Timing discrepancy between in Netlogo

Question

Can anyone explain why there is a performance difference between the following two segments? It's statistically significant that the second timer call reports a smaller number than the first timer call. My only thoughts would be that Netlogo could be caching the turtles in memory. Is this the expected behavior or is there a bug?

to setup
  clear-all
  crt 100
  let repetitions 10000

  ;;Timing assigning x to self
  reset-timer
  repeat repetitions
  [
   ask turtles
   [
     let x self 
   ] 

  ]
  show timer

  ;;Timing assigning x to who of self
  reset-timer
  repeat repetitions
  [
    ask turtles
    [
     let x [who] of self 
    ]
  ]
  show timer
end

score 6 · Accepted Answer · edited Mar 28 '16 at 15:49

This isn't because of anything in NetLogo itself, but rather because NetLogo runs on the JVM. The JVM learns to optimize code the more it runs it as part of its just-in-time compilation (JIT).

By the time the second segment is run, the JVM has had time to optimize many code paths that the two segments have in common. Indeed, switching the order of the segments, I got the following results:

observer> setup
observer: 0.203
observer: 0.094
observer> setup
observer: 0.136
observer: 0.098
observer> setup
observer: 0.13
observer: 0.097
observer> setup
observer: 0.119
observer: 0.095
observer> setup
observer: 0.13
observer: 0.09

Now the let x self code is faster (it's now the second thing that runs)! Notice also that both times decrease the more I ran setup. This is also due to the JVM's JIT.

Similarly, if I turn off view updates and run your original code, I get:

observer> setup
observer: 0.088
observer: 0.071
observer> setup
observer: 0.094
observer: 0.072
observer> setup
observer: 0.065
observer: 0.075
observer> setup
observer: 0.067
observer: 0.071
observer> setup
observer: 0.067
observer: 0.068

The let x self code starts out slower (for the reason above) and then becomes about the same speed, as one might expect. There are many possible reasons as to why this only happens with view updates off. NetLogo is doing a lot less with view updates off

The JVM's JIT is extremely optimized, yet complicated, and it can be hard to reason about. There's a lot to consider if you want to write truly correct micro-benchmarks.

Timing discrepancy between in Netlogo

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