By default, they take the usual value of "50% of physical RAM".
As /dev/shm can be filled by anyone, and tmpfs does not trigger the OOM killer (and
can hang the machine due to a lack of RAM), I need to configure that down
in order to avoid crashes.
There is still left the /var/run/nscd tmpfs filesystem, also created with 50%
of the RAM, but at least not writeable by anyone. We could find a reasonable
low value for that, or allow configuration.
svn path=/nixos/trunk/; revision=21140
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