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<H2><A NAME="SECTION04535000000000000000">Identify Performance Bottlenecks</A></H2>
<A NAME="subsecperfid"> </A>
<A NAME="4226"> </A><A NAME="4227"> </A>
<P>
The formulas mentioned in section <A HREF="node123.html#subsecestim">5.3.3</A>, in addition to
providing an estimate of performance, can help one identify whether
the performance is limited by computation, by the number
of messages, or by the volume of communication. Even if the estimate
is far from correct, the user may get some information about the
performance bottleneck by studying the computation and communication
estimates provided by those formulas.
<P>
Comparing the execution times of a problem of size <I>N</I> and one of size <I>N</I>/2
may also provide insight into the performance of the
ScaLAPACK routine being used. Let <IMG WIDTH=18 HEIGHT=23 ALIGN=MIDDLE ALT="tex2html_wrap_inline12212" SRC="img31.gif"> and <IMG WIDTH=30 HEIGHT=23 ALIGN=MIDDLE ALT="tex2html_wrap_inline12220" SRC="img33.gif"> be the time required
for a problem of size <I>N</I> and size <I>N</I>/2, respectively, on <I>P</I> processors.
<P>
<UL>
<LI> If <IMG WIDTH=87 HEIGHT=24 ALIGN=MIDDLE ALT="tex2html_wrap_inline17099" SRC="img419.gif">,
the physical memory of each node may be exceeded.
<LI> If <IMG WIDTH=84 HEIGHT=24 ALIGN=MIDDLE ALT="tex2html_wrap_inline17101" SRC="img420.gif">, the performance
may be limited by the rate at which flops
are performed. If the flop rate
is significantly less than expected,
the user should check the data distribution (try the standard data
distribution suggested in section <A HREF="node106.html#distmemcomp">5.1.1</A>)
and the underlying BLAS.
<LI> If <IMG WIDTH=84 HEIGHT=24 ALIGN=MIDDLE ALT="tex2html_wrap_inline17103" SRC="img421.gif">, the major performance
factor may be bandwidth (<IMG WIDTH=28 HEIGHT=27 ALIGN=MIDDLE ALT="tex2html_wrap_inline12064" SRC="img2.gif">). This is
what one should obtain for medium values of <I>N</I>.
<LI> If <IMG WIDTH=84 HEIGHT=24 ALIGN=MIDDLE ALT="tex2html_wrap_inline17109" SRC="img422.gif">, the major performance
factor may be latency (<IMG WIDTH=18 HEIGHT=23 ALIGN=MIDDLE ALT="tex2html_wrap_inline12208" SRC="img30.gif">). This is what
one should obtain for small values of <I>N</I>.
</UL>
This performance
analysis suggests which computer characteristic
is most likely limiting the performance. It
cannot say whether
one is getting good performance.
<P>
<BR> <HR>
<P><ADDRESS>
<I>Susan Blackford <BR>
Tue May 13 09:21:01 EDT 1997</I>
</ADDRESS>
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