On Sun, Sep 9, 2012 at 12:50 PM, Mark Phillips <mark@phillipsmarketing.biz> wrote:

I have been having some issues with an old server running a jsp applicaition (tomcat web server and java 1.5....I said it was old!). I looked at the partitions and found:

Last login: Sun Sep 9 11:57:49 2012 from 192.168.25.150
mark@gandalf:~$ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/hda6 110G 61G 44G 58% /
tmpfs 63M 0 63M 0% /dev/shm
/dev/hda1 30M 7.3M 21M 27% /boot
mark@gandalf:~$

tmpfs or /dev/shm is at 0%. Usually, a zero anywhere is bad thing....;) so should I do something about this?

This is 63M with 0 used above.

You can expand it for better performance, but I doubt that your utilization issues are /dev/shm kernel intermessage processing related. In a J2EE system, it would be memory, garbage collection, or a kernel based known memory error.

But if you are running Ultramonkey or a databasae, you could use all of /dev/shm during heavy network use, so you can easily expand /dev/shm (see below).

Reference: http://www.cyberciti.biz/tips/what-is-devshm-and-its-practical-usage.html

I goolged /dev/shm and understand that it is a ram disk for interprocess communications. Wonderful. Should I be worried it is at 0% Should I increase the size? Running top shows these resources in the system"

Tasks: 60 total, 1 running, 59 sleeping, 0 stopped, 0 zombie
Cpu(s): 1.0% us, 0.3% sy, 0.0% ni, 98.4% id, 0.0% wa, 0.3% hi, 0.0% si
Mem: 127152k total, 124740k used, 2412k free, 6896k buffers
Swap: 489940k total, 0k used, 489940k free, 45500k cached

If I should increase the size of /dev/shm, would I edit fstab and add this line

none /dev/shm tmpfs defaults,size=8G 0 0

and then

mount -o remount /dev/shm

You can use /dev/shm to improve the performance of application software such as Oracle or overall Linux system performance. On heavily loaded system, it can make tons of difference. For example VMware workstation/server can be optimized to improve your Linux host's performance (i.e. improve the performance of your virtual machines).

In this example, remount /dev/shm with 8G size as follows:
# mount -o remount,size=8G /dev/shm
To be frank, if you have more than 2GB RAM + multiple Virtual machines, this hack always improves performance. In this example, you will give you tmpfs instance on /disk2/tmpfs which can allocate 5GB RAM/SWAP in 5K inodes and it is only accessible by root:
# mount -t tmpfs -o size=5G,nr_inodes=5k,mode=700 tmpfs /disk2/tmpfs
Where,

-o opt1,opt2 : Pass various options with a -o flag followed by a comma separated string of options. In this examples, I used the following options:

remount : Attempt to remount an already-mounted filesystem. In this example, remount the system and increase its size.
size=8G or size=5G : Override default maximum size of the /dev/shm filesystem. he size is given in bytes, and rounded up to entire pages. The default is half of the memory. The size parameter also accepts a suffix % to limit this tmpfs instance to that percentage of your pysical RAM: the default, when neither size nor nr_blocks is specified, is size=50%. In this example it is set to 8GiB or 5GiB. The tmpfs mount options for sizing ( size, nr_blocks, and nr_inodes) accept a suffix k, m or g for Ki, Mi, Gi (binary kilo, mega and giga) and can be changed on remount.
nr_inodes=5k : 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.
mode=700 : Set initial permissions of the root directory.
tmpfs : Tmpfs is a file system which keeps all files in virtual memory.

How do I restrict or modify size of /dev/shm permanently?

You need to add or modify entry in /etc/fstab file so that system can read it after the reboot. Edit, /etc/fstab as a root user, enter:
# vi /etc/fstab
Append or modify /dev/shm entry as follows to set size to 8G

none      /dev/shm        tmpfs   defaults,size=8G        0 0

Save and close the file. For the changes to take effect immediately remount /dev/shm:
# mount -o remount /dev/shm

Thanks,

Mark

P.S. The issues I am having with the application may have nothing to do with this situation...could be some bad programming....ie a bug.

P.P.S. I am running Linux version 2.6.8-2-386 (horms@tabatha.lab.ultramonkey.org) (gcc version 3.3.5 (Debian 1:3.3.5-13)) (yes, I said it was old....) Consider it my contribution to keeping old hardware out of the dump!

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