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

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

--- Comment #16 from Seigo Tanimura <seigo.tanimura@gmail.com> ---
(In reply to Seigo Tanimura from comment #14)

* Notes on the comparision to kern.openfiles

I will assume that ~70% of kern.openfiles is for ZFS in the next test.  This
ratio has been observed from a preliminary test conducted separately from the
last one, where the vnode tag counting in enabled.

* Chart Archive: poudriere-bulk-2023-12-21_18h42m38s.7z
* Charts: vnode-vnodes.png, vnode-tags.png.

vnode-tags.png shows the vnode counts classified by its tags.  The section of
21 Dec 2023 15:00Z - 16:30Z has ~2.5M vnodes, out of which ~1.8M are for ZFS.
(The yellow area on vnode-vnodes.png)  2.5M / 1.8M == 0.72.

The vnode tag counting will be disabled in the next test (and so by default)
because it puts the load as heavy as the ZFS ARC pruning and is hence likely to
affect the test results.

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