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System adminsitrator Chris Siebenmann has found Modern versions of systemd can cause an unmount storm during shutdowns:
One of my discoveries about Ubuntu 20.04 is that my test machine can trigger the kernel's out of memory killing during shutdown. My test virtual machine has 4 GB of RAM and 1 GB of swap, but it also has 347 NFS[*] mounts, and after some investigation, what appears to be happening is that in the 20.04 version of systemd (systemd 245 plus whatever changes Ubuntu has made), systemd now seems to try to run umount for all of those filesystems all at once (which also starts a umount.nfs process for each one). On 20.04, this is apparently enough to OOM[**] my test machine.
[...] Unfortunately, so far I haven't found a way to control this in systemd. There appears to be no way to set limits on how many unmounts systemd will try to do at once (or in general how many units it will try to stop at once, even if that requires running programs). Nor can we readily modify the mount units, because all of our NFS mounts are done through shell scripts by directly calling
mount; they don't exist in/etc/fstabor as actual.mountunits.
[*] NFS: Network File System
[**] OOM Out of memory.
We've been here before and there is certainly more where that came from.
Previously:
(2020) Linux Home Directory Management is About to Undergo Major Change
(2019) System Down: A systemd-journald Exploit
(2017) Savaged by Systemd
(2017) Linux systemd Gives Root Privileges to Invalid Usernames
(2016) Systemd Crashing Bug
(2015) tmux Coders Asked to Add Special Code for systemd
(2016) SystemD Mounts EFI pseudo-fs RW, Facilitates Permanently Bricking Laptops, Closes Bug Invalid
(2015) A Technical Critique of Systemd
(2014) Devuan Developers Can Be Reached Via vua@debianfork.org
(2014) Systemd-resolved Subject to Cache Poisoning
(Score: 2) by bzipitidoo on Monday May 11 2020, @01:28PM
I daresay you are correct, and my somewhat hazy idea of what exactly PulseAudio does is a little off.
More generally, what has happened to our systems that 0.0625G RAM was once considered generous, and now, 4G is minimally adequate? Okay, yes, moving from ASCII to UTF-8 dramatically increased the memory needed for fonts, and library routines for handling text strings. And we are running much bigger screens-- 1280x1024 is kinda small by today's standards, and no one settles for just 256 colors any more. Wayland should be trimmer than XWindows, but dumping the 1980s graphics cruft that is now handled by monster graphics hardware doesn't seem to have gained back much. Moving from 32bit to 64bit was another memory eater. There are slimmer alternatives to glibc, such as uclibc and musl libc. And systemd, wouldn't surprise me if that has the largest footprint of all the init systems. Firefox too has grown, with ever more standards to follow. Really must have the Opus audio codec (added in Firefox 19), and now there's AV1 video in the works. Perhaps JPEG will at last be upgraded, to JPEG-XL. Somehow, now we can no longer completely control whether our browsers automatically play video, but back in the day there was a browser option to disable automatic downloading of mere images, to save bandwidth and all.