lightly loaded system eats swap space

Erich Dollansky freebsd.ed.lists at
Tue Jun 19 01:49:28 UTC 2018


On Mon, 18 Jun 2018 13:27:23 +0100
tech-lists <tech-lists at> wrote:

> On 18/06/2018 09:08, Erich Dollansky wrote:
> > 
> > On Sun, 17 Jun 2018 23:19:02 +0100
> > tech-lists <tech-lists at> wrote:
> >   
> >> freebsd-11-stable r333874, ZFS raidz1-0 (3x4TB disks), 128GB RAM,
> >> Swap: 4096M Total, 3502M Used, 594M Free, 85% Inuse  
> > 
> > this might not be related but I noticed that your swap space is
> > small compared to RAM size. I noticed on a much smaller Raspberry
> > Pi, that it runs into trouble when there is no swap even there is
> > enough RAM available. Is it easily possible for you to add some GB
> > of swap space and let the machine run then?
> > 
> > How much swap do the other machines have?  
> Yes, the machine with the problem uses the default 4GB swap. That's
> all the swap it has. The machine without issue has a swapfile
> installed on a SSD in addition to the default 4GB swap.
> problematic machine:
> Device          512-blocks     Used    Avail Capacity
> /dev/ada0p3        8388608     3.3G     714M    83%
> machine without a problem, it has swapfile installed:
> Device          512-blocks     Used    Avail Capacity
> /dev/ada0s1b       8262248     1.7G     2.2G    44%
> /dev/md0          65536000     1.9G      29G     6%
> Total             73798248     3.7G      32G    10%
> I added the swapfile a long time ago on this machine due to the same
> issue.

so, the same effect as on a small Raspberry.

It seems that you also use a memory disk for swap. Mine is backed by a
file via NFS.

> But my problem isn't so much an out of swapspace problem; all this
> is, is a symptom. My problem is "why is it swapping out at all on a
> 128GB system and why is what's swapped out not being swapped back in
> again".
I wondered even on the small Raspberry about this. The Raspberries come
with 1GB of RAM. Running just a compilation should never be the problem
but sometimes it is.

A very long time ago - and not on FreeBSD but maybe on a real BSD - I
worked with a system that swapped pages out just to bring it back as
one contiguous block. This made a difference those days. I do not know
if the code made it out of the university I was working at. I just
imagine now that the code made it out and is still in use with the
opposite effect.


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