<s>
Memory	B-General_Concept
scrubbing	I-General_Concept
consists	O
of	O
reading	O
from	O
each	O
computer	B-General_Concept
memory	I-General_Concept
location	O
,	O
correcting	O
bit	O
errors	O
(	O
if	O
any	O
)	O
with	O
an	O
error-correcting	B-Error_Name
code	O
(	O
ECC	O
)	O
,	O
and	O
writing	O
the	O
corrected	O
data	O
back	O
to	O
the	O
same	O
location	O
.	O
</s>
<s>
Due	O
to	O
the	O
high	O
integration	O
density	O
of	O
modern	O
computer	B-General_Concept
memory	I-General_Concept
chips	O
,	O
the	O
individual	O
memory	O
cell	O
structures	O
became	O
small	O
enough	O
to	O
be	O
vulnerable	O
to	O
cosmic	O
rays	O
and/or	O
alpha	O
particle	O
emission	O
.	O
</s>
<s>
This	O
can	O
be	O
a	O
problem	O
for	O
DRAM	O
and	O
SRAM	B-Architecture
based	O
memories	O
.	O
</s>
<s>
However	O
,	O
together	O
with	O
the	O
large	O
amount	O
of	O
memory	O
modern	O
computersespecially	O
serversare	O
equipped	O
with	O
,	O
and	O
together	O
with	O
extended	O
periods	O
of	O
uptime	B-Device
,	O
the	O
probability	O
of	O
soft	O
errors	O
in	O
the	O
total	O
memory	O
installed	O
is	O
significant	O
.	O
</s>
<s>
The	O
information	O
in	O
an	O
ECC	B-General_Concept
memory	I-General_Concept
is	O
stored	O
redundantly	B-General_Concept
enough	O
to	O
correct	O
single	O
bit	O
error	O
per	O
memory	O
word	O
.	O
</s>
<s>
Hence	O
,	O
an	O
ECC	B-General_Concept
memory	I-General_Concept
can	O
support	O
the	O
scrubbing	O
of	O
the	O
memory	O
content	O
.	O
</s>
<s>
Namely	O
,	O
if	O
the	O
memory	B-General_Concept
controller	I-General_Concept
scans	O
systematically	O
through	O
the	O
memory	O
,	O
the	O
single	O
bit	O
errors	O
can	O
be	O
detected	O
,	O
the	O
erroneous	O
bit	O
can	O
be	O
determined	O
using	O
the	O
ECC	O
checksum	B-Error_Name
,	O
and	O
the	O
corrected	O
data	O
can	O
be	O
written	O
back	O
to	O
the	O
memory	O
.	O
</s>
<s>
It	O
is	O
important	O
to	O
check	O
each	O
memory	O
location	O
periodically	O
,	O
frequently	O
enough	O
,	O
before	O
multiple	O
bit	O
errors	O
within	O
the	O
same	O
word	O
are	O
too	O
likely	O
to	O
occur	O
,	O
because	O
the	O
one	O
bit	O
errors	O
can	O
be	O
corrected	O
,	O
but	O
the	O
multiple	O
bit	O
errors	O
are	O
not	O
correctable	O
,	O
in	O
the	O
case	O
of	O
usual	O
(	O
as	O
of	O
2008	O
)	O
ECC	B-General_Concept
memory	I-General_Concept
modules	O
.	O
</s>
<s>
In	O
order	O
to	O
not	O
disturb	O
regular	O
memory	O
requests	O
from	O
the	O
CPU	B-General_Concept
and	O
thus	O
prevent	O
decreasing	O
performance	O
,	O
scrubbing	O
is	O
usually	O
only	O
done	O
during	O
idle	O
periods	O
.	O
</s>
<s>
For	O
many	O
servers	B-Application
,	O
the	O
scrub	O
period	O
can	O
be	O
configured	O
in	O
the	O
BIOS	B-Operating_System
setup	O
program	O
.	O
</s>
<s>
The	O
normal	O
memory	O
reads	O
issued	O
by	O
the	O
CPU	B-General_Concept
or	O
DMA	B-General_Concept
devices	O
are	O
checked	O
for	O
ECC	O
errors	O
,	O
but	O
due	O
to	O
data	B-General_Concept
locality	I-General_Concept
reasons	O
they	O
can	O
be	O
confined	O
to	O
a	O
small	O
range	O
of	O
addresses	O
and	O
keeping	O
other	O
memory	O
locations	O
untouched	O
for	O
a	O
very	O
long	O
time	O
.	O
</s>
<s>
On	O
some	O
systems	O
,	O
not	O
only	O
the	O
main	O
memory	O
(	O
DRAM-based	O
)	O
is	O
capable	O
of	O
scrubbing	O
but	O
also	O
the	O
CPU	B-General_Concept
caches	I-General_Concept
(	O
SRAM-based	O
)	O
.	O
</s>
<s>
Memory	B-General_Concept
scrubbing	I-General_Concept
increases	O
reliability	O
,	O
therefore	O
it	O
can	O
be	O
classified	O
as	O
a	O
RAS	B-General_Concept
feature	O
.	O
</s>
<s>
While	O
they	O
both	O
essentially	O
perform	O
memory	B-General_Concept
scrubbing	I-General_Concept
and	O
associated	O
error	B-Error_Name
correction	I-Error_Name
(	O
if	O
it	O
is	O
doable	O
)	O
,	O
the	O
main	O
difference	O
is	O
how	O
these	O
two	O
variants	O
are	O
initiated	O
and	O
executed	O
.	O
</s>
<s>
Patrol	O
scrubbing	O
runs	O
in	O
an	O
automated	O
manner	O
when	O
the	O
system	O
is	O
idle	O
,	O
while	O
demand	O
scrubbing	O
performs	O
the	O
error	B-Error_Name
correction	I-Error_Name
when	O
the	O
data	O
is	O
actually	O
requested	O
from	O
main	O
memory	O
.	O
</s>
