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memory_test.html
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<html>
<head>
<title>
MEMORY_TEST - How Big Can Arrays Be?
</title>
</head>
<body bgcolor="#EEEEEE" link="#CC0000" alink="#FF3300" vlink="#000055">
<h1 align = "center">
MEMORY_TEST <br> How Big Can Arrays Be?
</h1>
<hr>
<p>
<b>MEMORY_TEST</b>
is a FORTRAN90 program which
declares and uses a sequence of larger and larger arrays,
to see what the memory limits are on a given computer.
</p>
<p>
The program tries an increasing series of values of <b>N</b>, using
powers of 2, between limits that you set. At some point, the program
may ask for more memory than can be provided, and crash. This is one
way to find out what the memory ceiling is! The relevant power of 2
is likely to be in the 20's:
<table border="1">
<tr>
<th>Log(N)</th><th>N</th>
</tr>
<tr>
<td>20</td><td>1,048,576</td>
</tr>
<tr>
<td>21</td><td>2,097,152</td>
</tr>
<tr>
<td>22</td><td>4,194,304</td>
</tr>
<tr>
<td>23</td><td>8,388,608</td>
</tr>
<tr>
<td>24</td><td>16,777,216</td>
</tr>
<tr>
<td>25</td><td>33,554,432</td>
</tr>
<tr>
<td>26</td><td>67,108,864</td>
</tr>
<tr>
<td>27</td><td>134,217,728</td>
</tr>
<tr>
<td>28</td><td>268,435,456</td>
</tr>
<tr>
<td>29</td><td>536,870,912</td>
</tr>
<tr>
<td>30</td><td>1,073,741,824</td>
</tr>
</table>
</p>
<p>
Remember that your memory is probably described in terms of bytes,
but that integers and reals require 4 bytes, and double precision reals
require 8 bytes.
</p>
<h3 align = "center">
Problems with Large Arrays:
</h3>
<p>
The use of very large arrays in a perfectly legal program
can cause a failure that appears as a "segmentation fault".
</p>
<p>
This might be caused, for instance, by the use of automatic arrays
in a subprogram. An automatic array is one whose dimension
is passed into the subprogram as an argument, but whose memory
is only created when the subprogram is entered. Some compilers,
presumably expecting such arrays to be small, use memory from
the stack; if the available memory is exceeded, the program crashes.
</p>
<p>
You might be able to make such an error go away by replacing the
automatic arrays by explicitly allocated arrays. Their memory
comes from the heap, which is, typically, a more substantial
resource.
</p>
<p>
You could allocate the arrays in the calling program.
</p>
<p>
On Unix, you can determine the current stacksize, in kilobytes, by the
command
<pre>
ulimit -S -s
</pre>
My system reported the value as "8192". You can increase this limit to
a numeric value:
<pre>
ulimit -S -s 16384
</pre>
or simply ask that it be unlimited using the command:
<pre>
ulimit -s unlimited
</pre>
</p>
<p>
Alternatively, your FORTRAN compiler may have a switch that controls
the size of the stack, or that changes the method of getting memory
for automatic arrays. For the Intel <b>ifort</b> compiler, the
switch <b>-heap-arrays [n]</b> means that arrays larger than
<b>n</b> will be assigned memory from the heap rather than the stack.
</p>
<h3 align = "center">
Usage:
</h3>
<p>
<blockquote>
<b>memory_test</b> <i>log_n_min</i> <i>log_n_max</i>
</blockquote>
runs the program for sizes <b>N</b> = 2<sup>log_n_min</sup> to
2<sup>log_n_max</sup>.
</p>
<h3 align = "center">
Licensing:
</h3>
<p>
The computer code and data files described and made available on this web page
are distributed under
<a href = "../../txt/gnu_lgpl.txt">the GNU LGPL license.</a>
</p>
<h3 align = "center">
Languages:
</h3>
<p>
<b>MEMORY_TEST</b> is available in
<a href = "../../c_src/memory_test/memory_test.html">a C version</a> and
<a href = "../../cpp_src/memory_test/memory_test.html">a C++ version</a> and
<a href = "../../f_src/memory_test/memory_test.html">a FORTRAN90 version</a> and
<a href = "../../m_src/memory_test/memory_test.html">a MATLAB version</a>.
</p>
<h3 align = "center">
Related Data and Programs:
</h3>
<p>
<a href = "../../f_src/linpack_bench/linpack_bench.html">
LINPACK_BENCH</a>,
a FORTRAN90 program which
measures the time needed to factor and solve a linear system.
</p>
<p>
<a href = "../../f_src/machine/machine.html">
MACHINE</a>,
a FORTRAN90 library which
can return various machine constants.
</p>
<p>
<a href = "../../f_src/matmul/matmul.html">
MATMUL</a>,
a FORTRAN90 program which
is an interactive
matrix multiplication benchmark program.
</p>
<p>
<a href = "../../f77_src/mdbnch/mdbnch.html">
MDBNCH</a>,
a FORTRAN77 program which
is a benchmark
code for a molecular dynamics calculation.
</p>
<p>
<a href = "../../f_src/timer/timer.html">
TIMER</a>,
a FORTRAN90 program which
demonstrates how to compute CPU time or elapsed time.
</p>
<h3 align = "center">
Source Code:
</h3>
<p>
<ul>
<li>
<a href = "memory_test.f90">memory_test.f90</a>, the source code.
</li>
<li>
<a href = "memory_test.sh">memory_test.sh</a>,
commands to compile the source code.
</li>
</ul>
</p>
<h3 align = "center">
Examples and Tests:
</h3>
<p>
<ul>
<li>
<a href = "memory_test_output_aspen.txt">memory_test_output_aspen.txt</a>,
a run of the program on Aspen, a SUN workstation,
up to LOG(N) = 27.
</li>
<li>
<a href = "memory_test_output_chili.txt">memory_test_output_chili.txt</a>,
a run of the program on Chili, an ALPHA workstation,
up to LOG(N) = 27.
</li>
<li>
<a href = "memory_test_output_class15.txt">memory_test_output_class15.txt</a>,
a run of the program on Class15, an ALPHA workstation,
up to LOG(N) = 27.
</li>
<li>
<a href = "memory_test_output_g5.txt">memory_test_output_g5.txt</a>,
a run of the program on an Apple G5,
20 <= LOG(N) <= 27.
</li>
<li>
<a href = "memory_test_output_phoenix.txt">memory_test_output_phoenix.txt</a>,
a run of the program on (one node of) Phoenix, a LINUX cluster,
up to LOG(N) = 26.
</li>
<li>
<a href = "memory_test_output_quadrature.txt">memory_test_output_quadrature.txt</a>,
a run of the program on Quadrature, a LINUX workstation,
up to LOG(N) = 27.
</li>
</ul>
</p>
<h3 align = "center">
List of Routines:
</h3>
<p>
<ul>
<li>
<b>MAIN</b> is the main program for MEMORY_TEST.
</li>
<li>
<b>I4MAT_ALLOCATED_MEMORY_TEST</b> declares and uses an I4MAT of size N.
</li>
<li>
<b>I4VEC_MEMORY_ALLOCATED_TEST</b> uses an allocated I4VEC of size N.
</li>
<li>
<b>I4VEC_MEMORY_AUTOMATIC_TEST</b> uses an automatic I4VEC of size N.
</li>
<li>
<b>R4MAT_MEMORY_ALLOCATED_TEST</b> declares and uses an R4MAT of size N.
</li>
<li>
<b>R4VEC_MEMORY_ALLOCATED_TEST</b> uses an allocated R4VEC of size N.
</li>
<li>
<b>R4VEC_MEMORY_AUTOMATIC_TEST</b> uses an automatic R4VEC of size N.
</li>
<li>
<b>R8MAT_MEMORY_ALLOCATED_TEST</b> uses an allocated R8MAT of size N.
</li>
<li>
<b>R8VEC_MEMORY_ALLOCATED_TEST</b> uses an allocated R8VEC of size N.
</li>
<li>
<b>R8VEC_MEMORY_AUTOMATIC_TEST</b> uses an automatic R8VEC of size N.
</li>
<li>
<b>REAL_TIME</b> returns the real time in seconds.
</li>
<li>
<b>S_TO_I4</b> reads an integer value from a string.
</li>
<li>
<b>TIMESTAMP</b> prints the current YMDHMS date as a time stamp.
</li>
</ul>
</p>
<p>
You can go up one level to <a href = "../f_src.html">
the FORTRAN90 source codes</a>.
</p>
<hr>
<i>
Last revised on 17 January 2011.
</i>
<!-- John Burkardt -->
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