<s>
The	O
fat	B-Protocol
tree	I-Protocol
network	O
is	O
a	O
universal	O
network	O
for	O
provably	O
efficient	O
communication	O
.	O
</s>
<s>
It	O
was	O
invented	O
by	O
Charles	O
E	O
.	O
Leiserson	O
of	O
the	O
Massachusetts	O
Institute	O
of	O
Technology	O
in	O
1985	O
.	O
k-ary	O
n-trees	O
,	O
the	O
type	O
of	O
fat-trees	B-Protocol
commonly	O
used	O
in	O
most	O
high-performance	O
networks	O
,	O
were	O
initially	O
formalized	O
in	O
1997	O
.	O
</s>
<s>
In	O
a	O
tree	B-Application
data	I-Application
structure	I-Application
,	O
every	O
branch	O
has	O
the	O
same	O
thickness	O
,	O
regardless	O
of	O
their	O
place	O
in	O
the	O
hierarchy	O
—	O
they	O
are	O
all	O
"	O
skinny	O
"	O
(	O
skinny	O
in	O
this	O
context	O
means	O
low-bandwidth	O
)	O
.	O
</s>
<s>
In	O
a	O
fat	B-Protocol
tree	I-Protocol
,	O
branches	O
nearer	O
the	O
top	O
of	O
the	O
hierarchy	O
are	O
"	O
fatter	O
"	O
(	O
thicker	O
)	O
than	O
branches	O
further	O
down	O
the	O
hierarchy	O
.	O
</s>
<s>
Mesh	B-Architecture
and	O
hypercube	B-Architecture
topologies	O
have	O
communication	O
requirements	O
that	O
follow	O
a	O
rigid	O
algorithm	O
,	O
and	O
cannot	O
be	O
tailored	O
to	O
specific	O
packaging	O
technologies	O
.	O
</s>
<s>
Supercomputers	O
that	O
use	O
a	O
fat	B-Protocol
tree	I-Protocol
network	O
include	O
the	O
two	O
fastest	O
as	O
of	O
late	O
2018	O
,	O
Summit	B-Device
and	O
Sierra	B-Device
,	O
as	O
well	O
as	O
Tianhe-2	B-Device
,	O
the	O
Meiko	B-Device
Scientific	I-Device
CS-2	O
,	O
Yellowstone	B-Device
,	O
the	O
Earth	B-Device
Simulator	I-Device
,	O
the	O
Cray	B-Device
X2	I-Device
,	O
the	B-Device
Connection	I-Device
Machine	I-Device
CM-5	B-Device
,	O
and	O
various	O
Altix	B-Architecture
supercomputers	O
.	O
</s>
<s>
Mercury	B-Application
Computer	I-Application
Systems	I-Application
applied	O
a	O
variant	O
of	O
the	O
fat	B-Protocol
tree	I-Protocol
topology	O
—	O
the	O
hypertree	B-Protocol
network	I-Protocol
—	O
to	O
their	O
multicomputers	B-Operating_System
.	O
</s>
<s>
In	O
this	O
architecture	O
,	O
2	O
to	O
360	O
compute	O
nodes	O
are	O
arranged	O
in	O
a	O
circuit-switched	B-Protocol
fat	B-Protocol
tree	I-Protocol
network	O
.	O
</s>
<s>
Each	O
node	O
in	O
this	O
heterogeneous	O
system	O
could	O
be	O
an	O
Intel	B-General_Concept
i860	I-General_Concept
,	O
a	O
PowerPC	B-Architecture
,	O
or	O
a	O
group	O
of	O
three	O
SHARC	B-General_Concept
digital	B-Architecture
signal	I-Architecture
processors	I-Architecture
.	O
</s>
<s>
The	O
fat	B-Protocol
tree	I-Protocol
network	O
was	O
particularly	O
well	O
suited	O
to	O
fast	O
Fourier	O
transform	O
computations	O
,	O
which	O
customers	O
used	O
for	O
such	O
signal	O
processing	O
tasks	O
as	O
radar	B-Application
,	O
sonar	B-Application
,	O
and	O
medical	B-Application
imaging	I-Application
.	O
</s>
<s>
In	O
August	O
2008	O
,	O
a	O
team	O
of	O
computer	O
scientists	O
at	O
UCSD	O
published	O
a	O
scalable	O
design	O
for	O
network	O
architecture	O
that	O
uses	O
a	O
topology	O
inspired	O
by	O
the	O
fat	B-Protocol
tree	I-Protocol
topology	O
to	O
realize	O
networks	O
that	O
scale	O
better	O
than	O
those	O
of	O
previous	O
hierarchical	O
networks	O
.	O
</s>
<s>
This	O
topology	O
is	O
actually	O
a	O
special	O
instance	O
of	O
a	O
Clos	B-Device
network	I-Device
,	O
rather	O
than	O
a	O
fat-tree	B-Protocol
as	O
described	O
above	O
.	O
</s>
