Oasys LS-DYNA Environment 8.1 VOLUME 3 ... - Oasys Software

Oasys LS-DYNA Environment: User Guide (Version 8.1)
differ by as much as 10%.
Why do you get a difference ?
When any calculation is performed the answer is subject to numerical roundoffs. Normally these
roundoffs are not an issue but when calculations are performed over multiple CPU's the
roundoffs change due to the operations being performed in a different order.
Consider the following simple loop
DO I=1,80000
SUM = SUM + VAR(I)
END DO
One way to perform this loop in parallel on 4 CPU's is to break the problem done into 4 separate
loops and then sum together the results from the four loops.
(CPU 1) DO I=1,20000
SUM1 = SUM1 + VAR(I)
END DO
(CPU 2) DO I=20001,40000
SUM2 = SUM2 + VAR(I)
END DO
(CPU 3) DO I=40001,60000
SUM3 = SUM3 + VAR(I)
END DO
(CPU 4) DO I=60001,80000
SUM4 = SUM4 + VAR(I)
END DO
SUM = SUM1 + SUM2 + SUM3 +SUM4
Due to the different operation sequence the value of SUM will differ between the single CPU and
4 CPU calculations. If another variable, NORM, is now defined by subtracting SUM from a similar
value the roundoff, that is insignificant compared to SUM, becomes more significant. If in turn
NORM is used in another expression to divide another variable the results can be significantly
affected.
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