Re: page_waitqueue() considered harmful

[Linus Torvalds <torvalds@linux-foundation.org>]

On Mon, Sep 26, 2016 at 2:23 PM, Rik van Riel <riel@redhat.com> wrote:
>
> Why are we touching file pages at all during fork()?

This is *not* fork. This is fork/exec/exit. It's a real load, it's
just fairly concentrated by the scripts being many and small.

It's easy enough to try yourself:

   perf record -e cycles:pp make -j16 test

in the git tree.

> Could we get away with skipping copy_page_range on VMAs
> that do not have any anonymous pages?

You still have to duplicate those ranges, but that's fairly cheap.
>From what I can tell, the expensive part really is the "page in new
executable/library pages" and then tearing them down again (because it
was just a quick small git process or a very small shell script).

So forget about fork(). I'm not even sure how much of that there is,
it's possible that you end up having vfork() instead. It's exec/exit
that matters most in this load.

(You can see that in the profile with things like strnlen_user()
actually being fairly high on the profile too - mostly the environment
variables during exec).

            Linus

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