<s>
The	O
MESI	B-General_Concept
protocol	I-General_Concept
is	O
an	O
Invalidate-based	O
cache	B-General_Concept
coherence	I-General_Concept
protocol	I-General_Concept
,	O
and	O
is	O
one	O
of	O
the	O
most	O
common	O
protocols	O
that	O
support	O
write-back	O
caches	O
.	O
</s>
<s>
It	O
is	O
also	O
known	O
as	O
the	O
Illinois	B-General_Concept
protocol	I-General_Concept
(	O
due	O
to	O
its	O
development	O
at	O
the	O
University	O
of	O
Illinois	O
at	O
Urbana-Champaign	O
)	O
.	O
</s>
<s>
The	O
Illinois	B-General_Concept
Protocol	I-General_Concept
requires	O
a	O
cache	O
to	O
cache	O
transfer	O
on	O
a	O
miss	O
if	O
the	O
block	O
resides	O
in	O
another	O
cache	O
.	O
</s>
<s>
This	O
protocol	O
reduces	O
the	O
number	O
of	O
main	O
memory	O
transactions	O
with	O
respect	O
to	O
the	O
MSI	B-General_Concept
protocol	I-General_Concept
.	O
</s>
<s>
The	O
letters	O
in	O
the	O
acronym	O
MESI	B-General_Concept
represent	O
four	O
exclusive	O
states	O
that	O
a	O
cache	O
line	O
can	O
be	O
marked	O
with	O
(	O
encoded	O
using	O
two	O
additional	O
bits	O
)	O
:	O
</s>
<s>
The	O
MESI	B-General_Concept
protocol	I-General_Concept
is	O
defined	O
by	O
a	O
finite-state	B-Architecture
machine	I-Architecture
that	O
transitions	O
from	O
one	O
state	O
to	O
another	O
based	O
on	O
2	O
stimuli	O
.	O
</s>
<s>
The	O
bus	O
requests	O
are	O
monitored	O
with	O
the	O
help	O
of	O
Snoopers	B-General_Concept
,	O
which	O
monitor	O
all	O
the	O
bus	O
transactions	O
.	O
</s>
<s>
(	O
Such	O
Cache	O
to	O
Cache	O
transfers	O
can	O
reduce	O
the	O
read	O
miss	O
latency	B-Architecture
if	O
the	O
latency	B-Architecture
to	O
bring	O
the	O
block	O
from	O
the	O
main	O
memory	O
is	O
more	O
than	O
from	O
Cache	O
to	O
Cache	O
transfers	O
,	O
which	O
is	O
generally	O
the	O
case	O
in	O
bus	O
based	O
systems	O
.	O
</s>
<s>
(	O
Refer	O
image	O
above	O
for	O
MESI	B-General_Concept
state	O
diagram	O
)	O
.	O
</s>
<s>
The	O
bus	O
has	O
snoopers	B-General_Concept
on	O
both	O
sides	O
:	O
</s>
<s>
Each	O
Cache	O
block	O
has	O
its	O
own	O
4	O
state	O
finite-state	B-Architecture
machine	I-Architecture
(	O
refer	O
image	O
1.1	O
)	O
.	O
</s>
<s>
A	O
cache	O
that	O
holds	O
a	O
line	O
in	O
the	O
Modified	O
state	O
must	O
snoop	B-General_Concept
(	O
intercept	O
)	O
all	O
attempted	O
reads	O
(	O
from	O
all	O
the	O
other	O
caches	O
in	O
the	O
system	O
)	O
of	O
the	O
corresponding	O
main	O
memory	O
location	O
and	O
insert	O
the	O
data	O
that	O
it	O
holds	O
.	O
</s>
<s>
Note	O
:	O
The	O
term	O
snooping	O
referred	O
to	O
below	O
is	O
a	O
protocol	O
for	O
maintaining	O
cache	B-General_Concept
coherency	I-General_Concept
in	O
symmetric	O
multiprocessing	O
environments	O
.	O
</s>
<s>
All	O
the	O
caches	O
on	O
the	O
bus	O
monitor	O
(	O
snoop	B-General_Concept
)	O
the	O
bus	O
if	O
they	O
have	O
a	O
copy	O
of	O
the	O
block	O
of	O
data	O
that	O
is	O
requested	O
on	O
the	O
bus	O
.	O
</s>
<s>
A	O
Read	O
For	O
Ownership	O
(	O
RFO	O
)	O
is	O
an	O
operation	O
in	O
cache	B-General_Concept
coherency	I-General_Concept
protocols	O
that	O
combines	O
a	O
read	O
and	O
an	O
invalidate	O
broadcast	O
.	O
</s>
<s>
The	O
operation	O
is	O
issued	O
by	O
a	O
processor	O
trying	O
to	O
write	O
into	O
a	O
cache	O
line	O
that	O
is	O
in	O
the	O
shared	O
(	O
S	O
)	O
or	O
invalid	O
(	O
I	O
)	O
states	O
of	O
the	O
MESI	B-General_Concept
protocol	I-General_Concept
.	O
</s>
<s>
MESI	B-General_Concept
in	O
its	O
naive	O
,	O
straightforward	O
implementation	O
exhibits	O
two	O
particular	O
performance	O
issues	O
.	O
</s>
<s>
The	O
most	O
striking	O
difference	O
between	O
MESI	B-General_Concept
and	O
MSI	B-General_Concept
is	O
the	O
extra	O
"	O
exclusive	O
"	O
state	O
present	O
in	O
the	O
MESI	B-General_Concept
protocol	I-General_Concept
.	O
</s>
<s>
When	O
a	O
processor	O
needs	O
to	O
read	O
a	O
block	O
that	O
none	O
of	O
the	O
other	O
processors	O
have	O
and	O
then	O
write	O
to	O
it	O
,	O
two	O
bus	O
transactions	O
will	O
take	O
place	O
in	O
the	O
case	O
of	O
MSI	B-General_Concept
.	O
</s>
<s>
Thus	O
,	O
MESI	B-General_Concept
protocol	I-General_Concept
overcomes	O
this	O
limitation	O
by	O
adding	O
an	O
Exclusive	O
state	O
,	O
which	O
results	O
in	O
saving	O
a	O
bus	O
request	O
.	O
</s>
<s>
MSI	B-General_Concept
performs	O
much	O
worse	O
in	O
this	O
case	O
due	O
to	O
the	O
extra	O
bus	O
messages	O
.	O
</s>
<s>
Even	O
in	O
the	O
case	O
of	O
a	O
highly	O
parallel	O
application	O
with	O
minimal	O
sharing	O
of	O
data	O
,	O
MESI	B-General_Concept
is	O
far	O
faster	O
.	O
</s>
<s>
But	O
this	O
is	O
not	O
a	O
requirement	O
and	O
is	O
just	O
an	O
additional	O
overhead	O
caused	O
by	O
using	O
MESI	B-General_Concept
.	O
</s>
<s>
This	O
challenge	O
was	O
overcome	O
by	O
the	O
MOESI	B-General_Concept
protocol	I-General_Concept
.	O
</s>
<s>
In	O
case	O
of	O
S	O
(	O
Shared	O
State	O
)	O
,	O
multiple	O
snoopers	B-General_Concept
may	O
response	O
with	O
FlushOpt	O
with	O
the	O
same	O
data	O
(	O
see	O
the	O
example	O
above	O
)	O
.	O
</s>
<s>
The	O
F	O
state	O
in	O
MESIF	B-General_Concept
addresses	O
this	O
redundancy	O
.	O
</s>
