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<TITLE>7.5 Dummy Arguments and Modularity</TITLE>
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<B> Next:</B> <A NAME=tex2html3009 HREF="node88.html" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node88.html">7.6 Other HPF Features</A>
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<B> Previous:</B> <A NAME=tex2html3001 HREF="node86.html" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node86.html">7.4 Concurrency</A>
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<H1><A NAME=SECTION03450000000000000000>7.5 Dummy Arguments and Modularity</A></H1>
<P>
<A NAME=sechpfmod> </A>
<P>
<A NAME=11204> </A>
A large HPF program is typically constructed as a sequence of calls to
<A NAME=11205> </A>
subroutines and functions that implement different aspects of program
<A NAME=11206> </A>
logic. In the terminology used in Chapter <A HREF="node39.html#chapmod" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node39.html#chapmod">4</A>, the program is a
<A NAME=11208> </A>
sequential composition of program components. As discussed in
that chapter, one critical issue that arises when using
<A NAME=11209> </A>
sequential composition is the distribution of data structures that are
<A NAME=11210> </A>
shared by components.
<P>
Consider what happens when a subroutine is called in an HPF program.
For a particular computer and problem size, there is presumably a
distribution of that subroutine's dummy arguments and local variables
that is optimal in the sense that it minimizes execution time in that
subroutine. However, this optimal distribution may not correspond to the distribution
specified in the calling program for variables passed as arguments.
Hence, we have a choice of two different strategies at a subroutine
interface. These strategies, and the HPF mechanisms that support
them, are as follows.
<P>
<OL><LI>
We can ignore the distribution used in the calling program and specify
a local distribution that must apply, even if it requires a
(potentially expensive) remapping operation. For this purpose, we
use the distribution directives <tt> DISTRIBUTE</tt> and <tt> ALIGN</tt>
to specify the mapping of the dummy
arguments.
<P>
<LI>
We can use whatever data distribution is used in the calling program,
even though it may not be locally optimal. For this purpose, we
use the <tt> INHERIT</tt> directive.
<P>
</OL>
<P>
As noted in Chapter <A HREF="node39.html#chapmod" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node39.html#chapmod">4</A>, several tradeoffs
must be
evaluated when determining which strategy to adopt in a particular
circumstance. The cost of the remapping inherent in strategy 1 should
be weighed against the performance degradation that may occur if
strategy 2 is used. Similarly, the effort required to optimize a
subroutine for a particular distribution must be weighed against the
subroutine's execution cost and frequency of use. These tradeoffs are
more complex if a subroutine may be used in several contexts. In some
cases, it may be worthwhile for a subroutine to incorporate different
code for different distributions.
<P>
<H4><A NAME=SECTION03450010000000000000> Strategy 1: Remap Arguments.</A></H4>
<P>
Strategy 1 is straightforward to apply. Ordinary distribution
directives are applied to dummy arguments. As for any other variable,
<A NAME=11218> </A>
these directives recommend that the requested distribution hold. Any
necessary data movement is performed automatically when the subroutine
or function is called. (In the absence of a <tt> DISTRIBUTE</tt> or <tt>
ALIGN</tt> directive for a dummy argument, the compiler may choose to use
any distribution or alignment.) Any redistribution is undone upon
return from the subroutine, so any data movement costs introduced in
this way are incurred twice. The exception to this rule are arguments
used for input or output only, as specified by the use of the F90 <tt>
intent</tt> directive.
<P>
Program <A HREF="node87.html#proghpffft" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node87.html#proghpffft">7.5</A> illustrates some of the issues involved in
strategy 1. Arrays <tt> X</tt> and <tt> Y</tt> are distributed by rows and
columns in the calling program, respectively, while the dummy argument
<tt> Z</tt> of the subroutine <tt> fft</tt> is distributed by columns. Hence,
the first call to <tt> fft</tt> requires that two matrix transpose
operations be performed to convert from one distribution to the
other---one upon entry to and one upon exit from the routine. In
contrast, the second call to <tt> fft</tt> does not require any data
movement because the array <tt> Y</tt> is already distributed
appropriately.
<P>
<P><A NAME=proghpffft> </A><IMG BORDER=0 ALIGN=BOTTOM ALT="" SRC="img978.gif" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/img978.gif"><P><H4><A NAME=SECTION03450020000000000000> Strategy 2: Use Parent Mapping.</A></H4>
<P>
The second strategy is supported by the <tt> INHERIT</tt> directive, which,
however, does not form part of the HPF subset because of the difficulty of
<A NAME=11253> </A>
generating code that can handle multiple distributions. For that
reason, we do not consider this language feature in detail.
<P>
The following code fragment illustrates the use of <tt> INHERIT</tt>. This
is an alternative version of the <tt> fft</tt> routine in
Program <A HREF="node87.html#proghpffft" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node87.html#proghpffft">7.5</A>. The <tt> INHERIT</tt> directive indicates that
no remapping is to occur; hence, the two calls to <tt> fft</tt> in
Program <A HREF="node87.html#proghpffft" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node87.html#proghpffft">7.5</A> will execute with <tt> (BLOCK,*)</tt> and <tt>
(*,BLOCK)</tt> distribution, respectively.
<A NAME=11262> </A>
<P>
<PRE><TT>
<tt> subroutine fft(n, Z)</tt>
<P>
<tt> real Z(n,n)</tt>
<P>
<tt> ...</tt>
<P>
<tt> !HPF$</tt> <tt> INHERIT Z</tt> ! <tt> Z</tt> has parent mapping
<P>
<tt> ...</tt>
<P>
<tt> end</tt>
<P>
</TT></PRE>
<P>
<A NAME=11273> </A>
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<B> Next:</B> <A NAME=tex2html3009 HREF="node88.html" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node88.html">7.6 Other HPF Features</A>
<B>Up:</B> <A NAME=tex2html3007 HREF="node82.html" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node82.html">7 High Performance Fortran</A>
<B> Previous:</B> <A NAME=tex2html3001 HREF="node86.html" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node86.html">7.4 Concurrency</A>
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<I>© Copyright 1995 by <A href="msgs0.htm#6" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/tppmsgs/msgs0.htm#6">Ian Foster</a></I>
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