Mechanical APDL Basic Analysis Guide - Ansys

5.3.3. Disk Space (I/O) and Postprocessing Performance for Large Memory
Problems
I/O performance with large memory One of the hidden system benefits of large memory systems is the
ability to cache large I/O requests. Even for modest-sized ANSYS jobs, you can considerably reduce the cost
of I/O when the system free memory is larger than the sum of file sizes active in an ANSYS job. This feature,
often called buffer cache, is a system-tunable parameter and can effectively move all I/O traffic to memory
copy speeds. The system details are different for various vendors; consult your hardware manufacturer for
details on their systems. For most Linux versions and Windows X64 systems, the benefit of the system buffer
cache is automatic and does not require tuning. IBM and HP system caches may require some tuning; consult
your hardware vendor for details. A large memory system will often perform at almost in-core memory
performance with the sparse solver when the system memory size is larger than the matrix factorization file
(usually file.LN09 or file.LN07), even when the sparse solver runs in out-of-core mode.
5.3.4. Memory Usage on Windows 32-bit Systems
If you are running on a 32-bit Windows system, you may encounter memory problems due to Windows'
handling of contiguous memory blocks. Windows 32-bit systems limit the maximum continuous block of
memory to 2 GB; setting the /3GB switch will add another gigabyte of memory, but not contiguous with the
initial 2 GB. (See the ANSYS, Inc. Windows Installation Guide for information on setting the /3GB switch).
Running the PCG solver with the /3GB switch set will be sufficient in many situations, as will running the
sparse solver with a reasonably large -db setting and a -m setting of just 50 MB more than the -db setting.
However, to maximize your system's performance for large models, you need to:
1. Learn the largest -m you can use on your machine.
2. Learn how much memory solving your job will require.
3. Optimize your job and your system to take advantage of your system's capabilities.
Learn your -m limits To find out the largest -m setting you can use on your machine, use the following
procedure. The maximum number you come up with will be the upper bound on the largest contiguous
block of memory you can get on your system.
1. Open a command window and type:
ansys130 -m 1200 -db 64.
2. If that command successfully launches ANSYS, close ANSYS and repeat the above command, increasing
the -m value by 50 each time, until ANSYS issues an error message that it has insufficient memory and
fails to start. Be sure to specify the same -db value each time.
Ideally, you will be able to successfully launch ANSYS with a -m of 1700 or more, although 1400 is more
typical. A -m of 1200 indicates that you may have some DLLs in your user space; contact your system administrator
for suggestions on cleaning up your user space.
Learn your memory requirements ANSYS offers the BCSOPTION command to determine how much
memory your job will require (when running the shared-memory sparse solver). Use this command as a
habit to determine how much memory you need, and set your -m and -db appropriately. Too little memory,
and your job will not run. However, setting an unnecessarily high -m will prevent ANSYS from using available
memory to reduce I/O time. To use this command, add the following to your input:
BCSOPTION,,,,,,PERFORMANCE
5.3.4. Memory Usage on Windows 32-bit Systems
Release 13.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information
of ANSYS, Inc. and its subsidiaries and affiliates.
105