Why kinfo_getvmmap is sometimes so expensive?

[Yuri]

On 07/06/2016 17:19, Konstantin Belousov wrote:
> To calculate residency count for the process map entries, kernel has to
> iterate over all pages.  This operation was somewhat optimized in 10.3
> and HEAD, particularly for the large sparce mappings.  But for large populated
> mappings there is no other way then to check each page.
>
> You may confirm my hypothesis by setting sysctl
> kern.proc_vmmap_skip_resident_count to 0 and see whether the CPU
> consumption changed.  Of course, you will not get the resident count
> in the returned structure, after the knob is tweaked.


When people raise the question of why malloc library doesn't unmap the 
memory, developers there usually say that they call madvise(MADV_FREE) 
and this is as good as unmap. But this example shows that this isn't 
quite the case on the FreeBSD, and unmapping is better.


Yuri