[Bug 254639] Fork syscall performance

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254639

Mark Johnston <markj at FreeBSD.org> changed:

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                 CC|                            |markj at FreeBSD.org

--- Comment #1 from Mark Johnston <markj at FreeBSD.org> ---
How much RAM do you have?  Hopefully more than 100GB.

I suspect that we're spending a lot of time in pmap_copy(), which copies page
tables into the child process.  It only copies referenced PTEs, but in this
case we'll copy all of the page tables backing some_str.

You can try something like this to confirm:

# dtrace -q -n 'fbt::pmap_copy:entry /pid == $target/{self->ts = timestamp}
fbt::pmap_copy:return /self->ts/{@ = sum(timestamp - self->ts); self->ts = 0;}
tick-1s {printa("spent %@dns copying\n", @); clear(@);}' -c "python test.py"

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