50 percent swap used, but "ps auxww" output shows no processes swapped out

[Mark Millard]

Brandon Allbery allbery.b at gmail.com wrote on
Sat Feb 3 21:18:53 UTC 2018 :

> Swapping whole processes out is not really a thing any more. Individual
> pages are paged to/from memory; if a memory page has no backing file, it
> will be allocated a block in swap space as its backing storage.
> 
> (I'm not sure "W" status even means swap; I thought whole-process swapping
> wasn't even supported any more.)

From what I've seen on the lists there is a technical distinction
made between "kernel stacks for the process no longer memory resident"
(swapped out) and other pages for the process having paged to disk and
not being resident.

But many tools do not seem to present that point of view and still
reflect an older view in the terminology used, including in
documentation. One has to interpret what one is shown as I understand.

As an example, top can show RES being zero despite the kernel stacks
for the process not having been moved to disk. RES zero might not
mean what one might expect about "swapped out".

I do not know if a W after the first letter in state (STAT) for
"ps auxww" track the kernel-stacks' resident-vs-not status for the
process or not. (Matching your not sure status.)


> On Sat, Feb 3, 2018 at 4:14 PM, Michael Voorhis <mvoorhis at mcvau.net> wrote:
> 
> > Hi all,
> >
> > I've got an amd64 system running 11.1-STABLE r325027, with something
> > like 20G of swap. "swapinfo" shows that half the swap is used.
> >
> > So of course I'm curious to know which processes have been swapped
> > out. I'm not using any "tmpfs" filesystems; no ZFS, no huge amounts of
> > wired-down memory. The system's got 16 processors and 128G of RAM. "ps
> > auxww" output shows *no* processes that are swapped out (2nd character
> > in "STAT" field is "W"). Not a single one. The only process with a W in
> > the stat field at all is the "[intr]" kernel thread.
> >
> > What is using the swapspace

The so-called swapspace is really the paging/swap-space with
most of the use being paging typically. (As Brandon indicated.)

Once a page is paged out, if the process sticks around but
does not use or free the page, the page likely stays
paged-out. (I'm guessing some at the intended results for
default tuning --and that you probably are using default
tuning.) So the in-use swapspace is likely from one or
more existing processes that did page-outs earlier.

(Expect my descriptions to be over simplified, but hopefully
pointing in the right general direction.)

> > Please educate me.
> >


===
Mark Millard
marklmi at yahoo.com
( markmi at dsl-only.net is
going away in 2018-Feb, late)