[Bug 230402] With buildworld, the system can not use swap

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=230402

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

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                 CC|                            |marklmi26-fbsd at yahoo.com

--- Comment #2 from Mark Millard <marklmi26-fbsd at yahoo.com> ---
(In reply to Vladislav V. Prodan from comment #0)

Unfortunately, the message:

pid 9870 (c++), uid 0, was killed: out of swap space

can be misleading: that is not necessarily the actual
context.

The book "The Design and Implementation of the FreeBSD Operating System"
(2nd edition, 2014) states (page labeled 296):

QUOTE:
The FreeBSD swap-out daemon will not select a runnable processes to swap
out. So, if the set of runnable processes do not fit in memory, the
machine will effectively deadlock. Current machines have enough memory
that this condition usually does not arise. If it does, FreeBSD avoids
deadlock by killing the largest process. If the condition begins to arise
in normal operation, the 4.4BSD algorithm will need to be restored.
END QUOTE.

If there were no prior messages like:

sentinel kernel: swap_pager_getswapspace(32): failed

and tools such as swapinfo or top do not show low
swap space avaiable, then it is unlikely that "out of
swap space" is a correct wording in the message.

The arm list has lots of reports for this for RPI2 and
RPI3's (that are also 1 GiByte machines). In many cases
this is for head (12.0-CURRENT) but 11.x also shows
such issues as I understand. Also frequently UFS (so
no ARC memory use, for example).

Note: 4.4BSD is from long ago. The potential for the issue
is not new. What is new is building modern versions of
clang and other llvm materials.

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