Date: Wed, 12 Jan 2022 21:42:18 +0000 From: Ambert <ralph41096@protonmail.com> To: Peter Jeremy <peterj@freebsd.org> Cc: "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org> Subject: Re: Out-of-swap killer and SIGTERM signal Message-ID: <AiTm2ciCk5zelOVH-mLbfVX100TAB2cU6kB7xlJW5Y-ADMmAaMQAaHVxdYkXb0kqcKRBmXtDWpx2VuS15EZWQ3mQe2C0fYCmFGLx6yGof8o=@protonmail.com> In-Reply-To: <Yd5JNQyokaVrX6zb@server.rulingia.com> References: <ulzJKQSiG37ZtJR9pE-IasT-K1hqpq__htA_JbNJkDkVLD4MeGtXg59rltE6VhKB74U8ztlapijvd6i62-QJ_CVDrOdWubywhbWznmwVa1U=@protonmail.com> <Yd5JNQyokaVrX6zb@server.rulingia.com>
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On 2022-01-12, Peter Jeremy <peterj_at_freebsd.org> wrote:
> There have been lots of discussions about this in the past, starting
> in about 1998, (though I agree that it's been about 4 years since the
> last discussion). I suggest you search for "freebsd+sigdanger" for
> previous discussions.
Thank you for the keyword. After a search, I find that the previous
threads containing the keyword "sigdanger" discuss extensively two
subjects:
1) How to exclude a process from the reach of the OOM killer. And
how to ask the OOM killer to kill a given process first.
2) How to provide feedback about memory usage to processes, to give
them a hint that they should reduce their memory footprint, if
possible. This feedback can be a signal sent to all processes, or
system-wide flags readable by any process who cares about a
potential memory shortage.
Those two subjects are interesting, but I am talking about something
else. My suggestion is almost never mentionned in previous threads.
When it is mentionned [1] [2], there is no objection whatsoever, from
anyone. There is not even a comment about it.
Sending SIGTERM a few seconds before SIGKILL is useful because it
allows a condemned process to exit gracefully, just like it would
during a shutdown(8). And it is simple to implement. There is no need
to change the algorithm selecting condemned processes, and there is no
need to change a single line of code in userland. For the
administrator, only two tasks require extra work:
- set up a little bit of extra swap during the installation of
FreeBSD
- set a couple of sysctl values: the duration of the grace period
(vm.grace_period =3D zero milliseconds by default), and the amount of
extra swap that will not be usable normally (vm.grace_space =3D zero
bytes by default)
Excerpt from a historical thread:
On 1998-04-27, Jordan K. Hubbard <jkh_at_time.cdrom.com> wrote:
> All the SIGDANGER (Will Robinson) signal is meant to do is give a
> process a little _warning_ before it's chosen as the designated
> sacrifice for the evening and terminated in an untimely fashion.
>
> I don't think the question here is "is this a good idea" - it's a
> perfectly reasonable idea and one which has been proposed before.
> The question here is really "what are the proposed semantics of
> this mechanism?", e.g. how long do you wait from the time you
> SIGDANGER the process and actually shoot it down, and what
> happens if you're also critically short of resources and don't
> have much time to wait?
[1]
https://docs.freebsd.org/cgi/getmsg.cgi?fetch=3D209768+0+archive/1998/freeb=
sd-hackers/19980426.freebsd-hackers
[2]
https://lists.freebsd.org/pipermail/freebsd-current/2008-January/081743.htm=
l
Ambert
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