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Determining the I/O entitled memory for a shared memory partition

After you create a new logical partition that uses shared memory (hereafter referred to as a shared memory partition) or you dynamically add or remove a virtual adapter, you can use memory statistics that are displayed by the Hardware Management Console (HMC) to dynamically increase and decrease the amount of I/O entitled memory assigned to a shared memory partition.

The I/O entitled memory set for a shared memory partition needs to be high enough to ensure the progress of I/O operations, and low enough to ensure adequate memory use among all the shared memory partitions in the shared memory pool.

The operating system manages the I/O entitled memory allocated to a shared memory partition by distributing it among the I/O device drivers. The operating system monitors how the device drivers use the I/O entitled memory, and sends usage data to the HMC. You can view the data in the HMC and dynamically adjust the I/O entitled memory that is assigned to a shared memory partition.

To determine the I/O entitled memory for a shared memory partition, complete the following steps using the HMC:

  1. View information about the physical memory that is used by the shared memory partition for its I/O devices.
    1. In the navigation pane, expand Systems Management > Servers.
    2. Click the server on which the shared memory partition runs.
    3. In the work pane, select the shared memory partition and click Properties from the Tasks menu. The Partition Properties page is displayed.
    4. Click the Hardware tab.
    5. Click the Memory tab.
    6. Click Memory Statistics. The Memory Statistics panel is displayed.
  2. Determine whether you want to change the I/O entitled memory that is assigned to the shared memory partition, and the value to which you want to change it:
    • If the Maximum I/O Entitled Memory Used value is less than the Assigned I/O Entitled Memory value, the operating system is able to run all of the workload's I/O operations simultaneously without using all of its assigned I/O entitled memory. In this situation, you can decrease the Assigned I/O Entitled Memory value to the Maximum I/O Entitled Memory Used value and continue to maintain unconstrained I/O performance.
    • If the Maximum I/O Entitled Memory Used value is equal to the Assigned I/O Entitled Memory value, the I/O operations of the shared memory partition might or might not be constrained by the Assigned I/O Entitled Memory value as follows:
      • The Assigned I/O Entitled Memory value does not constrain the I/O operations: The operating system runs all of the workload's I/O operations simultaneously and uses all of the I/O entitled memory that is assigned to it. In this situation, the shared memory partition operates with the smallest amount of I/O entitled memory necessary to maintain unconstrained I/O performance.
      • The Assigned I/O Entitled Memory value constrains the I/O operations: The workload requires more physical memory for I/O operations than the Assigned I/O Entitled Memory value, so the operating system must delay some I/O operations so that the shared memory partition operates within the Assigned I/O Entitled Memory value. In this situation, you can increase the Assigned I/O Entitled Memory value so that it no longer constrains I/O performance.
      If you are unsure as to whether the Assigned I/O Entitled Memory value constrains the I/O operations of the shared memory partition, you can increase the I/O entitled memory that is assigned to the shared memory partition, reset the data collector, and view the memory statistics again. Repeat this process until the Maximum I/O Entitled Memory Used value no longer equals the Assigned I/O Entitled Memory value. You can also view statistics, for AIX®, IBM® i and Linux shared memory partitions, that show the number and frequency of delayed I/O operations. For instructions about how to view these statistics, see Performance considerations for shared memory partitions.
  3. Dynamically increase or decrease the I/O entitled memory that is assigned to the shared memory partition. For instructions, see Adding and removing I/O entitled memory dynamically to and from a shared memory partition. (Dynamically changing the I/O entitled memory also changes the I/O entitled memory mode to the manual mode.)
  4. Reset the data collector. From the Memory Statistics panel, click Reset Statistics and then click Close.
  5. Repeat this procedure until you are satisfied with the amount of I/O entitled memory that is assigned to the shared memory partition.

    For example, you create a shared memory partition with eight virtual adapters. You activate the shared memory partition and the HMC automatically assigns 128 MB of I/O entitled memory to the shared memory partition. After some time, you view the memory statistics for the shared memory partition and see that the Maximum I/O Entitled Memory Used value is 96 MB. You dynamically decrease the I/O entitled memory that is assigned to the shared memory partition from 128 MB to 96 MB and reset the data collector. After some time, you view the memory statistics for the shared memory partition and see that the Maximum I/O Entitled Memory Used value is 88 MB. Because 88 MB is similar to 96 MB, you decide to leave the I/O entitled memory assigned to 96 MB for the shared memory partition.

Examples

Creating a new shared memory partition

  1. You activate the new shared memory partition. The HMC automatically sets the I/O entitled memory for the shared memory partition.
  2. After some time, you view the memory statistics and see that the Maximum I/O Entitled Memory Used value is much less than the Assigned I/O Entitled Memory value.
  3. You dynamically decrease the I/O entitled memory of the shared memory partition to the Maximum I/O Entitled Memory Used value and reset the data collector. (Dynamically decreasing the I/O entitled memory also changes the I/O entitled memory mode to the manual mode.)
  4. After some time, you view the memory statistics again and determine that the new Maximum I/O Entitled Memory Used value is only slightly less than the new Assigned I/O Entitled Memory value, and no further adjustment is necessary.

Dynamically adding a virtual adapter to a shared memory partition in the auto I/O entitled memory mode

  1. You dynamically add a virtual adapter to a shared memory partition. The HMC automatically increases the I/O entitled memory that is assigned to the shared memory partition.
  2. After some time, you view the memory statistics and see that the Maximum I/O Entitled Memory Used value is much less than the Assigned I/O Entitled Memory value.
  3. You dynamically decrease the I/O entitled memory of the shared memory partition to the Maximum I/O Entitled Memory Used value and reset the data collector. (Dynamically decreasing the I/O entitled memory also changes the I/O entitled memory mode to the manual mode.)
  4. After some time, you view the memory statistics again and determine that the new Maximum I/O Entitled Memory Used value is only slightly less than the new Assigned I/O Entitled Memory value, and no further adjustment is necessary.

Dynamically adding a virtual adapter to a shared memory partition in the manual I/O entitled memory mode

  1. You ensure that the shared memory partition has enough I/O entitled memory to accommodate the new adapter by dynamically increasing the I/O entitled memory of the shared memory partition.
  2. You dynamically add a virtual adapter to the shared memory partition.
  3. After some time, you view the memory statistics and see that the Maximum I/O Entitled Memory Used value is much less than the Assigned I/O Entitled Memory value.
  4. You dynamically decrease the I/O entitled memory of the shared memory partition to the Maximum I/O Entitled Memory Used value and reset the data collector.
  5. After some time, you view the memory statistics again and determine that the new Maximum I/O Entitled Memory Used value is only slightly less than the new Assigned I/O Entitled Memory value, and no further adjustment is necessary.

Dynamically removing a virtual adapter from a shared memory partition

  1. You dynamically remove a virtual adapter from a shared memory partition. If the I/O entitled memory mode is in the auto mode, the HMC automatically decreases the I/O entitled memory that is assigned to the shared memory partition.
  2. You reset the data collector.
  3. After some time, you view the memory statistics and see that the Maximum I/O Entitled Memory Used value is much less than the Assigned I/O Entitled Memory value.
  4. You dynamically decrease the I/O entitled memory of the shared memory partition to the Maximum I/O Entitled Memory Used value and reset the data collector. (If the I/O entitled memory mode is in the auto mode, dynamically decreasing the I/O entitled memory also changes the I/O entitled memory mode to the manual mode.)
  5. After some time, you view the memory statistics again and determine that the new Maximum I/O Entitled Memory Used value is only slightly less than the new Assigned I/O Entitled Memory value, and no further adjustment is necessary.

The following example is another way to accomplish this example for AIX shared memory partitions:

  1. Determine the amount of physical memory that the virtual adapter (that you plan to remove) currently uses by running the lparstat command from the AIX command line.
  2. If the I/O entitled memory mode is in the auto mode, dynamically change the I/O entitled memory mode to the manual mode by running the chhwres command from the HMC command line.
  3. Using the HMC graphical interface, dynamically remove the virtual adapter.
  4. Using the HMC graphical interface, dynamically decrease the I/O entitled memory that is assigned to the shared memory partition by the amount that you identified in step 1.
Related concepts:
I/O entitled memory
Related tasks:
Managing virtual adapters dynamically

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Last updated: Thu, September 20, 2018