This adds a simple mmap microbenchmark that can be run on both OSv and
Linux.  The benchmark mmaps memory for various sizes and touches the
mmap'd memory in 4K increments to fault in memory.  The benchmark also
repeats the same tests using MAP_POPULATE for reference.

OSv page faults are slightly slower than Linux on first iteration but
faster on subsequent iterations after host operating system has faulted
in memory for the guest.

I've included full numbers on 2-core Sandy Bridge i7 for a OSv guest,
Linux guest, and Linux host below:

  OSv guest
  ---------

  Iteration 1

       time (seconds)
   MiB demand populate
     1 0.004  0.000
     2 0.000  0.000
     4 0.000  0.000
     8 0.001  0.000
    16 0.003  0.000
    32 0.007  0.000
    64 0.013  0.000
   128 0.024  0.000
   256 0.052  0.001
   512 0.229  0.002
  1024 0.587  0.005

  Iteration 2

       time (seconds)
   MiB demand populate
     1 0.001  0.000
     2 0.000  0.000
     4 0.000  0.000
     8 0.001  0.000
    16 0.002  0.000
    32 0.004  0.000
    64 0.010  0.000
   128 0.019  0.001
   256 0.036  0.001
   512 0.069  0.002
  1024 0.137  0.005

  Iteration 3

       time (seconds)
   MiB demand populate
     1 0.001  0.000
     2 0.000  0.000
     4 0.000  0.000
     8 0.001  0.000
    16 0.002  0.000
    32 0.005  0.000
    64 0.010  0.000
   128 0.020  0.000
   256 0.039  0.001
   512 0.087  0.002
  1024 0.138  0.005

  Iteration 4

       time (seconds)
   MiB demand populate
     1 0.001  0.000
     2 0.000  0.000
     4 0.000  0.000
     8 0.001  0.000
    16 0.002  0.000
    32 0.004  0.000
    64 0.012  0.000
   128 0.025  0.001
   256 0.040  0.001
   512 0.082  0.002
  1024 0.138  0.005

  Iteration 5

       time (seconds)
   MiB demand populate
     1 0.001  0.000
     2 0.000  0.000
     4 0.000  0.000
     8 0.001  0.000
    16 0.002  0.000
    32 0.004  0.000
    64 0.012  0.000
   128 0.028  0.001
   256 0.040  0.001
   512 0.082  0.002
  1024 0.166  0.005

  Linux guest
  -----------

  Iteration 1

       time (seconds)
   MiB demand populate
     1 0.001  0.000
     2 0.001  0.000
     4 0.002  0.000
     8 0.003  0.000
    16 0.005  0.000
    32 0.008  0.000
    64 0.015  0.000
   128 0.151  0.001
   256 0.090  0.001
   512 0.266  0.003
  1024 0.401  0.006

  Iteration 2

       time (seconds)
   MiB demand populate
     1 0.000  0.000
     2 0.000  0.000
     4 0.001  0.000
     8 0.001  0.000
    16 0.002  0.000
    32 0.005  0.000
    64 0.009  0.000
   128 0.019  0.001
   256 0.037  0.001
   512 0.072  0.003
  1024 0.144  0.006

  Iteration 3

       time (seconds)
   MiB demand populate
     1 0.000  0.000
     2 0.001  0.000
     4 0.001  0.000
     8 0.001  0.000
    16 0.002  0.000
    32 0.005  0.000
    64 0.010  0.000
   128 0.019  0.001
   256 0.037  0.001
   512 0.072  0.003
  1024 0.143  0.006

  Iteration 4

       time (seconds)
   MiB demand populate
     1 0.000  0.000
     2 0.001  0.000
     4 0.001  0.000
     8 0.001  0.000
    16 0.003  0.000
    32 0.005  0.000
    64 0.010  0.000
   128 0.020  0.001
   256 0.038  0.001
   512 0.073  0.003
  1024 0.143  0.006

  Iteration 5

       time (seconds)
   MiB demand populate
     1 0.000  0.000
     2 0.001  0.000
     4 0.001  0.000
     8 0.001  0.000
    16 0.003  0.000
    32 0.005  0.000
    64 0.010  0.000
   128 0.020  0.001
   256 0.037  0.001
   512 0.072  0.003
  1024 0.144  0.006

  Linux host
  ----------

  Iteration 1

       time (seconds)
   MiB demand populate
     1 0.000  0.000
     2 0.001  0.000
     4 0.001  0.000
     8 0.001  0.000
    16 0.002  0.000
    32 0.005  0.000
    64 0.009  0.000
   128 0.019  0.001
   256 0.035  0.001
   512 0.152  0.003
  1024 0.286  0.011

  Iteration 2

       time (seconds)
   MiB demand populate
     1 0.000  0.000
     2 0.000  0.000
     4 0.001  0.000
     8 0.001  0.000
    16 0.002  0.000
    32 0.004  0.000
    64 0.010  0.000
   128 0.018  0.001
   256 0.035  0.001
   512 0.192  0.003
  1024 0.334  0.011

  Iteration 3

       time (seconds)
   MiB demand populate
     1 0.000  0.000
     2 0.000  0.000
     4 0.001  0.000
     8 0.001  0.000
    16 0.002  0.000
    32 0.004  0.000
    64 0.010  0.000
   128 0.018  0.001
   256 0.035  0.001
   512 0.194  0.003
  1024 0.329  0.011

  Iteration 4

       time (seconds)
   MiB demand populate
     1 0.000  0.000
     2 0.000  0.000
     4 0.001  0.000
     8 0.001  0.000
    16 0.002  0.000
    32 0.004  0.000
    64 0.010  0.000
   128 0.018  0.001
   256 0.036  0.001
   512 0.138  0.003
  1024 0.341  0.011

  Iteration 5

       time (seconds)
   MiB demand populate
     1 0.000  0.000
     2 0.000  0.000
     4 0.001  0.000
     8 0.001  0.000
    16 0.002  0.000
    32 0.004  0.000
    64 0.010  0.000
   128 0.018  0.001
   256 0.035  0.001
   512 0.135  0.002
  1024 0.324  0.011

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>