[ET Trac] [Einstein Toolkit] #1302: Reduce time spent in deciding not to do output.

[Einstein Toolkit]

#1302: Reduce time spent in deciding not to do output.
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 Reporter:  diener                    |       Owner:  eschnett           
     Type:  enhancement               |      Status:  new                
 Priority:  major                     |   Milestone:                     
Component:  Carpet                    |     Version:  development version
 Keywords:  performance optimization  |  
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 I noticed, during some scaling tests, that for my code a significant
 amount of time was spent by CarpetIOASCII, even though I only had it
 activated and didn't actually request any output. The reason is that in
 order to figure out whether to do output or not, there is a loop over all
 grid variables and a routine (TimeToOutput) is called. In this
 routine there is a check if the out_dir and out_vars parameters have been
 steered and if so update some internal data structures. It should be
 sufficient to do this before entering the loop over grid variables. The
 same issue is present in CarpetIOScalar and CarpetIOHDF5. The attached
 patches for CarpetIOASCII, CarpetIOScalar and CarpetIOHDF5 moves this
 check outside of the loop over grid variables and in addition bypasses the
 loop completely if the out_vars parameter string is the empty string. Note
 that the code where I noticed this problem uses large vectors of 1D grid
 arrays, and it turns out that Cactus counts each vector element as a
 distinct grid variables and the length of the loop over grid variables in
 my case was close to 100.000, which might explain why nobody has noticed
 this before.

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Ticket URL: <https://trac.einsteintoolkit.org/ticket/1302>
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