Swap usage on Linux/AMD64: Have I goofed up a .config setting?

[Stuart Longland <stuartl@longlandclan.yi.org>]

Hi all,

I've been wondering this for some time (i.e. a good 5 years or so, never
had the time to go sit down and nut it out).  I haven't worried about it
since it normally is just a minor nit, but today I was doing something
where I really did need swap to work.

On i386 systems I used to allocate a *HUGE* swap partition and mount
tmpfs on /tmp.  This used to work really well, swap usage would grow as
I plonk files in /tmp, and when I reboot, it'd be nice and clean ready
to go.

When I tried this on AMD64, I found this no longer worked as expected.
Swap would be left alone, and sometimes RAM would become scarce once too
many files were deposited in /tmp.  This has been the case for me ever
since I got my first AMD64 machine (mid-2010).

Today, I was busy slicing up images for printing in The Gimp.  One that
has been giving me curry is this one:

https://publications.qld.gov.au/dataset/guide-to-queensland-roads/resource/f86c19ae-9adf-4100-bb59-5113548c361e

I rasterise that at 1440 DPI, then try to crop or print it.

I have 8GB RAM and my swap partition is 10GB.  Inspite of this, I'm not
seeing more than about 200MB of swap space used at any time, and The
Gimp crawls.  Not surprising that it's slow as I really need 16GB to
process something this size, but it is surprising that the 10GB of swap
doesn't get touched:

> RC=0 stuartl@vk4msl-mb ~ $ free
>              total       used       free     shared    buffers     cached
> Mem:       7915412    7684668     230744       6804      19264    2073856
> -/+ buffers/cache:    5591548    2323864
> Swap:     10664956     197432   10467524
> RC=0 stuartl@vk4msl-mb ~ $ uname -a
> Linux vk4msl-mb 3.16.2-vk4msl-mb #1 SMP PREEMPT Fri Sep 12 22:32:47 EST 2014 x86_64 Intel(R) Core(TM)2 Duo CPU P7350 @ 2.00GHz GenuineIntel GNU/Linux

I run vanilla kernels that I build myself and most of the settings come
from defconfig.  I can post my .config if people desire.

Until today I have not touched 'swappiness' (today I tried setting it to
80).

Is there some setting I've missed in .config that allows swap to be
utilised on AMD64?
-- 
Stuart Longland (aka Redhatter, VK4MSL)

I haven't lost my mind...
  ...it's backed up on a tape somewhere.