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Globally limiting precision or accuracy


Dear Group,

Situation
-  I have a large neural network model (my own code, not Wolfram's 
toolbox) that currently has a run time of about 20hours.
-  I am in the process of trying to profile and optimize the code 
(mostly matrix computations) to reduce the run time.
-  All of the computationas that I do are numerical.

Questions
(1) If I am not concerned about numerical accuracy beyond 3 decimal 
places for any of the computations in the model, can I improve 
performance by telling mathematica to globally restrict its accuracy (or 
precision) for all computations?

(2) If so, how do I do this?  Is it as simple as setting 
$MachinePrecision=3?  Is there a global way (rather than local use of 
N[]) to ensure that all computations are done numerically, and with 
machine precision?

Thanks,
Lee Newman


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