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

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

--- Comment #30 from Seigo Tanimura <seigo.tanimura@gmail.com> ---
(In reply to Allan Jude from comment #24)

> Something that might be worth looking at is these changes from upstream:
> https://github.com/openzfs/zfs/pull/15511

I have tested PR 15511 applied to the fix branch, but there were no significant
changes on the dnode sizes.

Result Chart Archive: (poudriere-bulk-openzfs-pr-11551-20240110.7z, Attachment
#247565)

- zfs-arc-dnode-size-before.png
  - The total dnode size in bytes, before applying PR 15511.
- zfs-arc-dnode-size-after.png
  - The total dnode size in bytes, after applying PR 15511.

Contrary to the test steps in PR 15511, the nullfs(5) vnodes used in my
poudriere setup keep the use counts on the lower ZFS vnodes.  This behaviour
makes the ARC-prunable vnodes much less compared to the total znode number. 
Please refer to zfs-znodes-and-dnodes.png in Attachment #247560; the area
between the blue and green traces are prunable.

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