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Starting from the 2nd call, setns() calls synchronize_rcu in kernel. Result -- 4 setns are 6% of 4 tests dump time.
Starting from the 2nd call, setns() calls synchronize_rcu in kernel. Result -- 4 setns are 6% of 4 tests dump time.
=== fopen/fgets/fclose ===
On some (all?) glibcs when doing fopen, fgets, fclose glibc mmap-s and then immediatelly unmaps one anon page. The thing is that for a tree of ~20 tasks these mmaps/munmaps take up to 8% of time and most likely work on the same virtual address. Knowing how kernel works, I can say that each access to this virtual address results in fresh page allocation. Keeping this page "in cache" might save some time.