On Mon, Sep 26, 2016 at 2:23 PM, Rik van Riel <riel@xxxxxxxxxx> 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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