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

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

--- Comment #151 from Olivier Certner <olce@FreeBSD.org> ---
(In reply to Nils Beyer from comment #150)

We would have to characterize the "misbehavior" you're seeing.  The cause of
the issue would be different, as what we have fixed so far is still in place.

Since you're reducing the ARC size a lot, to levels I've personally never used
and were advised against a long time ago, it's no surprise that ARC pruning
happens a lot more.

It's unclear if ARC pruning should run continuously or not in your case, as
this depends on lots of factors.  You might as well just be hitting an existing
limitation of how OpenZFS scales the pruning process.  I unfortunately didn't
take time to investigate any of the other issues I had spotted a year ago in
our interfacing with ZFS.  The only "related" fix I did was to solve a big
regression in vnode recycling in 14.2 (fixed in 14.3 and 15.0).

Out of curiosity, why would you want to reduce the ARC that much?

I probably have too much on my plate to promise to look into this again, but I
could try if the problem is very pressing and reasonably delimited.

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