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Old 11-23-2012, 02:10 PM   #1
Registered: Apr 2012
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SHMMAX paramater in linux

Hi to all;

Shared memory - (shmall, shmmax, and shmmni)


Can we set max value of the SHMMAX parameter is above 4GB.
if so , Keep in mind that the maximum value of the SHMMAX parameter is 4GB.

>> What is this formula >>

On a 32-bit Linux operating system, without Physical Address Extension (PAE), the physical memory is divided
into a 3GB user space and a 1GB kernel space.

It is therefore possible to create a 2.7GB SGA, but you will need make several changes at the Linux operating system
level by changing the mapped base.

In the case of a 2.7GB SGA, you would want to set the SHMMAX parameter to 3GB.
Tnanks & Regards
Thiyagusham. G

Last edited by colucix; 11-24-2012 at 02:50 AM. Reason: Changed tags to improve readability.
Old 11-25-2012, 08:31 AM   #2
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This is for an Oracle install, yes?

"On x86-64 platforms, SHMMAX can be much larger than 4GB since the virtual address space is not limited by 32 bits.".
NOTE: That says "can be" not "should be" or "has to be". The link also discusses the implications of not setting the correct SHMMAX size.

Good Luck.
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Old 11-25-2012, 02:03 PM   #3
Registered: Apr 2012
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Hi Habitual;

Thanks to given link. I have following question please
clarify it. From that links i have some question
Even i googled Some answers are couldd n't understand easily.
1 Main difference between virtual Address & Physical Address ?
2. Any difference between Virtual Space vs virtual address space ?

All operations performed by RAM.
3. How processess have physical address ?

4GB – 1byte” is related to the limitation on the 32 bit (x86) systems
4. here 1-byte what it indicates ?

SHMMAX is used for SGA , oracle processes only using this area
5. Won't LINUX process use SHMMAX area?

System allows unlimited allocation of virtual address space: [/B]
Each process having seperate address like arrays a[5465] Purpose of unlimited allocation setting why ?
PHP Code:
# cat /proc/sys/vm/overcommit_memory 
# ulimit -v

Last edited by thiyagusham; 11-25-2012 at 05:55 PM.
Old 11-25-2012, 05:01 PM   #4
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Every thing I ever knew and long since forgotten about Oracle I learned at


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