lightly loaded system eats swap space

[Kevin Oberman]

On Mon, Jun 18, 2018 at 5:27 AM, tech-lists <tech-lists at zyxst.net> wrote:

> On 18/06/2018 09:08, Erich Dollansky wrote:
>
>> Hi,
>>
>> On Sun, 17 Jun 2018 23:19:02 +0100
>> tech-lists <tech-lists at zyxst.net> 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?
>>
>
> Hi,
>
> 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.
>
> 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".
>
> thanks,
> --
> J.


Small correction. Your problem is  "why is it swapping out at all on a
128GB system ." Once pages are swapped out, they are never swapped back in
until/unless they are needed. There is no reason to waste time/disk
activity to swap pages back into memory unless they are required. RAM is
always more valuable than swap.

Ir is easy to write a process that eats up a large amount of memory, then
goes idle without freeing it. The memory will get pages out, fill swap,
and, unless the process terminates or becomes active, will consume up a
great deal of swap space "forever". Firefox is a good example of this. I
have to restart it every day or two and occasionally will run out of swap
which results in a nearly deadlocked system. It can take many minutes to
just kill firefox.
-- 
Kevin Oberman, Part time kid herder and retired Network Engineer
E-mail: rkoberman at gmail.com
PGP Fingerprint: D03FB98AFA78E3B78C1694B318AB39EF1B055683