I’ve installed Piwik some monthes ago, and after a server change, I’ve noticed that the /tmp/sessions is full of files (some of them are very old).
So, maybe, it’s possible to delete some of these files, for performance purpose of course ?
You’ll also want to check session.gc_probability is non-zero (which I mentioned in my first reply but omitted in your response). It seems Debian (not sure about derivatives) ships with this unset (thus, treated as zero, 0) … meaning garbage collection never runs (regardless of gc_divisor).