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<H1><A NAME=SECTION03570000000000000000>8.7 Performance Issues</A></H1>
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<A NAME=13364>&#160;</A>
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The performance analysis techniques developed in
Chapter <A HREF="node26.html#chapperf" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node26.html#chapperf">3</A> can be applied directly to MPI programs.  We
discuss some relevant costs here.
<P>
An <tt> MPI_SEND</tt>/<tt> MPI_RECV</tt> pair communicates a single message.
The cost of this communication can be modeled with
Equation <A HREF="node29.html#eq88" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node29.html#eq88">3.1</A>.
The cost
of the blocking <tt> MPI_PROBE</tt> operation will normally be similar to
that of an ordinary receive.  The cost of the nonblocking <tt>
MPI_IPROBE</tt> operation can vary significantly according to
implementation: in some implementations it may be negligible, while in
others it can be higher than an ordinary receive.
<P>
The performance of global operations is less straightforward to
analyze, as their execution can involve considerable internal
communication.  Implementations of these functions can normally be
expected to use efficient communication algorithms, such as the
hypercube algorithms described in Chapter <A HREF="node123.html#chapcube" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node123.html#chapcube">11</A>.  In the
absence of bandwidth limitations, these allow a barrier to complete in
<IMG BORDER=0 ALIGN=MIDDLE ALT="" SRC="img1047.gif" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/img1047.gif"> steps on
<em> P</em>
 processors, a broadcast of <em> N</em>
 words to proceed in
time <IMG BORDER=0 ALIGN=MIDDLE ALT="" SRC="img1048.gif" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/img1048.gif"> if <em> N</em>
 is small and in time <IMG BORDER=0 ALIGN=MIDDLE ALT="" SRC="img1049.gif" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/img1049.gif"> if <em> N</em>
 is large, and so on.  The 
costs associated with these algorithms are summarized in
Table <A HREF="node101.html#tabmpicomm" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node101.html#tabmpicomm">8.1</A>.  Remember that on many architectures,
bandwidth limitations can increase actual costs, especially for larger
messages (Section<A HREF="node33.html#secperfrefine" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node33.html#secperfrefine">3.7</A>).
<P>
<P><A NAME=13843>&#160;</A><IMG BORDER=0 ALIGN=BOTTOM ALT="" SRC="img1052.gif" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/img1052.gif">
<BR><STRONG>Table 8.1:</STRONG>  Communication costs associated with various MPI global
<A NAME=14216>&#160;</A>
operations when implemented using hypercube communication algorithms
on the idealized multicomputer architecture.  The term <IMG BORDER=0 ALIGN=MIDDLE ALT="" SRC="img1051.gif" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/img1051.gif">
represents the cost of a single reduction
operation.
<A NAME=tabmpicomm>&#160;</A><BR>
<P>
<P>
The <tt> MPI_COMM_DUP</tt>, <tt> MPI_COMM_SPLIT</tt>, and <tt>
MPI_COMM_FREE</tt> operations should not normally involve communication.
The cost of the <tt> MPI_INTERCOMM_CREATE</tt> operation is
implementation dependent, but will normally involve at least one
communication operation.  The cost of the <tt> MPI_INIT</tt> and <tt>
MPI_FINALIZE</tt> operations is implementation dependent and can be high.
However, these functions are called once only in a program execution.
Other functions can normally be expected to execute without
communication and with little local computation.
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<B> Next:</B> <A NAME=tex2html3186 HREF="node102.html" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node102.html">8.8 Case Study: Earth System Model</A>
<B>Up:</B> <A NAME=tex2html3184 HREF="node94.html" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node94.html">8 Message Passing Interface</A>
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