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

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

--- Comment #23 from Seigo Tanimura <seigo.tanimura@gmail.com> ---
(In reply to Mark Johnston from comment #1)

> > - vfs.vnode.vnlru.max_free_per_call=100000
> This value is 10 times larger than the default.  Why did you change this parameter?  What happens if you leave it at the default value?

Out of the tests so far, I now believe that the default value of
vfs.vnode.vnlru.max_free_per_call (10K) was chosen in order to polish the load
under the uncontrollable and unreasonable ARC pruning requests.

Now that the ARC pruning is precisely throttled for the efficient execution, it
should be all right to increase vfs.vnode.vnlru.max_free_per_call up to
vfs.vnode.param.limit and let ZFS determine the actual work load. 
vnode_list_mtx is an expensive lock, so once we acquire it, we should prune as
many vnodes as we can.

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