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Re: Eigensystem Eigenvalues fail for 34x34 sparse matric but not for 33x33

• To: mathgroup at smc.vnet.net
• Subject: [mg56433] Re: Eigensystem Eigenvalues fail for 34x34 sparse matric but not for 33x33
• From: Scott Hemphill <hemphill at hemphills.net>
• Date: Mon, 25 Apr 2005 01:30:48 -0400 (EDT)
• References: <d4fi04$frb$1@smc.vnet.net>
• Reply-to: hemphill at alumni.caltech.edu
• Sender: owner-wri-mathgroup at wolfram.com

barrowes at alum.mit.edu writes:

> After upgrading to 5.1.1, Eigensystem, Eigenvalues, and Eigenvectors
> fail for the following sparse matrix, but only if the square matrix
> size is >= 34x34. It works fine for 33x33. It works fine for all
> attemped sizes on this type of matrix in 5.1.
> 
> Consider the following 34x34 banded matrix:

Are you using a multiprocessor kernel?  Do you have an SMP machine, or
a single processor with HT?

If so, then you may be running into a case where Mathematica decides to
use multiple threads for matrix size >= 34x34.  If you are running too
new a glibc, then you are using an incompatible thread library.  You can
fix this by putting "export LD_ASSUME_KERNEL=2.4.1" in the script which
invokes Mathematica.

If in addition you only have a single processor with hyperthreading, you
may also which to put the line "export OMP_NUM_THREADS=1" in your script.
This will prevent Mathematica from using multiple threads, which in my
testing, actually slows Mathematica down slightly.

Scott
-- 
Scott Hemphill	hemphill at alumni.caltech.edu
"This isn't flying.  This is falling, with style."  -- Buzz Lightyear