<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
persistent	B-Library
memory	I-Library
is	O
any	O
method	O
or	O
apparatus	O
for	O
efficiently	O
storing	O
data	O
structures	O
such	O
that	O
they	O
can	O
continue	O
to	O
be	O
accessed	O
using	O
memory	B-General_Concept
instructions	I-General_Concept
or	O
memory	O
APIs	O
even	O
after	O
the	O
end	O
of	O
the	O
process	O
that	O
created	O
or	O
last	O
modified	O
them	O
.	O
</s>
<s>
Often	O
confused	O
with	O
non-volatile	B-General_Concept
random-access	I-General_Concept
memory	I-General_Concept
(	O
NVRAM	B-General_Concept
)	O
,	O
persistent	B-Library
memory	I-Library
is	O
instead	O
more	O
closely	O
linked	O
to	O
the	O
concept	O
of	O
persistence	B-Application
in	O
its	O
emphasis	O
on	O
program	O
state	O
that	O
exists	O
outside	O
the	O
fault	O
zone	O
of	O
the	O
process	O
that	O
created	O
it	O
.	O
</s>
<s>
Efficient	O
,	O
memory-like	O
access	O
is	O
the	O
defining	O
characteristic	O
of	O
persistent	B-Library
memory	I-Library
.	O
</s>
<s>
It	O
can	O
be	O
provided	O
using	O
microprocessor	B-Architecture
memory	B-General_Concept
instructions	I-General_Concept
,	O
such	O
as	O
load	O
and	O
store	O
.	O
</s>
<s>
It	O
can	O
also	O
be	O
provided	O
using	O
APIs	O
that	O
implement	O
remote	B-General_Concept
direct	I-General_Concept
memory	I-General_Concept
access	I-General_Concept
(	O
RDMA	B-General_Concept
)	O
actions	O
,	O
such	O
as	O
RDMA	B-General_Concept
read	O
and	O
RDMA	B-General_Concept
write	O
.	O
</s>
<s>
Other	O
low-latency	O
methods	O
that	O
allow	O
byte-grain	O
access	O
to	O
data	O
also	O
qualify	O
.	O
</s>
<s>
Persistent	B-Library
memory	I-Library
capabilities	O
extend	O
beyond	O
non-volatility	O
of	O
stored	O
bits	O
.	O
</s>
<s>
In	O
this	O
respect	O
,	O
persistent	B-Library
memory	I-Library
resembles	O
more	O
abstract	O
forms	O
of	O
computer	O
storage	O
,	O
such	O
as	O
file	B-Application
systems	I-Application
.	O
</s>
<s>
In	O
fact	O
,	O
almost	O
all	O
existing	O
persistent	B-Library
memory	I-Library
technologies	O
implement	O
at	O
least	O
a	O
basic	O
file	B-Application
system	I-Application
that	O
can	O
be	O
used	O
for	O
associating	O
names	O
or	O
identifiers	O
with	O
stored	O
extents	O
,	O
and	O
at	O
a	O
minimum	O
provide	O
file	B-Application
system	I-Application
methods	O
that	O
can	O
be	O
used	O
for	O
naming	O
and	O
allocating	O
such	O
extents	O
.	O
</s>
<s>
The	O
read-of-non-persistent-write	O
problem	O
is	O
found	O
for	O
lock-free	B-Operating_System
programs	O
on	O
persistent	B-Library
memory	I-Library
.	O
</s>
<s>
As	O
compare-and-swap	B-Operating_System
(	O
CAS	O
)	O
operations	O
do	O
not	O
persist	O
the	O
written	O
values	O
to	O
persistent	B-Library
memory	I-Library
,	O
the	O
modified	O
data	O
can	O
be	O
made	O
visible	O
by	O
the	O
cache	O
coherence	O
protocol	O
to	O
a	O
concurrent	O
observer	O
before	O
the	O
modified	O
data	O
can	O
be	O
observed	O
by	O
a	O
crash	O
observer	O
at	O
persistent	B-Library
memory	I-Library
.	O
</s>
<s>
If	O
a	O
power	O
failure	O
happens	O
right	O
after	O
the	O
write	O
is	O
made	O
visible	O
but	O
not	O
yet	O
persistent	O
,	O
the	O
read-of-non-persistent-write	O
problem	O
can	O
occur	O
,	O
i.e.	O
,	O
a	O
data	O
variable	O
that	O
is	O
modified	O
by	O
a	O
compare-and-swap	B-Operating_System
operation	O
can	O
be	O
made	O
visible	O
to	O
a	O
concurrent	O
observer	O
before	O
a	O
crash	O
observer	O
,	O
causing	O
potential	O
crash	O
inconsistencies	O
.	O
</s>
<s>
To	O
illustrate	O
the	O
problem	O
:	O
for	O
a	O
singly	O
linked	O
lock-free	B-Operating_System
list	O
,	O
a	O
node	O
can	O
be	O
inserted	O
by	O
a	O
producer	O
thread	O
A	O
after	O
the	O
head	O
node	O
,	O
the	O
next	O
pointer	O
of	O
the	O
head	O
node	O
gets	O
atomically	O
switched	O
(	O
CAS	O
)	O
to	O
point	O
to	O
the	O
new	O
node	O
A	O
,	O
however	O
,	O
this	O
CAS	O
is	O
not	O
persisted	O
.	O
</s>
<s>
The	O
read-of-non-persistent-write	O
problem	O
is	O
not	O
limited	O
to	O
lock-free	B-Operating_System
linked	O
lists	O
,	O
it	O
can	O
be	O
found	O
in	O
any	O
lock-free	B-Operating_System
data	I-Operating_System
structures	I-Operating_System
where	O
the	O
potential	O
gap	O
between	O
concurrent	O
visibility	O
and	O
persistent	O
visibility	O
can	O
exist	O
.	O
</s>
<s>
For	O
instance	O
,	O
a	O
similar	O
problem	O
can	O
occur	O
with	O
persistent	O
circular	O
buffers	B-General_Concept
.	O
</s>
