Question: 'Simplify' gives different values before/after gc


      I've noticed a strange phenomenon involving simplify: given the exact same input, its output varies between two different results depending on memory usage patterns.

      I've attached a sample code. There, I have an equation which should simplify to 0=0. If I run the code with garbage collection gc uncommented, it will correctly give 0=0. However, with gc commented out, it gives the unsimplified result result ~80% of the time (with 0=0 the remaining 20%):

2^(1/2)*k^(1/2)*(k*p*cos(psi__p)+(k^2*p^2+2*k*p*cos(psi__p)+1)^(1/2)+1)^(1/2)*t)) = 0

For my particular machine, it seems that when the memory usage is <98.4MB, it gives 0=0, and the unsimplied case otherwise.

      Any idea why this is? It seems odd that simplify would return different results depending on garbage collection, especially since use of gc is now discouraged.


A few notes:

  • It seems replacing gc with a simple call to kernelopts(memusage) also produces the correct output
  • This is a snippet of a larger body of code I wrote: there, even without gc, it will sporadically (with the exact same input) produce the correct answer ~50% of the time
  • Only the last 4 lines are relevant; the beginning of the attached code is simply to generate appropriate memory usage and doesn't affect the relevant, final 4 lines
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