SAP Extended memory on iSeries

[Former Member]

Hello, last week, checking a system with performance problems, I detected the customer has the parameter em/initial_size_MB (extended menory), in my opiniion, too high

The history of the parameter shows that the maximum ext. mem. reached is quite lower than the size of em/initial_size_MB parameter

Some colleages in Spain say it is not a problem, because SAP doesn't lock the extended memory not used, so the iSeries can use it

I'm not sure because if my colleages are true, then we always should give a very high value to em/initial_size_MB

SAP version is ECC 6.0 and iSeries version is V6R1

Does the parameter em/initial_size_MB lock or reserve the memory so the iSeries can't use it?

I've read oss 808.607, but I don't see my question clearly answered

Thanks in advance, Ximo Escobedo,

IBM Spain

[Former Member · Accepted solution]

Hi,

In addition to the em parameter also do increase the abap/heap_area_total and abap/buffersize parameters.

It will help if you set in accordance to whatever resources you have.

Regards,

Sai R.

[Former Member]

Hello Ximo,

the SAP system locks the whole space of size em/initial_size_MB at startup and clears it. If you run your system with the option 'memory mapped to file' (note 808607), this means that an unnamed file is the physical backbone of the memory. So after initialization, the file is paged out and from now on the SAP system only maps certain parts of this file to the application running in the work processes. So in one instant in time, only part of this large area is needed in the work processes. This number increases with the number of active work processes. Although you never need the whole size of em/initial_size_MB at one instant, the SAP system expects that area of this size is always available. So it is wise to not exceed the historical maximum of memory used by a large amount. Volker's recommendation with 30% is good.

If you use the option 'shared memory', similar consideration apply.

In summary, never define memory as bis as disp space allows it, but look at the past maximum values and stay close to them.

[Former Member]

Hi Volker, thank you for your answer

Then, if I understand "SAP dos not LOCK the space, but it increases the disk utilization. In theory, it should not have bad effects on the performance", in theory, if I have much free disk, I could define a great extended memory and did not affect to performance because of ram memory paging to disk

I trust in the rule of the thumb: the "useful value" you mention

I've never read something like "If you have free disk enough, define extended memory as big as you can"

Best Regards, Ximo Escobedo

[Former Member]

Hi Ximo,

SAP dos not LOCK the space, but it increases the disk utilization. In theory, it should not have bad effects on the performance.

But, I had this impression several times as well. Therefore, I always set it to a "useful value" which is about 30% more than the peak

... but be sure, to not reacch it - otherwise, everything runs into PRIV mode ;-(

Regards

Volker Gueldenpfennig, consolut international ag

http://www.consolut.net - http://www.4soi.de - http://www.easymarketplace.de