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Date: Sat, 14 Sep 2019 18:23:09 -0700 (PDT)
From: Don Lewis <truckman@FreeBSD.org>
Subject: Re: spurious out of swap kills
To: Mark Millard <marklmi@yahoo.com>
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On 13 Sep, Mark Millard wrote:
> Don Lewis truckman at FreeBSD.org wrote on
> Thu Sep 12 23:00:19 UTC 2019 :
> 
> . . .
>> Nevertheless, I see these errors,
>> with rustc being the usual victim:
>> 
>> Sep 11 23:21:43 zipper kernel: pid 16581 (rustc), jid 43, uid 65534, was killed: out of swap space
>> Sep 12 02:48:23 zipper kernel: pid 1209 (rustc), jid 62, uid 65534, was killed: out of swap space
> . . .
> 
> Unfortunately, the wording of this type of message is a misnomer for
> what drives the kills: it is actually driven by being unable to gain
> more free memory but FreeBSD will not swap-out processes that stay
> runnable (or are running), only ones that are waiting. Even a single
> process that stays runnable and keeps lots of RAM in the active
> category can lead to kills when swap is unused or little used. So the
> kill-behavior is very workload dependent.
> 
> Real "out of swap" conditions tend to also have messages
> similar to:
> 
> Aug  5 17:54:01 sentinel kernel: swap_pager_getswapspace(32): failed
> 
> If you are not seeing such messages, then it is likely that
> the mount of swap space still free is not the actual thing
> driving the kills.
> 
> Are you seeing "swap_pager_getswapspace(32): failed" messages?
> 
> (It used to be that the system simply leaves the dirty pages in
> memory when a swap_pager_getswapspace failed message is produced.
> Of itself, it did not cause a kill. I do not know about now.)

I'm only getting the "out of swap" error.  Yes, it is misleading because
there are still tens of GB of free swap.