Issues with vfs_cache_pressure

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1

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.

2

Please read this:

And thank you for pointing out that vm.vfs_cache_pressure is not a % any more (it can go > 100 nowadays)

:+1:

3

Probably better asked in a ubuntu forum when you’re using a ubuntu kernel and found a problem with it …

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4

Good catch, thanks.
I think there is no point in keeping this open.

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