Maximum address space question

[Craig A. Sharp]

We are running HPUX 11.00 and DB2. We are running out of shared memory as DB2 is running in 32 bit mode and we are not able to go to 64 bit mode due to the application using the database.

DB2 is able to use extended storage segments in memory but it is only able to do this if the environment has more memory than the maximum address space.

How do I determine the "Maximum Address Space" for our system.

We currently have 6 Gig of memory.

Thanks,

Craig

[John Poff]

Hi Craig,

How much shared memory is your DB2 database currently using? A 32-bit application will be able to address a maximum of 1.75 Gb of shared memory.

JP

[John Bolene]

Shared memory is limited to 1.75G.

This does not mean that you get this much, normally the max you can actually get is around 1.5G as you have to share with others.

You can only get the full 1.75G if you are the only user on the machine.

[James Murtagh]

Hi Craig,

Not really sure about what the extended storage segments feature is but...

32 bit processes have a 4GB address space broken into 4 equally sized 1GB quadrants, normally as follows:

Q1 : Text
Q2 : Data, Stack, Private regions
Q3 : Shared Memory, shlibs, mmaps
Q4 : Same as Q3 but last 0.25 GB used for IO.

Quadrants 1 and 2 are private to the process. Quadrants 3 and 4 are shared between ALL 32bit user processes.

There are two techniques currently emloyed to increase the shared memory space available to a process:

shmem_magic - use Q2 as an extra shared memory quadrant, moving the data area to share with text in quadrant 1.
memory windows - here the database has to be broken down into <= 1GB chunks, but you can start many of these overcoming the overall limit of 1.75GB shared memory.

Is this what you were asking?

Regards,

James.

[James R. Ferguson]

Hi Craig:

Unless you use "Memory Windows" (see below), a 32-bit kernel severely restricts the amount of memory a program can address or use. See this white paper for more information:

http://docs.hp.com/hpux/onlinedocs/os/memwn1_4.pdf

Regards!

...JRF...

[John Palmer]

Have a look at the Memory Management and Memory Windows White Papers. These are:

/usr/share/doc/mem_mgt.txt
/usr/share/doc/mem_wndws.txt

Also there are many pertinent threads already in the forums, try the search facility.

Regards,
John