ScaLAPACK Archives

[Scalapack] Scalapack errorneous output

The reason one may expect different answers from LAPACK and ScaLAPACK
includes the facts that
(1) floating point operations may be done in a different order, due to 
parallelism,
and since floating point summation (for example) is not associative, the 
rounding
errors will differ, and you'll get different answers, and
(2) in some cases different algorithms are used in LAPACK and ScaLAPACK 
since the
most efficient parallel algorithm and most efficient sequential 
algorithm are not
necessarily the same.

Depending on the condition number of your problem, either of these may 
lead to
large or small changes in the output. And of course there may be bugs 
somewhere ...

Jim Demmel


On 5/6/15 7:54 AM, julie wrote:
Dear Suzy,
 You can send it to the mailing list: scalapack@Domain.Removed 
<mailto:scalapack@Domain.Removed> (just reply to my email )
Sincerely,
Julie
On May 5, 2015, at 12:42 PM, Susini de Silva <susini86@Domain.Removed 
<mailto:susini86@Domain.Removed>> wrote:

Hi!

This is a question regarding implementation of Scalapack on an 
algorithm that currently runs correctly using Lapack.

I have been spending a good amount of time editing this very large 
code that uses multiple LAPACK calls to use corresponding scalapack 
calls instead. I'm making sure the operand arrays are distributed, 
array descriptors are used before every call. However the final 
output of the Scalapack version of my code turned out to be 
significantly different from the final output of Lapack version. So I 
decided to cross compare the outputs after every Lapack/Scalapack 
call. I do see a slight difference. Is this possible? Has anyone seen 
this issue before, or do you know why this could happen?

Please, I'm at a dead-end at the moment :shock: , and any 
comments/suggestions would be greatly helpful!

I just made an account on Lapack User Forums and tried to post this. 
However it was flagged as spam even before I submitted it. I'd be 
really grateful if you can help me with this, or direct me to the 
right resources.

Thanks!
Suzy
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