13.2. Memory Management

The API provides two choices for a host's memory management:

Kernel SamePage Merging (KSM) - The host kernel reduces references to memory pages from multiple identical pages to a single page reference. This helps with optimization for memory density.
Transparent Hugepage support - Hugepage support expands the size of memory pages beyond the standard 4kB limit. This reduces memory consumption and increases host performance.

Kernel SamePage Merging (KSM) uses the ksm element, which is a read-only element.

Example 13.5. KSM memory management

<host id="2ab5e1da-b726-4274-bbf7-0a42b16a0fc3"
  href="/rhevm-api/hosts/2ab5e1da-b726-4274-bbf7-0a42b16a0fc3">
    <name>zig</name>
    <status>UP</status>
    ...
    <ksm>true</ksm>
</host>

Transparent Hugepage support uses the transparent_hugepages element and updates with a PUT request:

Example 13.6. Transparent Hugepage memory management

PUT /rhevm-api/hosts/2ab5e1da-b726-4274-bbf7-0a42b16a0fc3 HTTP/1.1
Accept: application/xml
Content-Type: application/xml

<host>
    <transparent_hugepages>true</transparent_hugepages>
</host>

This request also sets the status of ksm to the opposite status. For example, if transparent_hugepages is set to true, the API sets ksm to false.

Example 13.7. Transparent Hugepage set to true, which sets KSM to false

<host id="2ab5e1da-b726-4274-bbf7-0a42b16a0fc3"
  href="/rhevm-api/hosts/2ab5e1da-b726-4274-bbf7-0a42b16a0fc3">
    <name>zig</name>
    <status>UP</status>
    ...
    <transparent_hugepages>true</transparent_hugepages>
    <ksm>false</ksm>
</host>

If transparent_hugepages is set to false, the API sets ksm to true.

Example 13.8. Transparent Hugepage set to false, which sets KSM to true

<host id="2ab5e1da-b726-4274-bbf7-0a42b16a0fc3"
  href="/rhevm-api/hosts/2ab5e1da-b726-4274-bbf7-0a42b16a0fc3">
    <name>zig</name>
    <status>UP</status>
    ...
    <transparent_hugepages>false</transparent_hugepages>
    <ksm>true</ksm>
</host>

Availability of Transparent Hugepage support is found in the capabilities collection. See Section 7.1.2, “Features”.