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Postings from March 2017
On Sat, Mar 11, 2017 at 1:32 PM, Ævar Arnfjörð Bjarmason
<avarab@gmail.com> wrote:
> On a related note. At Booking.com in our web applications we don't run
> destruction beyond just destroying the Plack $env, then we
> POSIX::_exit(). See https://github.com/unbit/uwsgi/pull/1392/files &
> https://github.com/unbit/uwsgi/issues/1384 for why, but tl;dr: With
> forked processes it creates a CPU/Memory DoS attack on the kernel as
> it frantically tries to unshare all the pages whose refcounts change.
>
> So uWSGI has an option now to simply skip the perl_destruct() &
> perl_free() phases, which from my looking at the source & testing it
> seemed like the sane solution:
> https://github.com/avar/uwsgi/blob/bafc14c80771a2c55063c3c9cc9c9dd0377a0294/plugins/psgi/psgi_plugin.c#L860
>
> I can't think of a use-case for this, but is it a good idea to provide
> some alternative to POSIX::_exit() in this case, i.e. is there some
> subset of perl_{destruct,free}() that we could run that wouldn't free
> everything, but e.g. just enough to run any DESTROY handlers?
> Currently if we have some DESTROY handler hanging off anything but
> $env it simply won't execute when we teardown the process.
>
> I don't know if that's possible or even a mad idea, but since you're
> poking this code I thought I'd bring this use-case to your attention.
Perl already has a concept of destruct level, current valid values are
0, 1, and 2.
0 destructs much of the interpreter and all objects, this is the
default on unthreaded perls.
1 destructs a bit more of the interpreter, in particular the stashes.
This is the default in an embeded perl with multiplicity enabled.
2 destructs absolutely everything, including non-object cycles and the
global string table. threads.pm sets it to this value.
I'm not sure what the value for PL_perl_destruct_level is in your
particular case, lowering it may help. I can imagine introducing a -1
level that does little more than run END blocks and flush IO buffers.
Leon
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