[Numpy-discussion] Apparently non-deterministic behaviour of complex array multiplication

[kneil]

OK - here is the status update on this problem:
1. To test for bad RAM, I saved the code plus data onto a usb drive and
manually transferred it to a colleague's desktop machine with same specs as
mine.  The NaN values continued to appear at random, so it is unlikely to be
bad RAM - unless its contagious.

2. Here is how I am temporarily working around the problem:  Right before
performing the multiplication of X*X.H where X=asmatrix(Y), I save X to file
using np.save('X',X).   Then I reload it via X=np.load('X.npy').  Cast as
matrix: X=asmatrix(X); and carry on: S=X*X.H.  I have not seen any NaN
although I have only run it a few times... but it seems to work.

I'd be very curious to hear any ideas on why this problem exists in the
first place, and why save/load gets me around it.

For the record I am running ubuntu 11.04, have 16GB RAM (not 12) and use
python2.7.
cheers, 
Karl

Hi Pierre,

I confirmed with the guy who put together the machine that it is non-ECC
RAM.  You know, now that i think about it, this machine seems to crash a
fair amount more often than its identical twin which sits on a desk near me. 
I researched memtest a bit... downloaded and compiled it, but I do not quite
understand the finer points of using it... it seems that I want to remove my
RAM cards and test them one at a time.  Do you know a good reference for
using it.  

I think at this point the best thing to do will be to dump my data/code to
portable HDD and load it on the other computer with same specs as this one. 
If it runs without generating any NaN then I will proceed to a full memtest.

Thanks for the advice.
-Karl

On Thu, Dec 1, 2011 at 2:47 PM, kneil <magnetotellurics at gmail.com> wrote:

Are you using non-ECC RAM, by chance?  (Though if you have >4GB of ram, I
can't imagine that you wouldn't be using ECC...)

Alternately, have you run memtest lately?  That sound suspiciously like bad
ram...



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