<s>
A	O
matrix	B-Operating_System
clock	I-Operating_System
is	O
a	O
mechanism	O
for	O
capturing	O
chronological	O
and	O
causal	O
relationships	O
in	O
a	O
distributed	B-Architecture
system	I-Architecture
.	O
</s>
<s>
Matrix	B-Operating_System
clocks	I-Operating_System
are	O
a	O
generalization	O
of	O
the	O
notion	O
of	O
vector	B-Operating_System
clocks	I-Operating_System
.	O
</s>
<s>
A	O
matrix	B-Operating_System
clock	I-Operating_System
maintains	O
a	O
vector	B-Data_Structure
of	O
the	O
vector	B-Operating_System
clocks	I-Operating_System
for	O
each	O
communicating	O
host	O
.	O
</s>
<s>
This	O
allows	O
establishing	O
a	O
lower	O
bound	O
on	O
what	O
other	O
hosts	O
know	O
,	O
and	O
is	O
useful	O
in	O
applications	O
such	O
as	O
checkpointing	B-General_Concept
and	O
garbage	B-General_Concept
collection	I-General_Concept
.	O
</s>
