[Gimp-developer] GIMP 2.9 doesn't seem to release RAM

[Elle Stone <ellestone ninedegreesbelow com>]

My system has 32GB RAM. I'm running Gentoo Linux and using GIMP 2.9 
updated very recently.
I opened five or six large (1 to 2 GB each) XCF files and then closed 
all but one of the files (not having an image browser that can produce a 
thumbnail for GIMP 2.9 XCF files, I was trying to figure out which file 
was which).
After closing all but one of the files, GIMP was still using 27GB 
virtual/25GB resident, and 79% of total RAM. I tried to decompose a 
layer in the single remaining open file and got the following two error 
messages:
Unable to run plug-in "decompose"
(/home/elle/code/gimplch/install/lib/gimp/2.0/plug-ins/decompose)

Failed to fork (Cannot allocate memory)
Execution error for 'Decompose':
Failed to run plug-in "decompose"

Closing GIMP and then restarting and reopening just the one file 
resulted in much less RAM usage, and there was no problem with 
decomposing the layer.
There are several RAM-related open bug reports, but all of them seem to 
be about Windows and/or GIMP 2.8.
Tying up all that memory means other applications sometimes don't have 
enough temp space to function properly. For example I repeated the "open 
a bunch of XCF files and close all but one" procedure to verify that it 
really is a reproducible problem, and Thunderbird couldn't save a copy 
of this email as a draft message.
I think maybe GIMP (and/or GEGL?) really isn't releasing RAM when it 
should. But maybe also I have some incompatible settings:
1. In System Resources in GIMP Preferences, I have the tile cache size 
set to 15GB - too much?
2. I allocated 12GB as a dynamically expanding tmp file in fstab (tmpfs 
/tmp tmpfs noatime,nodiratime,size=12G 0 0).
3. I use the following lines to start GIMP:
GEGL_USE_OPENCL=no
export GEGL_USE_OPENCL
GEGL_SWAP=RAM
export GEGL_SWAP

Are the above three settings for RAM usage perhaps in conflict with one 
another?
Best,
Elle