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<H1><A NAME="SECTION03320000000000000000"></A><A NAME="secblasport"></A><A NAME="7605"></A>
<BR>
The BLAS as the Key to Portability
</H1>
<P>
How then can we hope to be able to achieve sufficient control over
vectorization, data movement, and parallelism in portable Fortran code,
to obtain the levels of performance that machines can offer?
<P>
The LAPACK strategy for combining
efficiency with portability<A NAME="7606"></A> is to construct the software
as much as possible
out of calls to the BLAS (Basic Linear Algebra Subprograms); the BLAS are
used
as building blocks.
<P>
The efficiency<A NAME="7607"></A> of LAPACK software depends on efficient
implementations
of the BLAS being provided by computer vendors (or others) for their
machines.
Thus the BLAS form a low-level interface between LAPACK software and
different machine architectures.
Above this level, almost all of the LAPACK software is truly portable.
<P>
There are now three levels of BLAS:
<P>
<DL>
<DT><STRONG>Level 1 BLAS [<A
HREF="node151.html#blas1">78</A>]:</STRONG>
<DD>for vector operations, such as
<!-- MATH
$y \leftarrow \alpha x + y$
-->
<IMG
WIDTH="92" HEIGHT="30" ALIGN="MIDDLE" BORDER="0"
SRC="img207.png"
ALT="$y \leftarrow \alpha x + y$">
<P>
<DT><STRONG>Level 2 BLAS [<A
HREF="node151.html#blas2">42</A>]:</STRONG>
<DD>for matrix-vector operations, such as
<!-- MATH
$y \leftarrow \alpha A x + \beta y$
-->
<IMG
WIDTH="116" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img208.png"
ALT="$y \leftarrow \alpha A x + \beta y$">
<P>
<DT><STRONG>Level 3 BLAS [<A
HREF="node151.html#blas3">40</A>]:</STRONG>
<DD>for matrix-matrix operations, such as
<!-- MATH
$C \leftarrow \alpha A B + \beta C$
-->
<IMG
WIDTH="129" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img209.png"
ALT="$C \leftarrow \alpha A B + \beta C$">
<P>
</DL>
<P>
Here, <B><I>A</I></B>, <B><I>B</I></B> and <B><I>C</I></B> are matrices, <B><I>x</I></B> and <B><I>y</I></B> are vectors, and <IMG
WIDTH="16" HEIGHT="16" ALIGN="BOTTOM" BORDER="0"
SRC="img49.png"
ALT="$\alpha$">
and
<IMG
WIDTH="15" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img50.png"
ALT="$\beta$">
are scalars.
<P>
The Level 1 BLAS<A NAME="7613"></A> are used in LAPACK,
but for convenience rather than
for performance: they perform an insignificant fraction of the computation,
and they cannot achieve high efficiency on most modern supercomputers.
<P>
The Level 2 BLAS<A NAME="7614"></A> can achieve near-peak performance
on many vector processors,
such as a single processor of a CRAY Y-MP, CRAY C90, or CONVEX C4 machine.
However on other vector processors, such as a CRAY 2,
or a RISC workstation or PC with one more levels of cache,
their performance is limited by the rate of data movement between different
levels of memory.
<P>
This limitation is overcome by the Level 3 BLAS<A NAME="7615"></A>,
which perform <B><I>O</I>(<I>n</I><SUP>3</SUP>)</B>
floating-point operations on <B><I>O</I>(<I>n</I><SUP>2</SUP>)</B> data, whereas the Level 2 BLAS
perform only <B><I>O</I>(<I>n</I><SUP>2</SUP>)</B> operations on <B><I>O</I>(<I>n</I><SUP>2</SUP>)</B> data.
<P>
The BLAS also allow us to exploit parallelism in a way that is transparent
to the software that calls them.
Even the Level 2 BLAS offer some scope for exploiting parallelism, but
greater
scope is provided by the Level 3 BLAS, as Table <A HREF="node65.html#tabsgiblas">3.1</A>
illustrates.
<P>
<BR>
<DIV ALIGN="CENTER">
<A NAME="tabsgiblas"></A>
<P>
<DIV ALIGN="CENTER">
(all matrices are of order 1000; <B><I>U</I></B> is upper triangular)
</DIV>
<P>
<DIV ALIGN="CENTER">
<BR>
</DIV>
<P>
<DIV ALIGN="CENTER"><A NAME="7618"></A>
<TABLE CELLPADDING=3 BORDER="1">
<CAPTION><STRONG>Table 3.1:</STRONG>
Speed in megaflops of Level 2 and Level 3 BLAS operations on an
SGI Origin 2000</CAPTION>
<TR><TD ALIGN="LEFT">Number of processors:</TD>
<TD ALIGN="RIGHT">1</TD>
<TD ALIGN="RIGHT">2</TD>
<TD ALIGN="RIGHT">4</TD>
<TD ALIGN="RIGHT">8</TD>
<TD ALIGN="RIGHT">16</TD>
</TR>
<TR><TD ALIGN="LEFT">Level 2:
<!-- MATH
$y \leftarrow \alpha A x + \beta y$
-->
<IMG
WIDTH="116" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img208.png"
ALT="$y \leftarrow \alpha A x + \beta y$"></TD>
<TD ALIGN="RIGHT">210</TD>
<TD ALIGN="RIGHT">154</TD>
<TD ALIGN="RIGHT">385</TD>
<TD ALIGN="RIGHT">493</TD>
<TD ALIGN="RIGHT">163</TD>
</TR>
<TR><TD ALIGN="LEFT">Level 3:
<!-- MATH
$C \leftarrow \alpha A B + \beta C$
-->
<IMG
WIDTH="129" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img209.png"
ALT="$C \leftarrow \alpha A B + \beta C$"></TD>
<TD ALIGN="RIGHT">555</TD>
<TD ALIGN="RIGHT">1021</TD>
<TD ALIGN="RIGHT">1864</TD>
<TD ALIGN="RIGHT">3758</TD>
<TD ALIGN="RIGHT">4200</TD>
</TR>
<TR><TD ALIGN="LEFT">Level 2:
<!-- MATH
$x \leftarrow U x$
-->
<IMG
WIDTH="65" HEIGHT="16" ALIGN="BOTTOM" BORDER="0"
SRC="img210.png"
ALT="$x \leftarrow U x$"></TD>
<TD ALIGN="RIGHT">162</TD>
<TD ALIGN="RIGHT">152</TD>
<TD ALIGN="RIGHT">148</TD>
<TD ALIGN="RIGHT">153</TD>
<TD ALIGN="RIGHT">137</TD>
</TR>
<TR><TD ALIGN="LEFT">Level 3:
<!-- MATH
$B \leftarrow U B$
-->
<IMG
WIDTH="74" HEIGHT="16" ALIGN="BOTTOM" BORDER="0"
SRC="img211.png"
ALT="$B \leftarrow U B$"></TD>
<TD ALIGN="RIGHT">518</TD>
<TD ALIGN="RIGHT">910</TD>
<TD ALIGN="RIGHT">1842</TD>
<TD ALIGN="RIGHT">3519</TD>
<TD ALIGN="RIGHT">5171</TD>
</TR>
<TR><TD ALIGN="LEFT">Level 2:
<!-- MATH
$x \leftarrow U^{-1} x$
-->
<IMG
WIDTH="83" HEIGHT="19" ALIGN="BOTTOM" BORDER="0"
SRC="img212.png"
ALT="$x \leftarrow U^{-1} x$"></TD>
<TD ALIGN="RIGHT">172</TD>
<TD ALIGN="RIGHT">80</TD>
<TD ALIGN="RIGHT">140</TD>
<TD ALIGN="RIGHT">168</TD>
<TD ALIGN="RIGHT">17</TD>
</TR>
<TR><TD ALIGN="LEFT">Level 3:
<!-- MATH
$B \leftarrow U^{-1} B$
-->
<IMG
WIDTH="92" HEIGHT="19" ALIGN="BOTTOM" BORDER="0"
SRC="img213.png"
ALT="$B \leftarrow U^{-1} B$"></TD>
<TD ALIGN="RIGHT">519</TD>
<TD ALIGN="RIGHT">967</TD>
<TD ALIGN="RIGHT">1859</TD>
<TD ALIGN="RIGHT">3507</TD>
<TD ALIGN="RIGHT">5078</TD>
</TR>
</TABLE>
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<ADDRESS>
<I>Susan Blackford</I>
<BR><I>1999-10-01</I>
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