5.2.1. Processor Sharing Mode

With the time slice method of the POWER hypervisor, every LPAR is guaranteed its entitlement. For example, if you have only 3 LPARs on a managed system with only one physical processor, LPAR1 with an entitlement of 0.15, LPAR2 with an entitlement of 0.10 and LPAR3 with an entitlement of 0.15, then this is a total of 0.15 + 0.10 + 0.15 = 0.40. This guarantees 4 ms for the 3 LPARs in every 10 ms time interval, but there are still 6 ms that are not guaranteed to any LPAR, as shown in Figure 5.6.

Unallocated time in the hypervisor time slice method
Figure 5.6: Unallocated time in the hypervisor time slice method

This unallocated CPU time could be divided among the 3 active LPARs in order to achieve better performance. That would correspond to an increase in CPU time of 150%!

The processor sharing mode (attribute sharing_mode) can be used to configure whether an LPAR may receive shares of unallocated or unused CPU time. The following two values are possible for shared processor LPARs:

    • cap: The CPU time is limited to the guaranteed entitlement. No additional shares will be allocated to the LPAR, even if available.
    • uncap: In addition to the guaranteed entitlement, the LPAR can receive additional CPU shares from unused processor resources. This is the default for shared processor LPARs.

The sharing mode can be specified when creating an LPAR:

$ lpar create -m ms02 lpar12 desired_procs=2 desired_proc_units=0.8 sharing_mode=cap
    > lpar12

The sharing mode can only be changed in the profile, so you cannot change the sharing mode from cap to uncap or vice versa while an LPAR is running. The easiest way to change the sharing mode in the profile is to use the “lpar chproc” command, specifying the “-p” option and the profile name:

$ lpar -p standard chproc lpar11 sharing_mode=cap

For the change to take effect, the LPAR must be shut down and reactivated by specifying the changed profile.