We have issue with temfs filesystem on red hat linux

Hi experts,

We have issue with tempfs filesystem on redhat Linux box.
Memory = 20GB
SWAP = 41 GB
tmpfs = 11 Gb

Some times the tmpfs (/dev/shm) is filled up with 90 to 95 % and SAP application responding very very slow.  

1. How exactly shared memory is works?
2. What is thumb rule for creating shared memory ?
3. What is solution for above issue?

Regards.
Sk.
MaddyUNIXAsked:
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legolasthehansyCommented:
/dev/shm a.k.a the shared memory is concept which shares memory between programs. Very rarely you would see issues with /dev/shm on Linux OS'
What is your 'top' like? Does your SAP app configured to use /dev/shm and what are its settings. I haven't used SAP at all so I won't be able to tell you what settings need to be tweaked. Perhaps other experts can help..
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MaddyUNIXAuthor Commented:
Hi Lego,

Yes, SAP app configured to use /dev/shm.
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legolasthehansyCommented:
Do you know if a setting has been configured for the app to use an amount of memory from /dev/shm. If not, check logs for the app, /var/og/messages, dmesg any logs for some clues. A trivial error can be useful for debug.
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MaddyUNIXAuthor Commented:
Hi lego,

No clues, in logs... I need to know.  

1. How exactly shared memory is works?
2. What is thumb rule for creating shared memory ?
3. What is solution for above issue?

SK
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legolasthehansyCommented:
Hi,

Here are some pointers on how to push through after going through some sites. It seems SAP needs the /dev/shm to be 75% RAM + Swap.

Warning: tmpfs at /dev/shm is configured quite small with 1024 MB!
 Minimum value is 2048 MB.
 Recommended size is 75 % of RAM + swap. In your case you are just above 50%.

Here is another link on how a user was guided on checking certain settings.

http://forums.sdn.sap.com/thread.jspa?threadID=1920244&start=0&tstart=0

For your questions,
Shared memory is a concept for data sharing between programs.
There isn't any rule for creating /dev/shm unless specified or unless there is a reason to take advantage of its capabilities.
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MaddyUNIXAuthor Commented:
Hi lego,

I have few more qustions:

1.  In 22GB of RAM , if i allocate tempfs (/dev/shm) 11GB . The 11GB shm is fully occupied in RAM (ramdisk) or it will extend on demand or allocated 11GB reserved.

2. What disadvantages we have configuring shm? What are the limitations?

3. It shm is 80 to 90 % full what will happens?

Thanks;

SK
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