Public bug reported:
I have an old workstation running kernel
Linux desktop 5.4.0-74-lowlatency #83~18.04.1-Ubuntu SMP PREEMPT Tue May
11 17:19:13 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
and when I run
echo 1 > /proc/sys/vm/vfs_cache_pressure
to allow directory structure to be cached better and then run
rsync path/to/A path/to/B
multiple times where both A and B contain approximately 6.5 million
files (~2.5 TB per directory) I randomly get long pauses (1-15 seconds)
which freeze both the desktop and audio output. I get nothing in "sudo
journalctl -f" until some software starts to fail (e.g. pulseaudio emits
a warning about too high latency). With low "vfs_cache_pressure" the
expected behavior would be to be able to keep dentries for those 13
million files and the repeated rsync calls should be pretty fast and not
cause high I/O nor memory management load.
I've been running with the above setting of "1" for years and I'm pretty
sure I haven't noticed this issue before kernel 5.4 series so I think
the kernel behavior has degraded.
I'm pretty sure I'm not running out of RAM so this is probably a change
in the dentry cache behavior or some other part of the kernel memory
management.
$ free -m
total used free shared buff/cache available
Mem: 31807 9649 6445 3262 15712 6252
Swap: 1590 1453 136
ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: linux-lowlatency-hwe-18.04-edge 5.4.0.74.83~18.04.67
ProcVersionSignature: Ubuntu 5.4.0-74.83~18.04.1-lowlatency 5.4.114
Uname: Linux 5.4.0-74-lowlatency x86_64
ApportVersion: 2.20.9-0ubuntu7.24
Architecture: amd64
CurrentDesktop: MATE
Date: Mon Jun 14 15:34:23 2021
EcryptfsInUse: Yes
InstallationDate: Installed on 2019-01-05 (890 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
SourcePackage: linux-meta-hwe-5.4
UpgradeStatus: No upgrade log present (probably fresh install)
** Affects: linux-meta-hwe-5.4 (Ubuntu)
Importance: Undecided
Status: New
** Tags: amd64 apport-bug bionic
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https://bugs.launchpad.net/bugs/1931871
Title:
low 'vfs_cache_pressure' causes bad real world latency
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