[Bug 275594] High CPU usage by arc_prune; analysis and fix

From: <bugzilla-noreply_at_freebsd.org>
Date: Fri, 19 Jan 2024 08:35:12 UTC
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=275594

Thomas Mueller <thmu7@freenet.de> changed:

           What    |Removed                     |Added
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                 CC|                            |thmu7@freenet.de

--- Comment #32 from Thomas Mueller <thmu7@freenet.de> ---
After upgrading from 12-STABLE to 13-STABLE (due to ports changes
triggered by 12 being EOLed), I'm now observing high CPU usage of
kernel{arc_prune} on 13-STABLE too.

This is on a system with 16GB of RAM which boots from a SATA attached SSD
with UFS containing the OS, /usr/src, and /usr/local. A ZFS pool containing
data, home, poudriere jails and data, /usr/ports is located on a GELI
encrypted 1TB NVMe. ARC is limited to 2GB.
This system is used as desktop development system.

The arc_prune high CPU usage appears to be triggered by heavy file I/O on
the UFS file systems, for example git operations on /usr/src. 
Once, when /usr/ports was also still on UFS, the
  "Inspecting ports tree for modifications to git checkout..."
step of poudriere took more than 3 hours!

Questions:
Would migrating to ZFS on root mitigate the issues?
Is 13-STABLE in focus for this PR?

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