<s>
A	O
victim	B-General_Concept
cache	I-General_Concept
is	O
a	O
small	O
,	O
usually	O
fully	O
associative	O
cache	B-General_Concept
placed	O
in	O
the	O
refill	O
path	O
of	O
a	O
CPU	B-General_Concept
cache	I-General_Concept
that	O
stores	O
all	O
the	O
blocks	O
evicted	O
from	O
that	O
level	O
of	O
cache	B-General_Concept
,	O
originally	O
proposed	O
in	O
1990	O
.	O
</s>
<s>
In	O
modern	O
architectures	O
,	O
this	O
function	O
is	O
typically	O
performed	O
by	O
Level	O
3	O
or	O
Level	O
4	O
caches	B-General_Concept
.	O
</s>
<s>
Victim	O
caching	B-General_Concept
is	O
a	O
hardware	O
technique	O
to	O
improve	O
performance	O
of	O
caches	B-General_Concept
proposed	O
by	O
Norman	O
Jouppi	O
.	O
</s>
<s>
A	O
victim	B-General_Concept
cache	I-General_Concept
is	O
a	O
hardware	O
cache	B-General_Concept
designed	O
to	O
decrease	O
conflict	O
misses	O
and	O
improve	O
hit	O
latency	O
for	O
direct-mapped	O
caches	B-General_Concept
.	O
</s>
<s>
It	O
is	O
employed	O
at	O
the	O
refill	O
path	O
of	O
a	O
Level	O
1	O
cache	B-General_Concept
,	O
such	O
that	O
any	O
cache-line	O
which	O
gets	O
evicted	O
from	O
the	O
cache	B-General_Concept
is	O
cached	O
in	O
the	O
victim	B-General_Concept
cache	I-General_Concept
.	O
</s>
<s>
Thus	O
,	O
the	O
victim	B-General_Concept
cache	I-General_Concept
gets	O
populated	O
only	O
when	O
data	O
is	O
thrown	O
out	O
of	O
Level	O
1	O
cache	B-General_Concept
.	O
</s>
<s>
In	O
case	O
of	O
a	O
miss	O
in	O
Level	O
1	O
,	O
the	O
missed	O
entry	O
is	O
looked	O
up	O
in	O
the	O
victim	B-General_Concept
cache	I-General_Concept
.	O
</s>
<s>
If	O
the	O
resulting	O
access	O
is	O
a	O
hit	O
,	O
the	O
contents	O
of	O
the	O
Level	O
1	O
cache-line	O
and	O
the	O
matching	O
victim	B-General_Concept
cache	I-General_Concept
line	O
are	O
swapped	O
.	O
</s>
<s>
Though	O
initially	O
proposed	O
by	O
Jouppi	O
to	O
improve	O
cache	B-General_Concept
performance	O
of	O
a	O
direct-mapped	O
cache	B-General_Concept
Level	O
1	O
,	O
modern	O
day	O
microprocessors	O
with	O
multi-level	O
cache	B-General_Concept
hierarchy	O
employ	O
Level	O
3	O
or	O
Level	O
4	O
cache	B-General_Concept
to	O
act	O
as	O
victim	B-General_Concept
cache	I-General_Concept
for	O
the	O
cache	B-General_Concept
lying	O
above	O
it	O
in	O
the	O
memory	O
hierarchy	O
.	O
</s>
<s>
Intel	O
's	O
Crystal	O
Well	O
of	O
its	O
Haswell	O
processors	O
introduced	O
an	O
on-package	O
Level	O
4	O
cache	B-General_Concept
which	O
serves	O
as	O
a	O
victim	B-General_Concept
cache	I-General_Concept
to	O
processor	O
's	O
Level	O
3	O
cache	B-General_Concept
.	O
</s>
<s>
A	O
4	O
–	O
12MB	O
Level	O
3	O
cache	B-General_Concept
is	O
used	O
as	O
a	O
victim	B-General_Concept
cache	I-General_Concept
in	O
POWER5	B-Device
(	O
IBM	O
)	O
microprocessors	O
.	O
</s>
<s>
The	O
problem	O
of	O
increasing	O
memory	O
latency	O
,	O
relative	O
to	O
processor	O
speed	O
,	O
has	O
been	O
dealt	O
with	O
by	O
adding	O
high	O
speed	O
cache	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
Direct-mapped	O
caches	B-General_Concept
have	O
faster	O
access	O
time	O
than	O
set-associative	O
caches	B-General_Concept
.	O
</s>
<s>
However	O
,	O
when	O
multiple	O
cache	B-General_Concept
blocks	I-General_Concept
in	O
memory	O
map	O
to	O
the	O
same	O
cache-line	O
in	O
a	O
direct-mapped	O
cache	B-General_Concept
,	O
they	O
end	O
up	O
evicting	O
each	O
other	O
when	O
anyone	O
of	O
them	O
is	O
accessed	O
.	O
</s>
<s>
This	O
is	O
known	O
as	O
the	O
cache-conflict	O
problem	O
.	O
</s>
<s>
This	O
problem	O
is	O
resolved	O
by	O
increasing	O
the	O
associativity	O
of	O
the	O
cache	B-General_Concept
.	O
</s>
<s>
Thus	O
,	O
for	O
solving	O
the	O
cache	B-General_Concept
conflict	I-General_Concept
problem	O
for	O
a	O
cache	B-General_Concept
with	O
limited	O
associativity	O
,	O
a	O
victim	B-General_Concept
cache	I-General_Concept
is	O
employed	O
.	O
</s>
<s>
The	O
behavior	O
of	O
a	O
victim	B-General_Concept
cache	I-General_Concept
in	O
its	O
respective	O
interaction	O
with	O
the	O
corresponding	O
level	O
cache	B-General_Concept
is	O
given	O
below	O
:	O
</s>
<s>
Cache	B-General_Concept
Miss	O
,	O
Victim	O
Hit	O
:	O
The	O
block	O
is	O
in	O
the	O
victim	B-General_Concept
cache	I-General_Concept
and	O
the	O
one	O
in	O
the	O
cache	B-General_Concept
are	O
replaced	O
with	O
each	O
other	O
.	O
</s>
<s>
This	O
new	O
entry	O
in	O
victim	B-General_Concept
cache	I-General_Concept
becomes	O
the	O
most	O
recently	O
used	O
block	O
.	O
</s>
<s>
Cache	B-General_Concept
Miss	O
,	O
Victim	O
Miss	O
:	O
The	O
block	O
is	O
brought	O
to	O
cache	B-General_Concept
from	O
next	O
level	O
.	O
</s>
<s>
The	O
block	O
evicted	O
from	O
the	O
cache	B-General_Concept
gets	O
stored	O
in	O
Victim	B-General_Concept
cache	I-General_Concept
.	O
</s>
<s>
Suppose	O
a	O
direct-mapped	O
L1	O
cache	B-General_Concept
with	O
blocks	O
A	O
,	O
B	O
pointing	O
to	O
the	O
same	O
set	O
.	O
</s>
<s>
It	O
is	O
linked	O
to	O
a	O
2	O
entry	O
fully	O
associative	O
victim	B-General_Concept
cache	I-General_Concept
with	O
blocks	O
C	O
,	O
D	O
in	O
it	O
.	O
</s>
<s>
From	O
the	O
diagram	O
,	O
we	O
can	O
see	O
that	O
,	O
in	O
case	O
of	O
victim	B-General_Concept
cache	I-General_Concept
(	O
VC	O
)	O
hit	O
,	O
blocks	O
A	O
and	O
B	O
are	O
swapped	O
.	O
</s>
<s>
Hence	O
,	O
it	O
gives	O
an	O
illusion	O
of	O
associativity	O
to	O
the	O
direct-mapped	O
L1	O
cache	B-General_Concept
,	O
in	O
turn	O
reducing	O
the	O
conflict	O
misses	O
.	O
</s>
<s>
In	O
case	O
of	O
two	O
caches	B-General_Concept
,	O
L1	O
and	O
L2	O
with	O
exclusive	O
cache	B-General_Concept
policy	O
(	O
L2	O
does	O
not	O
cache	B-General_Concept
same	O
the	O
memory	O
locations	O
as	O
L1	O
)	O
,	O
L2	O
acts	O
as	O
the	O
victim	B-General_Concept
cache	I-General_Concept
for	O
L1	O
.	O
</s>
<s>
While	O
measuring	O
performance	O
improvement	O
by	O
using	O
victim	B-General_Concept
cache	I-General_Concept
,	O
Jouppi	O
assumed	O
a	O
Level-1	O
direct-mapped	O
cache	B-General_Concept
augmented	O
with	O
a	O
fully	O
associative	O
cache	B-General_Concept
.	O
</s>
<s>
For	O
the	O
test	O
suite	O
used	O
by	O
him	O
,	O
on	O
an	O
average	O
39%	O
of	O
the	O
Level-1	O
data	B-General_Concept
cache	I-General_Concept
misses	O
are	O
found	O
to	O
be	O
conflict	O
misses	O
,	O
while	O
on	O
an	O
average	O
29%	O
of	O
the	O
Level-1	O
instruction	O
misses	O
are	O
found	O
to	O
be	O
conflict	O
misses	O
.	O
</s>
<s>
Since	O
conflict	O
misses	O
amount	O
to	O
large	O
percentage	O
of	O
total	O
misses	O
,	O
therefore	O
providing	O
additional	O
associativity	O
by	O
augmenting	O
the	O
Level	O
1	O
cache	B-General_Concept
with	O
a	O
victim	B-General_Concept
cache	I-General_Concept
is	O
bound	O
to	O
improve	O
total	O
miss	B-General_Concept
rate	I-General_Concept
significantly	O
.	O
</s>
<s>
Experimental	O
results	O
are	O
deduced	O
by	O
considering	O
a	O
32-Kb	O
Direct-Mapped	O
,	O
2-way	O
and	O
fully	O
associative	O
cache	B-General_Concept
augmented	O
with	O
a	O
256	O
block	O
(	O
8KB	O
)	O
victim	B-General_Concept
cache	I-General_Concept
and	O
running	O
on	O
it	O
8	O
randomly	O
selected	O
SPEC95	O
Benchmarks	O
.	O
</s>
<s>
While	O
the	O
results	O
cannot	O
be	O
generalized	O
for	O
all	O
benchmarks	O
,	O
adding	O
a	O
victim	B-General_Concept
cache	I-General_Concept
provides	O
a	O
miss	B-General_Concept
rate	I-General_Concept
reduction	O
ranging	O
from	O
10%	O
to	O
100%	O
for	O
all	O
the	O
cache	B-General_Concept
configuration	O
.	O
</s>
<s>
The	O
returns	O
although	O
seem	O
to	O
level	O
off	O
beyond	O
victim	B-General_Concept
cache	I-General_Concept
size	O
of	O
50	O
blocks	O
,	O
thus	O
proving	O
Jouppi	O
's	O
observation	O
that	O
victim	B-General_Concept
cache	I-General_Concept
benefits	O
reach	O
a	O
plateau	O
after	O
the	O
first	O
few	O
victim	O
blocks	O
.	O
</s>
<s>
Miss	B-General_Concept
rate	I-General_Concept
reduction	O
for	O
a	O
64KB	O
cache	B-General_Concept
size	O
are	O
found	O
to	O
be	O
significantly	O
lower	O
,	O
proving	O
that	O
victim	O
caching	B-General_Concept
is	O
not	O
indefinitely	O
scalable	O
.	O
</s>
<s>
While	O
comparing	O
various	O
cache	B-General_Concept
configuration	O
it	O
was	O
found	O
that	O
in	O
certain	O
cases	O
adding	O
a	O
small	O
victim	B-General_Concept
cache	I-General_Concept
can	O
give	O
performance	O
benefit	O
equivalent	O
to	O
that	O
observed	O
by	O
multiplying	O
the	O
cache	B-General_Concept
size	O
by	O
2	O
.	O
</s>
