<s>
Automatically	B-Language
Tuned	I-Language
Linear	I-Language
Algebra	I-Language
Software	I-Language
(	O
ATLAS	O
)	O
is	O
a	O
software	B-Library
library	I-Library
for	O
linear	B-Language
algebra	I-Language
.	O
</s>
<s>
It	O
provides	O
a	O
mature	O
open	B-Application
source	I-Application
implementation	O
of	O
BLAS	B-Application
APIs	B-Application
for	O
C	B-Language
and	O
Fortran77	B-Application
.	O
</s>
<s>
ATLAS	O
is	O
often	O
recommended	O
as	O
a	O
way	O
to	O
automatically	O
generate	O
an	O
optimized	O
BLAS	B-Application
library	O
.	O
</s>
<s>
While	O
its	O
performance	O
often	O
trails	O
that	O
of	O
specialized	O
libraries	O
written	O
for	O
one	O
specific	O
hardware	B-Device
platform	I-Device
,	O
it	O
is	O
often	O
the	O
first	O
or	O
even	O
only	O
optimized	O
BLAS	B-Application
implementation	O
available	O
on	O
new	O
systems	O
and	O
is	O
a	O
large	O
improvement	O
over	O
the	O
generic	O
BLAS	B-Application
available	O
at	O
Netlib	B-Application
.	O
</s>
<s>
ATLAS	O
runs	O
on	O
most	O
Unix-like	O
operating	O
systems	O
and	O
on	O
Microsoft	B-Application
Windows	I-Application
(	O
using	O
Cygwin	B-Language
)	O
.	O
</s>
<s>
It	O
is	O
released	O
under	O
a	O
BSD-style	B-Operating_System
license	I-Operating_System
without	O
advertising	O
clause	O
,	O
and	O
many	O
well-known	O
mathematics	O
applications	O
including	O
MATLAB	B-Language
,	O
Mathematica	B-Language
,	O
Scilab	B-Application
,	O
SageMath	B-Application
,	O
and	O
some	O
builds	O
of	O
GNU	B-Language
Octave	I-Language
may	O
use	O
it	O
.	O
</s>
<s>
ATLAS	O
provides	O
a	O
full	O
implementation	O
of	O
the	O
BLAS	B-Application
APIs	B-Application
as	O
well	O
as	O
some	O
additional	O
functions	O
from	O
LAPACK	B-Application
,	O
a	O
higher-level	O
library	O
built	O
on	O
top	O
of	O
BLAS	B-Application
.	O
</s>
<s>
In	O
BLAS	B-Application
,	O
functionality	O
is	O
divided	O
into	O
three	O
groups	O
called	O
levels	O
1	O
,	O
2	O
and	O
3	O
.	O
</s>
<s>
Every	O
ATLAS	O
level	O
1	O
BLAS	B-Application
function	O
has	O
its	O
own	O
kernel	O
.	O
</s>
<s>
Since	O
it	O
would	O
be	O
difficult	O
to	O
maintain	O
thousands	O
of	O
cases	O
in	O
ATLAS	O
there	O
is	O
little	O
architecture	O
specific	O
optimization	O
for	O
Level	O
1	O
BLAS	B-Application
.	O
</s>
<s>
Instead	O
multiple	O
implementation	O
is	O
relied	O
upon	O
to	O
allow	O
for	O
compiler	B-Application
optimization	I-Application
to	O
produce	O
high	O
performance	O
implementation	O
for	O
the	O
system	O
.	O
</s>
<s>
All	O
routines	O
in	O
the	O
ATLAS	O
level	O
2	O
BLAS	B-Application
are	O
built	O
from	O
two	O
Level	O
2	O
BLAS	B-Application
kernels	O
:	O
</s>
<s>
Most	O
of	O
the	O
Level	O
3	O
BLAS	B-Application
is	O
derived	O
from	O
GEMM	O
,	O
so	O
that	O
is	O
the	O
primary	O
focus	O
of	O
the	O
optimization	O
.	O
</s>
<s>
Run	O
the	O
risk	O
of	O
TLB	B-Architecture
issues	O
,	O
bad	O
strides	O
,	O
etc	O
.	O
</s>
<s>
The	O
actual	O
decision	O
is	O
made	O
through	O
a	O
simple	O
heuristic	B-Algorithm
which	O
checks	O
for	O
"	O
skinny	O
cases	O
"	O
.	O
</s>
<s>
When	O
integrating	O
the	O
ATLAS	O
BLAS	B-Application
with	O
LAPACK	B-Application
an	O
important	O
consideration	O
is	O
the	O
choice	O
of	O
blocking	O
factor	O
for	O
LAPACK	B-Application
.	O
</s>
<s>
If	O
the	O
ATLAS	O
blocking	O
factor	O
is	O
small	O
enough	O
the	O
blocking	O
factor	O
of	O
LAPACK	B-Application
could	O
be	O
set	O
to	O
match	O
that	O
of	O
ATLAS	O
.	O
</s>
<s>
To	O
take	O
advantage	O
of	O
recursive	O
factorization	O
,	O
ATLAS	O
provides	O
replacement	O
routines	O
for	O
some	O
LAPACK	B-Application
routines	O
.	O
</s>
<s>
These	O
simply	O
overwrite	O
the	O
corresponding	O
LAPACK	B-Application
routines	O
from	O
Netlib	B-Application
.	O
</s>
