O
least	O
when	O
running	O
on	O
SPARC-based	O
platforms	O
.	O
</s>
<s>
The	O
fast-path	O
used	O
a	O
unique	O
"	O
half-trap	O
"	O
to	O
avoid	O
much	O
of	O
the	O
context	B-Operating_System
switching	I-Operating_System
overhead	O
which	O
plagued	O
Mach	B-Operating_System
systems	O
.	O
</s>
<s>
Instead	O
of	O
saving	O
out	O
all	O
of	O
the	O
processor	O
state	O
—	O
the	O
normal	O
procedure	O
in	O
the	O
case	O
of	O
a	O
trap	O
into	O
the	O
kernel	O
—	O
Spring	O
only	O
saved	O
out	O
the	O
top	O
16	O
SPARC	B-Architecture
registers	O
,	O
a	O
number	O
which	O
was	O
defined	O
by	O
specific	O
implementation	O
details	O
of	O
the	O
SPARC	B-Architecture
architecture	O
.	O
</s>
<s>
The	O
other	O
portions	O
of	O
the	O
register	O
stack	O
were	O
rendered	O
invisible	O
to	O
the	O
receiver	O
using	O
the	O
SPARC	B-Architecture
's	O
WIM	O
instruction	O
,	O
providing	O
some	O
level	O
of	O
security	O
.	O
</s>
<s>
The	O
fast-path	O
strongly	O
resembles	O
a	O
classic	O
procedure	O
call	O
within	O
a	O
single	O
application	O
,	O
which	O
uses	O
register	B-General_Concept
windows	I-General_Concept
on	O
the	O
SPARC	B-Architecture
,	O
adding	O
some	O
MMU	O
work	O
to	O
move	O
the	O
context	O
from	O
one	O
program	O
to	O
another	O
.	O
</s>
<s>
On	O
32-bit	O
SPARC	B-Architecture
V8	I-Architecture
systems	O
,	O
a	O
complete	O
round-trip	O
call	O
using	O
the	O
fast-path	O
took	O
just	O
over	O
100	O
instructions	O
,	O
making	O
it	O
many	O
times	O
faster	O
than	O
a	O
typical	O
Mach	B-Operating_System
call	O
.	O
</s>
<s>
It	O
remains	O
unclear	O
whether	O
or	O
not	O
the	O
fast-path	O
could	O
be	O
implemented	O
on	O
other	O
machines	O
,	O
so	O
the	O
overall	O
performance	O
improvement	O
of	O
Spring	O
is	O
difficult	O
to	O
compare	O
with	O
Mach	B-Operating_System
,	O
which	O
was	O
typically	O
measured	O
on	O
IA-32	B-Device
systems	O
.	O
</s>
<s>
Specifically	O
,	O
a	O
full	O
syscall	O
took	O
under	O
20µs	O
on	O
a	O
486DX-50	O
for	O
existing	O
BSD	B-Operating_System
Unix	I-Operating_System
systems	O
,	O
and	O
114µs	O
under	O
Mach	B-Operating_System
.	O
</s>
<s>
This	O
led	O
to	O
a	O
performance	O
hit	O
of	O
50%	O
or	O
more	O
,	O
and	O
doomed	O
most	O
Mach	B-Operating_System
projects	O
.	O
</s>
<s>
In	O
contrast	O
,	O
Spring	O
using	O
the	O
fast-path	O
boasted	O
an	O
IPC	O
time	O
of	O
only	O
11µs	O
on	O
a	O
SPARCstation	B-Architecture
2	I-Architecture
.	O
</s>
<s>
Another	O
key	O
area	O
of	O
improvement	O
in	O
Spring	O
was	O
the	O
implementation	O
of	O
the	O
virtual	B-Architecture
memory	I-Architecture
(	O
VM	O
)	O
system	O
,	O
also	O
part	O
of	O
the	O
kernel	O
.	O
</s>
<s>
Virtual	B-Architecture
memory	I-Architecture
is	O
a	O
system	O
which	O
ties	O
together	O
the	O
physical	O
random-access	B-Architecture
memory	I-Architecture
(	O
RAM	B-Architecture
)	O
in	O
a	O
machine	O
,	O
the	O
MMU	O
,	O
and	O
the	O
disk	O
system	O
to	O
create	O
the	O
illusion	O
that	O
every	O
program	O
on	O
the	O
system	O
has	O
its	O
own	O
block	O
of	O
RAM	B-Architecture
equal	O
to	O
the	O
maximum	O
the	O
machine	O
and	O
operating	B-General_Concept
system	I-General_Concept
can	O
support	O
.	O
</s>
<s>
The	O
most	O
prevalent	O
memory	O
addressing	O
model	O
in	O
computers	O
and	O
operating	B-General_Concept
systems	I-General_Concept
in	O
use	O
in	O
the	O
1980s	O
and	O
1990s	O
,	O
was	O
32-bit	O
,	O
providing	O
access	O
to	O
a	O
theoretical	O
limit	O
of	O
4GiB	O
of	O
memory	O
,	O
but	O
until	O
the	O
early	O
2000s	O
,	O
only	O
relatively	O
expensive	O
computers	O
would	O
have	O
that	O
much	O
physical	O
RAM	B-Architecture
.	O
</s>
<s>
The	O
VM	O
system	O
creates	O
the	O
illusion	O
of	O
more	O
by	O
using	O
the	O
hard	B-Device
disk	I-Device
as	O
a	O
backing	B-General_Concept
store	I-General_Concept
,	O
an	O
area	O
of	O
much	O
slower	O
memory	O
used	O
to	O
offload	O
inactive	O
portions	O
of	O
RAM	B-Architecture
.	O
</s>
<s>
In	O
traditional	B-Application
Unix	I-Application
systems	O
VM	O
is	O
a	O
part	O
of	O
the	O
kernel	O
,	O
as	O
are	O
the	O
disk	O
and	O
memory	O
handlers	O
it	O
ties	O
together	O
.	O
</s>
<s>
Under	O
Mach	B-Operating_System
the	O
decision	O
of	O
where	O
to	O
place	O
the	O
VM	O
system	O
is	O
not	O
so	O
obvious	O
—	O
although	O
the	O
kernel	O
is	O
in	O
control	O
of	O
RAM	B-Architecture
and	O
the	O
MMU	O
,	O
the	O
disk	O
handlers	O
are	O
part	O
of	O
external	O
client	O
programs	O
.	O
</s>
<s>
To	O
solve	O
this	O
problem	O
Mach	B-Operating_System
3	O
introduced	O
a	O
new	O
two-layer	O
VM	O
system	O
,	O
with	O
control	O
of	O
the	O
actual	O
VM	O
system	O
in	O
the	O
kernel	O
,	O
which	O
would	O
then	O
ask	O
an	O
external	O
client-space	O
pager	O
to	O
interact	O
with	O
the	O
disk	O
system	O
to	O
physically	O
copy	O
memory	O
around	O
.	O
</s>
<s>
Unfortunately	O
this	O
proved	O
to	O
be	O
a	O
serious	O
performance	O
issue	O
,	O
requiring	O
several	O
trips	O
in	O
and	O
out	O
of	O
the	O
kernel	O
(	O
with	O
resulting	O
context	B-Operating_System
switches	I-Operating_System
along	O
with	O
it	O
)	O
as	O
the	O
various	O
layers	O
of	O
the	O
VM	O
system	O
called	O
each	O
other	O
.	O
</s>
<s>
The	O
Spring	O
team	O
had	O
the	O
advantage	O
of	O
being	O
able	O
to	O
examine	O
what	O
went	O
wrong	O
with	O
the	O
Mach	B-Operating_System
model	O
and	O
fix	O
it	O
.	O
</s>
<s>
The	O
result	O
was	O
a	O
much	O
more	O
cleanly	O
separated	O
system	O
of	O
address	O
spaces	O
in	O
programs	O
,	O
mapped	O
by	O
the	O
VM	O
into	O
various	O
memory	O
objects	O
,	O
which	O
were	O
in	O
turn	O
managed	O
by	O
a	O
pager	O
for	O
backing	B-General_Concept
store	I-General_Concept
handling	O
.	O
</s>
<s>
In	O
exchange	O
the	O
pager	O
was	O
passed	O
a	O
cache	B-General_Concept
manager	O
from	O
the	O
VM	O
,	O
which	O
was	O
responsible	O
for	O
keeping	O
track	O
of	O
clean/dirty	O
status	O
of	O
the	O
local	O
cache	B-General_Concept
of	O
that	O
memory	O
object	O
.	O
</s>
<s>
In	O
the	O
end	O
the	O
basic	O
system	O
had	O
pagers	O
which	O
were	O
in	O
charge	O
of	O
the	O
memory	O
,	O
and	O
address	O
spaces	O
which	O
were	O
in	O
charge	O
of	O
the	O
caches	B-General_Concept
.	O
</s>
<s>
Since	O
programs	O
could	O
share	O
the	O
memory	O
objects	O
,	O
and	O
microkernel	B-Operating_System
systems	O
like	O
Spring	O
are	O
based	O
on	O
the	O
idea	O
of	O
copying	O
memory	O
around	O
,	O
Spring	O
allowed	O
programs	O
sharing	O
memory	O
in	O
this	O
fashion	O
to	O
share	O
it	O
in	O
the	O
VM	O
system	O
as	O
well	O
.	O
</s>
<s>
Thus	O
under	O
Mach	B-Operating_System
if	O
a	O
network	O
file	B-Application
server	I-Application
is	O
handing	O
data	O
to	O
a	O
program	O
both	O
programs	O
will	O
end	O
up	O
using	O
up	O
memory	O
in	O
the	O
VM	O
system	O
,	O
whereas	O
under	O
Spring	O
the	O
two	O
would	O
naturally	O
share	O
the	O
same	O
memory	O
objects	O
,	O
as	O
the	O
pager	O
implementing	O
that	O
memory	O
object	O
would	O
simply	O
return	O
another	O
handle	O
to	O
the	O
same	O
memory	O
.	O
</s>
<s>
Only	O
inside	O
the	O
VM	O
would	O
they	O
be	O
considered	O
different	O
objects	O
,	O
and	O
would	O
be	O
handled	O
by	O
separate	O
cache	B-General_Concept
managers	O
.	O
</s>
<s>
Therefore	O
,	O
the	O
data	O
would	O
only	O
be	O
cached	O
in	O
RAM	B-Architecture
once	O
.	O
</s>
<s>
In	O
theory	O
this	O
could	O
lead	O
to	O
considerably	O
better	O
real-world	O
RAM	B-Architecture
usage	O
.	O
</s>
<s>
Additionally	O
,	O
the	O
use	O
of	O
external	O
pagers	O
with	O
a	O
well	O
defined	O
API	B-General_Concept
allowed	O
the	O
system	O
to	O
be	O
cleanly	O
separated	O
when	O
this	O
was	O
needed	O
.	O
</s>
<s>
For	O
applications	O
like	O
file	B-Application
servers	I-Application
,	O
web	B-Application
servers	I-Application
and	O
database	B-Application
management	I-Application
systems	I-Application
,	O
custom	O
VMs	O
and	O
file	B-Application
systems	I-Application
often	O
lead	O
to	O
dramatically	O
improved	O
performance	O
.	O
</s>
<s>
Most	O
operating	B-General_Concept
systems	I-General_Concept
include	O
a	O
variety	O
of	O
naming	O
services	O
.	O
</s>
<s>
The	O
most	O
basic	O
example	O
is	O
a	O
file	B-Application
system	I-Application
,	O
in	O
which	O
the	O
files	O
are	O
internally	O
referred	O
to	O
by	O
a	O
"	O
handle	O
"	O
,	O
a	O
small	O
number	O
,	O
while	O
a	O
separate	O
directory	O
gives	O
the	O
files	O
names	O
with	O
which	O
the	O
users	O
interact	O
.	O
</s>
<s>
The	O
same	O
kind	O
of	O
name/identifier	O
dichotomy	O
occurs	O
many	O
other	O
parts	O
of	O
the	O
typical	O
Unix	B-Application
system	I-Application
;	O
printers	O
are	O
named	O
in	O
the	O
etc/printcap	O
file	O
,	O
small	O
numbers	O
and	O
strings	O
in	O
the	O
environment	O
variables	O
,	O
and	O
network	O
locations	O
in	O
DNS	O
.	O
</s>
<s>
Each	O
of	O
these	O
systems	O
provided	O
its	O
own	O
names	O
,	O
with	O
a	O
custom	O
API	B-General_Concept
,	O
making	O
the	O
different	O
objects	O
appear	O
completely	O
different	O
even	O
in	O
concept	O
.	O
</s>
<s>
Other	O
systems	O
had	O
attempted	O
to	O
add	O
naming	O
systems	O
to	O
existing	O
Unix	B-Application
systems	I-Application
,	O
but	O
generally	O
these	O
were	O
"	O
covers	O
"	O
over	O
the	O
existing	O
functionality	O
which	O
simply	O
collected	O
up	O
all	O
the	O
names	O
from	O
these	O
various	O
services	O
and	O
presented	O
them	O
in	O
one	O
collection	O
.	O
</s>
<s>
Only	O
in	O
a	O
completely	O
new	O
operating	B-General_Concept
system	I-General_Concept
could	O
one	O
hope	O
to	O
provide	O
a	O
universal	O
service	O
.	O
</s>
<s>
For	O
instance	O
,	O
Plan	B-Operating_System
9	I-Operating_System
used	O
the	O
file	B-Application
system	I-Application
as	O
a	O
universal	O
naming	O
service	O
;	O
everything	O
from	O
printers	O
to	O
windows	O
could	O
be	O
accessed	O
by	O
name	O
through	O
the	O
file	B-Application
system	I-Application
.	O
</s>
<s>
This	O
is	O
an	O
extension	O
of	O
the	O
original	O
Unix	B-Application
concept	O
,	O
one	O
which	O
had	O
slowly	O
disappeared	O
as	O
more	O
and	O
more	O
functionality	O
had	O
been	O
added	O
over	O
the	O
years	O
.	O
</s>
<s>
Mach	B-Operating_System
did	O
not	O
have	O
a	O
naming	O
service	O
of	O
any	O
sort	O
for	O
its	O
ports	O
.	O
</s>
<s>
Spring	O
's	O
approach	O
essentially	O
inverted	O
the	O
Plan	B-Operating_System
9	I-Operating_System
system	O
:	O
under	O
Spring	O
the	O
file	B-Application
system	I-Application
was	O
one	O
example	O
of	O
a	O
server	O
which	O
used	O
the	O
single	O
unified	O
name	O
service	O
.	O
</s>
<s>
Other	O
servers	O
would	O
then	O
"	O
bind	O
"	O
the	O
names	O
they	O
knew	O
into	O
the	O
system	O
,	O
the	O
printer	O
server	O
would	O
produce	O
a	O
list	O
of	O
printers	O
,	O
the	O
file	B-Application
system	I-Application
would	O
bind	O
in	O
the	O
directories	O
of	O
attached	O
disks	O
.	O
</s>
<s>
In	O
this	O
way	O
a	O
mapping	O
of	O
all	O
the	O
objects	O
on	O
the	O
system	O
was	O
built	O
up	O
,	O
potentially	O
at	O
runtime	O
,	O
and	O
could	O
be	O
accessed	O
in	O
a	O
file-like	O
fashion	O
very	O
similar	O
to	O
Plan	B-Operating_System
9	I-Operating_System
.	O
</s>
<s>
All	O
of	O
these	O
could	O
be	O
accessed	O
using	O
a	O
single	O
API	B-General_Concept
,	O
although	O
the	O
system	O
also	O
provided	O
a	O
variety	O
of	O
stub	O
libraries	O
to	O
make	O
it	O
appear	O
as	O
classical	O
services	O
as	O
well	O
,	O
notably	O
in	O
the	O
Unix	B-Application
emulation	O
server	O
.	O
</s>
<s>
Since	O
doors	B-General_Concept
,	O
the	O
real	O
accessors	O
in	O
Spring	O
,	O
were	O
handed	O
out	O
by	O
the	O
name	O
service	O
,	O
the	O
server	O
included	O
a	O
complete	O
access	O
control	O
list-based	O
permission	O
checking	O
system	O
.	O
</s>
<s>
So	O
in	O
addition	O
to	O
providing	O
permissions	O
on	O
the	O
file	B-Application
system	I-Application
,	O
under	O
Spring	O
any	O
object	O
could	O
be	O
controlled	O
using	O
the	O
same	O
set	O
of	O
permissions	O
and	O
user	O
interface	O
.	O
</s>
<s>
Contrast	O
this	O
with	O
Windows	B-Device
NT	I-Device
for	O
instance	O
,	O
which	O
includes	O
about	O
a	O
dozen	O
permissioning	O
systems	O
(	O
file	B-Application
system	I-Application
,	O
DCOM	O
,	O
SQL	O
access	O
,	O
IIS	O
,	O
etc	O
.	O
</s>
<s>
For	O
instance	O
,	O
if	O
a	O
user	O
asked	O
the	O
file	B-Application
server	I-Application
to	O
access	O
a	O
file	O
,	O
the	O
system	O
nameserver	O
would	O
pass	O
along	O
the	O
request	O
to	O
the	O
file	B-Application
system	I-Application
,	O
which	O
would	O
immediately	O
honor	O
it	O
.	O
</s>
<s>
Contexts	O
were	O
also	O
names	O
,	O
and	O
thus	O
similar	O
to	O
the	O
file	B-Application
system	I-Application
concept	O
of	O
a	O
directory	O
.	O
</s>
<s>
In	O
this	O
manner	O
Spring	O
allowed	O
file	O
directories	O
to	O
be	O
"	O
unioned	O
"	O
,	O
a	O
useful	O
feature	O
lacking	O
from	O
traditional	O
Unixen	B-Operating_System
.	O
</s>
<s>
Spring	O
did	O
not	O
include	O
a	O
built-in	O
object	B-Application
persistence	I-Application
system	O
,	O
but	O
the	O
name	O
service	O
was	O
persistent	O
and	O
could	O
be	O
used	O
to	O
find	O
objects	O
in	O
this	O
sort	O
of	O
manner	O
.	O
</s>
<s>
In	O
fact	O
the	O
separation	O
of	O
name	O
spaces	O
would	O
allow	O
this	O
to	O
be	O
separated	O
out	O
to	O
the	O
service	O
which	O
actually	O
implemented	O
the	O
naming	O
of	O
doors	B-General_Concept
,	O
making	O
implementation	O
considerably	O
easier	O
.	O
</s>
<s>
These	O
two	O
concepts	O
were	O
combined	O
to	O
produce	O
the	O
Spring	O
file	B-Application
system	I-Application
.	O
</s>
<s>
Key	O
to	O
the	O
Spring	O
file	B-Application
system	I-Application
's	O
operation	O
was	O
tight	O
integration	O
with	O
the	O
VM	O
system	O
.	O
</s>
<s>
Since	O
it	O
was	O
"	O
known	O
"	O
that	O
the	O
VM	O
system	O
would	O
be	O
managing	O
the	O
local	O
cache	B-General_Concept
of	O
the	O
data	O
from	O
the	O
file	B-Application
system	I-Application
,	O
the	O
file	B-Application
system	I-Application
was	O
reduced	O
to	O
a	O
command	O
structure	O
only	O
,	O
and	O
was	O
its	O
own	O
pager	O
.	O
</s>
<s>
That	O
is	O
,	O
the	O
file	B-Application
system	I-Application
was	O
responsible	O
for	O
loading	O
and	O
saving	O
data	O
from	O
memory	O
objects	O
when	O
needed	O
,	O
but	O
caching	B-General_Concept
of	O
that	O
data	O
would	O
be	O
handled	O
for	O
it	O
by	O
the	O
VM	O
.	O
</s>
<s>
As	O
mentioned	O
before	O
,	O
this	O
means	O
that	O
under	O
Spring	O
a	O
file	O
only	O
exists	O
in	O
RAM	B-Architecture
in	O
one	O
place	O
,	O
no	O
matter	O
how	O
it	O
is	O
being	O
shared	O
by	O
the	O
programs	O
in	O
the	O
system	O
.	O
</s>
<s>
Spring	O
used	O
two	O
sorts	O
of	O
file	B-Application
systems	I-Application
,	O
a	O
local	O
file	B-Application
system	I-Application
which	O
was	O
similar	O
to	O
most	O
common	O
Unix	B-Application
systems	I-Application
,	O
as	O
well	O
as	O
a	O
caching	B-General_Concept
file	B-Application
system	I-Application
for	O
network	O
devices	O
.	O
</s>
<s>
The	O
caching	B-General_Concept
system	O
demonstrates	O
the	O
utility	O
of	O
Spring	O
's	O
VM/pager	O
split	O
,	O
using	O
the	O
same	O
physical	O
memory	O
from	O
the	O
VM	O
which	O
it	O
would	O
have	O
to	O
use	O
normally	O
,	O
the	O
CFS	O
short-circuited	O
all	O
read	O
requests	O
to	O
the	O
local	O
cache	B-General_Concept
,	O
and	O
did	O
lazy	O
write-backs	O
every	O
30	O
seconds	O
to	O
the	O
source	O
file	B-Application
system	I-Application
.	O
</s>
<s>
This	O
would	O
be	O
particularly	O
notable	O
if	O
common	O
Unix	B-Application
directories	O
were	O
being	O
loaded	O
over	O
the	O
network	O
,	O
the	O
normal	O
setup	O
for	O
labs	O
of	O
workstations	B-Device
.	O
</s>
<s>
Most	O
Unix	B-Application
systems	I-Application
use	O
similar	O
caching	B-General_Concept
mechanisms	O
for	O
the	O
same	O
performance	O
reasons	O
,	O
but	O
would	O
end	O
up	O
using	O
RAM	B-Architecture
twice	O
,	O
once	O
in	O
the	O
cache	B-General_Concept
,	O
and	O
again	O
in	O
the	O
programs	O
using	O
it	O
.	O
</s>
<s>
The	O
Spring	O
file	B-Application
system	I-Application
is	O
also	O
a	O
name	O
service	O
context	O
provider	O
,	O
lazily	O
mapping	O
directories	O
from	O
the	O
on-disk	O
structure	O
into	O
new	O
contexts	O
in	O
the	O
name	O
service	O
.	O
</s>
<s>
These	O
could	O
then	O
be	O
accessed	O
using	O
the	O
universal	O
naming	O
API	B-General_Concept
,	O
or	O
alternately	O
via	O
a	O
Unix	B-Application
emulation	O
library	O
which	O
presented	O
them	O
as	O
a	O
traditional	B-Application
unix	I-Application
file	B-Application
system	I-Application
.	O
</s>
<s>
Note	O
that	O
Spring	O
's	O
use	O
of	O
the	O
term	O
file	B-Application
system	I-Application
is	O
somewhat	O
confusing	O
.	O
</s>
<s>
Spring	O
also	O
needed	O
to	O
support	O
existing	O
Unix	B-Application
applications	O
,	O
the	O
basis	O
of	O
Sun	O
's	O
business	O
.	O
</s>
<s>
To	O
do	O
this	O
,	O
Spring	O
also	O
shipped	O
with	O
two	O
key	O
extensions	O
:	O
a	O
Unix	B-Application
process	O
server	O
which	O
mimicked	O
a	O
full	O
Unix	B-Application
,	O
and	O
a	O
re-write	O
of	O
the	O
standard	O
libc	O
library	O
called	O
libue	O
which	O
redirected	O
Unix	B-Application
kernel	O
requests	O
to	O
various	O
servers	O
.	O
</s>
<s>
For	O
instance	O
,	O
a	O
Unix	B-Application
application	O
which	O
required	O
file	O
or	O
network	O
services	O
would	O
be	O
directed	O
to	O
the	O
associated	O
Spring	O
server	O
,	O
while	O
one	O
which	O
wanted	O
to	O
list	O
the	O
currently	O
running	O
programs	O
would	O
be	O
directed	O
to	O
the	O
Unix	B-Application
process	O
server	O
.	O
</s>
<s>
Running	O
a	O
Unix	B-Application
application	O
under	O
Spring	O
required	O
that	O
it	O
be	O
re-linked	O
against	O
libue	O
;	O
the	O
system	O
shipped	O
with	O
the	O
majority	O
of	O
basic	O
Unix	B-Application
utilities	O
and	O
an	O
X11	O
server	O
relinked	O
and	O
ready	O
to	O
use	O
.	O
</s>
<s>
Sun	O
has	O
added	O
a	O
"	O
Unixified	O
"	O
version	O
of	O
Doors	B-General_Concept
to	O
Solaris	B-Application
.	O
</s>
<s>
In	O
the	O
years	O
since	O
the	O
Spring	O
system	O
work	O
ended	O
,	O
work	O
on	O
operating	B-General_Concept
systems	I-General_Concept
in	O
general	O
has	O
essentially	O
ended	O
.	O
</s>
<s>
With	O
the	O
market	O
quickly	O
stratifying	O
into	O
a	O
world	O
dominated	O
by	O
Windows	O
and	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
there	O
appear	O
to	O
be	O
only	O
niche	O
markets	O
open	O
for	O
any	O
other	O
system	O
.	O
</s>
<s>
Additionally	O
,	O
the	O
poor	O
performance	O
of	O
Mach	B-Operating_System
3	O
seems	O
to	O
have	O
taken	O
the	O
wind	O
out	O
of	O
the	O
sails	O
of	O
many	O
projects	O
.	O
</s>
<s>
One	O
in	O
particular	O
,	O
the	O
L4	B-Operating_System
microkernel	I-Operating_System
,	O
shares	O
many	O
features	O
with	O
Spring	O
's	O
kernel	O
.	O
</s>
<s>
L4	B-Operating_System
has	O
,	O
so	O
far	O
,	O
concentrated	O
almost	O
solely	O
on	O
the	O
kernel	O
itself	O
;	O
there	O
is	O
nothing	O
analogous	O
to	O
Spring	O
's	O
naming	O
service	O
,	O
security	O
model	O
,	O
or	O
file	B-Application
system	I-Application
.	O
</s>
