Limiting How Much Memory a Storage Pool Can Allocate

Published: October 26, 2011

Hey, Joe:

I want to run the i OS Performance Adjuster to automatically allocate system memory to my subsystems. But when I turn it on, most of the memory goes to the *INTERACT storage pool used by the QINTER subsystem. I want to take care of interactive users, but I also want some memory left over for other subsystems. How do I limit *INTERACT from using too much memory?

--Jens

This sometimes happens with using the i operating system automatic performance adjustment feature. By default in the i 6.1 operating system, the Performance Adjustment (QPFRADJ) system value is set to "2". This tells the system to automatically adjust the allocated system memory each storage pool uses at the following times.

• To set up the storage pools when the system performs an initial program load (IPL).
• Periodically while the system is running, making adjustments to the storage pools based on current activity. This allows the system to allocate memory more efficiently as it is needed.

But sometimes the system works a little too efficiently, allocating too much memory to one storage pool, such as you're seeing with *INTERACT. When that happens, other storage pools may not be able to get the memory they need quickly enough to process their jobs.

Fortunately, the i operating system provides a mechanism for handling this situation: minimum and maximum pool sizes. You can adjust your storage pool memory allocation based on a minimum and maximum percentage of available system memory. Here’s how it works.

Open a green screen session and type in the Work with Shared Pools (WRKSHRPOOL) command. This will give you a screen that looks like this:

This screen displays all the configured shared storage pools that are available on your system. By default, the *INTERACT storage pool controls the amount of memory allocated to the QINTER subsystem. To control how much memory *INTERACT can use, press the F11 key to look at your shared storage pools tuning data. You'll see a screen that looks like this:

This screen controls the individual tuning data parameters you can set for your calculated storage pools (those pools that have a paging option of *CALC on the initial WRKSHRPOOL screen).

For your question, the settings you're interested in are the two columns labeled Minimum and Maximum under the Size % heading. For dynamic storage pool adjustment, these columns control how much memory each pool can allocate from your partition's main storage size (system memory). Looking at this screen, you'll see that by default, the performance adjuster will always assign a *INTERACT value that is between 10 and 100 percent of available memory. This is because *INTERACT's minimum size percentage is set to 10, and its maximum size percentage is set to 100.

Basically, the default setup allows *INTERACT to allocate as much system memory as it can get. This is what you want to control when the performance adjustment feature is turned on.

It's easy to set limits on *INTERACT's storage pool size. To tell the performance adjuster never to assign more than 33 percent of available system memory to *INTERACT (1/3), tab over to the "100" value for the *INTERACT’s maximum size percentage. Change that value to 33, and press ENTER. Your screen will now look like this.

With this change, you've changed your *INTERACT authorized memory range from 10 to 100 percent of system memory to 10 to 33 percent of memory. Now the performance adjuster will never give more than 33 percent of available memory to *INTERACT. This should leave more memory to allocate to other storage pools.

Also note that the tuning data section of this screen is color coded. Default values are displayed in white text while changed values are displayed in green text. It's easy to reset a changed minimum or maximum size percentage back to its default value. Simply tab over to the green value you want to change, type "*DFT" in the field, and press ENTER. The pool's size percentage value will then be reset to its default.

Your storage pool's minimum and maximum size percentages can also be changed in System i Navigator (OpsNav). To change these values in OpsNav, open the Work Management→Memory Pools→Shared Pools node on your target partition. This will display all your shared storage pools. Right click on the pool that you want to change your minimum or maximum size percentage value for and select Open. Select the Tuning tab off the Properties screen that appears. You'll see a screen that looks something like the screen below. Change the values listed on this screen.

If you want to reset the storage pool' settings back to its default values, simply click on the Reset to Defaults button.

And that' all there is to changing default shared storage pool sizes on either the green screen or in OpsNav.

For more information on working with shared storage pools, see the articles listed below.

HTH

--Joe

RELATED STORIES

Allocating Enough System Storage to a Subsystem

Tuning i5/OS Storage Pools for Performance

More Information on Fixed Storage and WRKSYSACT

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