[Bug 224080] ZFS: "arc_dnlc_evicts_thr" + "arc_reclaim_thread" - high CPU load during poudriere build...

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=224080

            Bug ID: 224080
           Summary: ZFS: "arc_dnlc_evicts_thr" + "arc_reclaim_thread" -
                    high CPU load during poudriere build...
           Product: Base System
           Version: CURRENT
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: nbe at renzel.net

On 12-CURRENT r326527

Running a poudriere build results in high CPU load (up to 100%) for the ZFS
kernel threads "arc_dnlc_evicts_thr" and "arc_reclaim_thread":
------------------------------------------------------------------------------------------------
last pid:  8708;  load averages:  5.53,  1.49,  0.55  up 0+00:01:05    14:09:07 
787 processes: 18 running, 728 sleeping, 2 zombie, 39 waiting                   

Mem: 379M Active, 72M Inact, 765M Wired, 14G Free
ARC: 250M Total, 22M MFU, 178M MRU, 314K Anon, 6575K Header, 43M Other
     100M Compressed, 292M Uncompressed, 2.91:1 Ratio
Swap: 32G Total, 32G Free

  PID USERNAME      PRI NICE   SIZE    RES STATE   C   TIME    WCPU COMMAND
   17 root           -8    -     0K   192K CPU6    6   0:18  70.65%
zfskern{arc_dnlc_evicts_thr}
 1328 root           52    0 14300K  5280K piperd  0   0:09  22.36% sh
   11 root          155 ki31     0K   128K RUN     5   0:21  16.46% idle{idle:
cpu5}
   11 root          155 ki31     0K   128K RUN     7   0:28  16.16% idle{idle:
cpu7}
   11 root          155 ki31     0K   128K RUN     0   0:29  15.48% idle{idle:
cpu0}
   11 root          155 ki31     0K   128K RUN     1   0:28  15.38% idle{idle:
cpu1}
   17 root           -8    -     0K   192K arc_re  1   0:09  14.79%
zfskern{arc_reclaim_thread}
   11 root          155 ki31     0K   128K RUN     6   0:28  14.45% idle{idle:
cpu6}
   11 root          155 ki31     0K   128K RUN     4   0:27  14.36% idle{idle:
cpu4}
   11 root          155 ki31     0K   128K RUN     2   0:28  14.06% idle{idle:
cpu2}
   11 root          155 ki31     0K   128K RUN     3   0:28  13.77% idle{idle:
cpu3}
 1513 root           52    0 13024K  3968K nanslp  4   0:00   0.49% sh
   15 root          -16    -     0K    48K -       3   0:00   0.10% cam{doneq1}
    0 root          -16    -     0K  9024K swapin  4   0:28   0.00%
kernel{swapper}
    0 root            8    -     0K  9024K -       3   0:00   0.00%
kernel{thread taskq}
   12 root          -96    -     0K   624K WAIT    5   0:00   0.00%
intr{irq281: hdac1}
   12 root          -60    -     0K   624K WAIT    0   0:00   0.00% intr{swi4:
clock (0)}
   15 root          -16    -     0K    48K -       6   0:00   0.00%
cam{scanner}
------------------------------------------------------------------------------------------------

I have 16GB of RAM and ARC is not limited - so there should be no memory
pressure.

That behaviour does not exist on 11-STABLE (r326088). There, both mentioned
threads only reaches about 1-2% at maximum (similar CPU).

Any ideas how to fix that?

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