Oracle SGA size

[Unix Engineering]

Hello,

The Oracle installation manual says that the maximum allowable shared memory is 1.75GB. Does that apply to the maximum size of the SGA? Must the SGA be less than 1.75GB?

[Stefan Farrelly]

There have been a lot of long detailed answers to this question. Please search the forum archives for shared memory and you will find the answers you seek.

[John Palmer]

Hi,

Yes if you are using 32 bit Oracle, No if you are using 64 bit Oracle.

As Stefan says, a forum search will give you several previous threads.

Regards,
John

[Unix Engineering]

Does each instance of Oracle get it's own separate shared memory segment for an SGA or do all the instances' SGA's use only 1 shared memory segment?

For example:
Will this work for an HP11 Oracle 7.3.3.4 (32bit) database with 2 instances:
databaseA: SGA size set to 1GB
databaseB: SGA size set to 1GB.
Or does that exceed the 1.8G shared memory segment rule (assuming 4G swap and 4G physical memory)?

Thanks again!

[John Palmer · Accepted solution]

Hi,

The 32 bit model for shared memory only allows 1.75Gb system wide.

However at 11.00, HP have implemented a system called 'Memory Windows' which allow many sets of 1.75Gb shared memory, each shared by a number of processes which are in the same 'window'.

For details, there are a couple of .txt files in /usr/share/doc (on an 11.00 system) called mem_mgt.txt and mem_wndows.txt.

Also a forum search will give you more information.

Regards,
John

[CHRIS ANORUO]

Hi,

Check the attached, it is from an ealrier posting done by Stefan.
I hope it helps.

[Unix Engineering]

If there is a system wide limit of 1.75GB memory for 32 bit apps, why would HP sell a K570 HP10.20 system with 4G of memory? If what you say is true, wouldn't that other 2.25GB of memory be unusable?

[John Palmer]

You can use the rest for memory that is not shared.

Regards,
John

[Alexander M. Ermes]

Working with Oracle 32-bit the SGA is limited to 1.75 GB, even if you run several databases. If you do not use the 'Memory windows' of HP-UX, you are really in trouble.
That also counts, if you run several databases in 32-bit mode. Then you have to add up the size of the SGA's to avoid using more than 1.75 GB. Try to run Memory windows.
Kind regards
Alexander M. Ermes