What the heck is /dev/shm ?

It is tempfs (what's tempfs)?

From: http://lxr.linux.no/#linux+v2.6.35.5/Documentation/filesystems/tmpfs.txt#L57

tmpfs has three mount options for sizing:

size:      The limit of allocated bytes for this tmpfs instance. The 
           default is half of your physical RAM without swap. If you
           oversize your tmpfs instances the machine will deadlock
           since the OOM handler will not be able to free that memory.
nr_blocks: The same as size, but in blocks of PAGE_CACHE_SIZE.
nr_inodes: The maximum number of inodes for this instance. The default
           is half of the number of your physical RAM pages, or (on a
           machine with highmem) the number of lowmem RAM pages,
           whichever is the lower.

These parameters accept a suffix k, m or g for kilo, mega and giga and
can be changed on remount.  The size parameter also accepts a suffix %
to limit this tmpfs instance to that percentage of your physical RAM:
the default, when neither size nor nr_blocks is specified, is size=50%

If nr_blocks=0 (or size=0), blocks will not be limited in that instance;
if nr_inodes=0, inodes will not be limited.  It is generally unwise to
mount with such options, since it allows any user with write access to
use up all the memory on the machine; but enhances the scalability of
that instance in a system with many cpus making intensive use of it.


tmpfs has a mount option to set the NUMA memory allocation policy for
all files in that instance (if CONFIG_NUMA is enabled) - which can be
adjusted on the fly via 'mount -o remount ...'

mpol=default             use the process allocation policy
                         (see set_mempolicy(2))
mpol=prefer:Node         prefers to allocate memory from the given Node
mpol=bind:NodeList       allocates memory only from nodes in NodeList
mpol=interleave          prefers to allocate from each node in turn
mpol=interleave:NodeList allocates from each node of NodeList in turn
mpol=local               prefers to allocate memory from the local node

NodeList format is a comma-separated list of decimal numbers and ranges,
a range being two hyphen-separated decimal numbers, the smallest and
largest node numbers in the range.  For example, mpol=bind:0-3,5,7,9-15

A memory policy with a valid NodeList will be saved, as specified, for
use at file creation time.  When a task allocates a file in the file
system, the mount option memory policy will be applied with a NodeList,
if any, modified by the calling task's cpuset constraints
[See Documentation/cgroups/cpusets.txt] and any optional flags, listed
below.  If the resulting NodeLists is the empty set, the effective memory
policy for the file will revert to "default" policy.

NUMA memory allocation policies have optional flags that can be used in
conjunction with their modes.  These optional flags can be specified
when tmpfs is mounted by appending them to the mode before the NodeList.
See Documentation/vm/numa_memory_policy.txt for a list of all available
memory allocation policy mode flags and their effect on memory policy.

        =static         is equivalent to        MPOL_F_STATIC_NODES
        =relative       is equivalent to        MPOL_F_RELATIVE_NODES

For example, mpol=bind=static:NodeList, is the equivalent of an
allocation policy of MPOL_BIND | MPOL_F_STATIC_NODES.

Note that trying to mount a tmpfs with an mpol option will fail if the
running kernel does not support NUMA; and will fail if its nodelist
specifies a node which is not online.  If your system relies on that
tmpfs being mounted, but from time to time runs a kernel built without
NUMA capability (perhaps a safe recovery kernel), or with fewer nodes
online, then it is advisable to omit the mpol option from automatic
mount options.  It can be added later, when the tmpfs is already mounted
on MountPoint, by 'mount -o remount,mpol=Policy:NodeList MountPoint'.


To specify the initial root directory you can use the following mount
options:

mode:   The permissions as an octal number
uid:    The user id 
gid:    The group id

These options do not have any effect on remount. You can change these
parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem.


So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs'
will give you tmpfs instance on /mytmpfs which can allocate 10GB
RAM/SWAP in 10240 inodes and it is only accessible by root.


Author:
   Christoph Rohland <cr@sap.com>, 1.12.01
Updated:
   Hugh Dickins, 4 June 2007
Updated:
   KOSAKI Motohiro, 16 Mar 2010