[Nix-dev] Re: Performance impact of Nix

[Ludovic Courtès]

Hello!

ludo at gnu.org (Ludovic Courtès) writes:

> In a recent discussion, Ralf Wildenhues [0] rightfully wondered about
> the performance impact of Nix' file system layout and/or that of the
> symlink indirection in profiles.

I did a quick experiment with the attached Guile script (run as "sudo
guile test-symlink.scm"), which shows these timings for stat(2) of a
regular file vs. stat(2) of a symlink pointing to that file:

  * regular file
  clock utime stime cutime cstime gctime
   1.14  0.74  0.38   0.00   0.00   0.21
  * symlink
  clock utime stime cutime cstime gctime
   1.46  0.86  0.59   0.00   0.00   0.30

Here it's 28% slower to stat(2) a symlink that to stat a plain regular
file.

I'm not sure how much can be extrapolated from this result, but it's
probably safe to say that symlinks do add noticeable overhead.

As for possible solutions, I don't have any idea.  Hard links won't work
across file systems, and it's not obvious that some unionfs would
perform better.  (GNU/Hurd has "firm links", which would be handy
here...)

Thanks,
Ludo'.

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