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

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

Mark Millard <marklmi26-fbsd@yahoo.com> changed:

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                 CC|                            |marklmi26-fbsd@yahoo.com

--- Comment #72 from Mark Millard <marklmi26-fbsd@yahoo.com> ---
(In reply to Anton Saietskii from comment #71)

One of the things that can happen during low free RAM is
for inactive processes to have their kernel stack swapped
out. That can include the processes for interacting with
the system, output and input. Until the kernel stacks from
the relevant processes are swapped back in, interaction is
blocked via those processes.

Do you know what kind of hang you are seeing? Might it be
an example of such?

For reference, in /etc/sysctl.conf I use:

#
# Together this pair avoids swapping out the process kernel stacks.
# This avoids processes for interacting with the system from being
# hung-up by such.
vm.swap_enabled=0
vm.swap_idle_enabled=0

Both are writable live:

# sysctl -Wd vm.swap_enabled vm.swap_idle_enabled
vm.swap_enabled: Enable entire process swapout
vm.swap_idle_enabled: Allow swapout on idle criteria

but are not tunables ( avoid in /boot/loader.conf ):

# sysctl -Td vm.swap_enabled vm.swap_idle_enabled
#

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