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

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

--- Comment #101 from Olivier Certner <olce@FreeBSD.org> ---
Hi Seigo,

Sorry, I had other things to look for in the meantime, so did not make much
progress up to now.  That said, from now on I should be able to work on it near
to full-time.

(In reply to Seigo Tanimura from comment #99)

That is my first target indeed.  Seems fairly simple.

(In reply to Seigo Tanimura from comment #100)

Great.

First of all, let me say I've never personally experienced the bug reported
here.  I'm using poudriere for big builds from time to time, and with ZFS, but
on machines with plenty of RAM, so that may explain why.  Bottom-line is that I
don't have any reproducer on my own at the moment.  Will try to spin up some VM
with a limited amount of RAM, limit the number of vnodes in the system and the
ARC, and see how that goes.

Second, it seems that this problem materializes enough and with enough severity
that it is quite urgent to find/commit fixes for it.  It might be that some of
the changes proposed here cannot be imported as is because of subtle impacts or
design issues (I'm just saying this generally, this may not apply in the end). 
If it happens to be the case, we'll work on proposing alternatives easier to
review/get accepted, and defer the other changes.  Longer term, the vnode
recycling mechanism will probably be completely re-thinked, but hopefully the
whole of it shouldn't be a requirement to solve the case at hand.

So, please bear a little with me until I get up to speed with all your patches
and results, and the reports of testers in this bug and some connected ones.

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