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

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=275594

--- Comment #152 from Nils Beyer <nbe@vkf-renzel.de> ---
I intend to use that system as a poudriere builder. 8 fast cores (16 threads)
-> 16 build threads. But only 32GB of RAM, so I HAVE to turn down ARC to use
max 2GB because I need the rest for "poudriere" and TMPFS.

The 512MB is just an example how to get fast results. You can set it to 1GB or
2GB - it will happen eventually - especially during compilation orgies.

The "old" and original FreeBSD ZFS implementation did NOT show this behaviour.
I.e.: I could turn down ARC to 512MB (on a 4GB laptop) and do ZFS scrubs and
compilations as much as I wanted - yes, disks accesses became very excessively,
but at least it accessed the disks and did not wait for "arc_prune" and friends
to finish whatever they were busy with.

It definitely has gotten worse since the import of the Linux ZFS code base...

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