Session files

Clean up is determined by the PHP runtime configuration items gc_probability divided by gc_divisor. That ratio defines the probability that the GC (garbage collection) process is started on any session initialization. So, given 1/100 there is a 1% chance that the GC process starts. (docs)

I've highlighted probability and chance to point out there is no guarantee GC will happen once every 100 times per the 1/100 example. It could happen twice in a row and then not again for 200 (and potentially many more) session initializations.