DataBase - Recommeded SHMALL Linux kernel parm for DB2 Linux 9.5

Asked By Mark A

11-Mar-10 04:43 PM

Here is the recommendation for SHMALL in the Redbook entitled Up and Running
with DB2 on Linux (SG24-6899-01) that covers V9.5:

more physical memory than 8 GB for DB2, increase the value of this parameter
to approximately 90% of your computer’s physical memory."

Here is the recommendation from the current DB2 9.5 InfoCenter for SHMALL on
Linux:

IPC kernel parameter: kernel.shmall (SHMALL)
Enforced minimum setting: no value is enforced
Recommended minimum value: 2 * <size of RAM in bytes> (setting is in 4K
pages)
Example minimum value (system with 16GB of RAM: 8388608 (4K pages) [which is
32GB]

So the Redbook effectively states SHMALL should be 90% of real memory on a
system with more than 8 GB, and the DB2 LUW 9.5 InfoCenter says it should be
two times the amount of real memory for Linux. Anyone have any info on this
discrepancy, or any other ideas.

12-Mar-10 04:28 AM

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When you check the DB2 9.7 InfoCenter in the section "Configuring for
good performance", it is the 90% rule. Go with that.

http://publib.boulder.ibm.com/infocenter/db2luw/v9r7/topic/com.ibm.db2.luw.=
admin.perf.doc/doc/c0054689.html

Henrik

12-Mar-10 04:32 PM

That's also in line with this "Best Practices" document (based on 9.5):
http://download.boulder.ibm.com/ibmdl/pub/software/dw/dm/db2/bestpractices/DB2BP_System_Performance_1008I.pdf

Unfortunately it contradicts with what is written on this IC9.7 page:
http://publib.boulder.ibm.com/infocenter/db2luw/v9r7/topic/com.ibm.db2.luw.qb.server.doc/doc/c0057140.html
(As far as I can tell, there is no 9.5 equivalent for this page)

To add to the confusion, Mark Mulligan offers yet another alternative on his
blog on Automatic Memory Management:
http://it.toolbox.com/blogs/db2-luw-tips/db2-luw-automatic-memory-management-configuration-tips-on-linux-servers-31337

Quoting Mark M.:
LINUX Configuration Tip: If you want to use the DB2 LUW Automatic Memory
Manager feature on a LINUX server then the maximum segment size (SHMMAX)
should be configured to the same size as the maximum total shared memory
(SHMALL) and the maximum total shared memory (SHMALL) should be configured
to the total physical memory on the server.

DB2 Configuration Tip: If the maximum segment size is less than the maximum
total shared memory for LINUX then you should avoid using AUTOMATIC on
configuration parameters that manage the amount of shared memory that DB2
will use or you will experience problems if DB2 exceeds these limits. The
same is true for the maximum total shared memory if it is set to something
smaller than the maximum physical memory on the server. The SHMMAX and
SHMALL LINUX configuration values limit the amount of shared memory DB2 can
use on a LINUX server. If DB2 is configured to exceed these limits or if the
DB2 automatic memory manager adjusts DB2 shared memory so that it exceeds
these limits then LINUX will stop the DB2 threads when these limits are
exceeded.

As the InfoCenter (at least for 9.7) contradicts itself, I would suggest to
the OP to use the "Feedback" option that is offered.


Cheers!

--
Jeroen

12-Mar-10 04:48 PM

Actually, the content of above IC9.7 page is copied from the "Best
Practices" document (page 6-15).

--
Jeroen

12-Mar-10 04:51 PM

I meant to say there is a 9.5 equivalent for this page, but not for the page
offered by Henrik.
Sorry for the extra confusion...

--
Jeroen