MEMORY_TEST
How Big Can Arrays Be?

MEMORY_TEST 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.

The program tries an increasing series of values of N, 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:

Log(N)	N
20	1,048,576
21	2,097,152
22	4,194,304
23	8,388,608
24	16,777,216
25	33,554,432
26	67,108,864
27	134,217,728
28	268,435,456
29	536,870,912
30	1,073,741,824

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.

Problems with Large Arrays:

The use of very large arrays in a perfectly legal program can cause a failure that appears as a "segmentation fault".

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.

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.

You could allocate the arrays in the calling program.

On Unix, you can determine the current stacksize, in kilobytes, by the command

        ulimit -S -s
      

        ulimit -S -s 16384
      

        ulimit -s unlimited
      

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 ifort compiler, the switch -heap-arrays [n] means that arrays larger than n will be assigned memory from the heap rather than the stack.

Usage:

memory_test log_n_min log_n_max

Licensing:

The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.

Languages:

MEMORY_TEST is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version.

Related Data and Programs:

LINPACK_BENCH, a FORTRAN90 program which measures the time needed to factor and solve a linear system.

MACHINE, a FORTRAN90 library which can return various machine constants.

MATMUL, a FORTRAN90 program which is an interactive matrix multiplication benchmark program.

MDBNCH, a FORTRAN77 program which is a benchmark code for a molecular dynamics calculation.

TIMER, a FORTRAN90 program which demonstrates how to compute CPU time or elapsed time.

Source Code:

• memory_test.f90, the source code.

Examples and Tests:

• memory_test_aspen.txt, a run of the program on Aspen, a SUN workstation, up to LOG(N) = 27.
• memory_test_chili.txt, a run of the program on Chili, an ALPHA workstation, up to LOG(N) = 27.
• memory_test_class15.txt, a run of the program on Class15, an ALPHA workstation, up to LOG(N) = 27.
• memory_test_g5.txt, a run of the program on an Apple G5, 20 <= LOG(N) <= 27.
• memory_test_phoenix.txt, a run of the program on (one node of) Phoenix, a LINUX cluster, up to LOG(N) = 26.
• memory_test_quadrature.txt, a run of the program on Quadrature, a LINUX workstation, up to LOG(N) = 27.

List of Routines:

• MAIN is the main program for MEMORY_TEST.
• I4MAT_ALLOCATED_MEMORY_TEST declares and uses an I4MAT of size N.
• I4VEC_MEMORY_ALLOCATED_TEST uses an allocated I4VEC of size N.
• I4VEC_MEMORY_AUTOMATIC_TEST uses an automatic I4VEC of size N.
• R4MAT_MEMORY_ALLOCATED_TEST declares and uses an R4MAT of size N.
• R4VEC_MEMORY_ALLOCATED_TEST uses an allocated R4VEC of size N.
• R4VEC_MEMORY_AUTOMATIC_TEST uses an automatic R4VEC of size N.
• R8MAT_MEMORY_ALLOCATED_TEST uses an allocated R8MAT of size N.
• R8VEC_MEMORY_ALLOCATED_TEST uses an allocated R8VEC of size N.
• R8VEC_MEMORY_AUTOMATIC_TEST uses an automatic R8VEC of size N.
• REAL_TIME returns the real time in seconds.
• S_TO_I4 reads an integer value from a string.
• TIMESTAMP prints the current YMDHMS date as a time stamp.

You can go up one level to the FORTRAN90 source codes.