First benchmark is slower than the rest

[mooreryan]

I've noticed that when running benchmarks, the first one that is specified is slower than the rest, even when the same function with the same inputs is benchmarked repeatedly.

Here is a simple example:

open! Core

let rec fib n = if n < 2 then n else fib (n - 1) + fib (n - 2)

let () =
  let bench name f = Core_bench.Bench.Test.create ~name f in
  Command_unix.run
    (Core_bench.Bench.make_command
       [
         bench "fib 1" (fun () -> fib 10);
         bench "fib 2" (fun () -> fib 10);
         bench "fib 3" (fun () -> fib 10);
       ])

And the result of running time dune exec ./bench/a.exe -- -ascii with a couple different values for fib to see if the behavior changes when shorter or longer running functions are being benched.

  Name    Time/Run   Percentage  
 ------- ---------- ------------ 
  fib 1    30.12ns      100.00%  
  fib 2    18.49ns       61.39%  
  fib 3    18.41ns       61.11%  


  Name    Time/Run   Percentage  
 ------- ---------- ------------ 
  fib 1   317.15ns      100.00%  
  fib 2   216.46ns       68.25%  
  fib 3   215.50ns       67.95%  


  Name    Time/Run   Percentage  
 ------- ---------- ------------ 
  fib 1    35.66us      100.00%  
  fib 2    26.90us       75.44%  
  fib 3    26.97us       75.63%  


  Name    Time/Run   Percentage  
 ------- ---------- ------------ 
  fib 1     4.42ms      100.00%  
  fib 2     3.31ms       74.88%  
  fib 3     3.32ms       75.00%  

Is there something wrong with the way I'm setting up the benchmarks that could be causing this?