Bug#388128: [Pkg-uml-pkgs] Bug#388128: user-mode-linux: eats up the whole /dev/shm

[Mattia Dongili]

Hello

On Mon, Sep 18, 2006 at 08:57:57PM +0200, Andreas Beckmann wrote:
> Package: user-mode-linux
> Version: 2.6.17-1um-2
> Severity: normal
> 
> Hi,
> 
> running several uml guests can consume the whole /dev/shm quickly: e.g.
> host mem: 1GB
> /dev/shm: 507 MB (default tmpfs size)
> 2x linux mem=256m: 512MB ==> greater than /dev/shm
> One uml guest crashed, the other could be shut down, but this took
> nearly forever.

Yes, that's something that hit me too. I'm running on a 256M host and
firing up up to 3 UML guests with mem=128M usually. I did notice that
with 2.6.17 the situation got a definitely worse.

Something that helps is obviously increasing /dev/shm size (I doubled it
and I'm now an happy citizen).

> Obviously the 'physical memory' used by the uml guests is taken from
> /dev/shm and won't be freed again.

not nice and definitely not that obvious. This could be considered a
bug, but I think upstream is well aware of it.

Anyway, could you check if you still have that UML instance running?
I just reproduced something similar here and 'killall -9 linux' frees
all memory:

$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/hda1             2.8G  1.4G  1.5G  49% /
tmpfs                 128M  128M     0 100% /dev/shm

$ killall -9 linux

$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/hda1             2.8G  1.4G  1.5G  49% /
tmpfs                 128M  4.0K  128M   1% /dev/shm

> linux --help says "mem= ... is not releated to the amount of memory in
> the host. ... can be more ... will be swapped out."
> Even using half of the host memory seems to be impossible under this
> situation.
> Is there a way to reduce /dev/shm usage?
> 
> The host has enough swap (4GB) and tmpfs space available at /tmp (3GB).
> More could be made available, if neccessary.

You need to increase /dev/shm size, default is half of the available ram
but you can edit /etc/default/tmpfs and put e.g.:
SHM_SIZE=536870912
for 512M (it's in bytes).
Or try 
   mount -t tmpfs tmpfs /dev/shm -o remount,size=536870912
before, to test it.

-- 
mattia
:wq!