[Bug 274698] arc_prune consuming 100% CPU

From: <bugzilla-noreply_at_freebsd.org>
Date: Tue, 24 Oct 2023 16:45:40 UTC
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=274698

            Bug ID: 274698
           Summary: arc_prune consuming 100% CPU
           Product: Base System
           Version: 15.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: markj@FreeBSD.org
                CC: mav@FreeBSD.org

In the past couple of months I see a new problem: ZFS' arc_prune thread will
occasionally start consuming a CPU and run for hours.  Looking at "vmstat -z |
grep taskq", I can see that a huge number of task allocations are pending, and
they are slowly draining.

It looks like back-to-back calls to arc_evict() can queue up a silly amount of
work.  The Linux implementation of arc_prune_async() is careful to avoid this
problem.

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