Re: [PATCH 1/2 v2] fdmap(2)

[Andy Lutomirski <luto@amacapital.net>]

On Sun, Sep 24, 2017 at 1:06 PM, Alexey Dobriyan <adobriyan@gmail.com> wrote:
> From: Aliaksandr Patseyenak <Aliaksandr_Patseyenak1@epam.com>
>
> Implement system call for bulk retrieveing of opened descriptors
> in binary form.
>
> Some daemons could use it to reliably close file descriptors
> before starting. Currently they close everything upto some number
> which formally is not reliable. Other natural users are lsof(1) and CRIU
> (although lsof does so much in /proc that the effect is thoroughly buried).
>
> /proc, the only way to learn anything about file descriptors may not be
> available. There is unavoidable overhead associated with instantiating
> 3 dentries and 3 inodes and converting integers to strings and back.
>
> Benchmark:
>
>         N=1<<22 times
>         4 opened descriptors (0, 1, 2, 3)
>         opendir+readdir+closedir /proc/self/fd vs fdmap
>
>         /proc 8.31 ą 0.37%
>         fdmap 0.32 ą 0.72%

This doesn't have the semantic problem that pidmap does, but I still
wonder why this can't be accomplished by adding a new file in /proc.