13-CURRENT: several GB swap being used despite plenty of free RAM

[Cy Schubert]

In message <F5ACF6D0-DBD7-416F-9AAC-7709771FE545 at yahoo.com>, Mark 
Millard via f
reebsd-hackers writes:
> On 2018-Nov-17, at 16:13, Mark Johnston <markj at freebsd.org> wrote:
> > 
> > On Sat, Nov 17, 2018 at 04:59:48PM -0700, Ian Lepore wrote:
> >> On Sat, 2018-11-17 at 22:52 +0100, Wojciech Puchar wrote:
> >>> freebsd will not swap with that lots of free ram.
> >>> but it's 90GB free NOW, how about before?
> >>> 
> >> 
> >> Your information is outdated. For at least a couple years now (since
> >> approximately the 10.1 - 10.2 timeframe is my vague estimate), freebsd
> >> will page out application memory that hasn't been referenced for some
> >> time, even when the system has no shortage of free memory at all.
> > 
> > No, FreeBSD will only ever swap when there is a free page shortage.  The
> > difference is that we now slowly age unreferenced pages into the
> > inactive queue, which makes them candidates for pageout and subsequent
> > eviction.  With pageout_update_period=0, anonymous memory won't get
> > paged out unless there's a shortage of inactive pages, or an application
> > calls madvise(MADV_DONTNEED) on a range of memory (which moves any
> > backing pages to the inactive queue).
>
> Swapping is built on top of paging as I understand. The system
> can page without swapping but can not swap without (effectively)
> paging to implement the swapping, if I understand right. If I
> understand right, swapped-out means that kernel stacks have
> been written out and have to be loaded back in RAM before the
> process/threads can even run. (I might not understand.)
>
> If I've got that right, are there distinctions here for
> paging that is not part of swapping vs. actual swapping
> (and its use of paging)? Saying that something does not
> swap does not necessarily imply that it does not page:
> it still could have paging activity that does not include
> moving the kernel stacks for the process to backing media?

This is generally the old-school definition, IIRC third year comp sci, 
original BSD definition, which was also the way IBM defined it for MVS. 
(IBM also virtually swapped out address spaces, not written to DASD, as 
a means to deny CPU cycles through the scheduler not given the chance 
to consider the tasks (threads) in the address space).

You can disable swapping by setting vm.swap_enabled=0.

>
> At times I have trouble interpreting when wording goes back
> and forth between swapping and paging, both for the intended
> meaning and for the technical implications.
>
> >> The advice I was recently given to revert to the old behavior is:
> >> 
> >>   sysctl vm.pageout_update_period=0
> >> 
> >> I've been using it on a couple systems here for a few days now, and so
> >> far results are promising, I am no longer seeing gratuitous swapfile
> >> usage on systems that have so much free physical ram that they should
> >> never need to page anything out. I haven't yet pushed one of those
> >> systems hard enough to check what happens when they do need to start
> >> proactively paging out inactive memory due to shortages -- it could be
> >> that turning off the new behavior has downsides for some workloads.
>
>
>
>
> ===
> Mark Millard
> marklmi at yahoo.com
> ( dsl-only.net went
> away in early 2018-Mar)
>
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-- 
Cheers,
Cy Schubert <Cy.Schubert at cschubert.com>
FreeBSD UNIX:  <cy at FreeBSD.org>   Web:  http://www.FreeBSD.org

	The need of the many outweighs the greed of the few.