What are shares?

Resource shares are the minimum amounts of a resource assigned to each PRM group in a PRM configuration file (default name /etc/prmconf). For FSS PRM groups, you can assign CPU and real memory shares, although only CPU share assignments are required. For PSET PRM groups, you can only assign real memory in shares. For both types of groups, you can also specify shared memory allocations.

In addition to minimum amounts, you can specify maximum amounts of of some resources that PRM groups can use. For FSS PRM groups, you can specify maximum amounts of CPU and memory resources. For PSET PRM groups, you can assign a maximum amount of memory; however, the maximum amount of CPU resources available to the PRM group is based on the number of cores assigned to the group. You can assign a maximum amount (known as caps) of memory to a PSET PRM group. Shared memory allocations are static in size, so no caps are needed.

How shares work

A share is a guaranteed minimum when the system is at peak load. When the system is not at peak load, PRM shares are not enforced—unless CPUCAPON mode is enabled, in which case CPU shares are always enforced.

Valid values for shares are integers from one to MAXINT (the maximum integer value allowed for the system). PRM calculates the sum of the shares, then allocates a percentage of the system resource to each PRM group based on its shares relative to the sum.

Table 3 (page 19) shows how shares determine CPU resource percentage. The total number of shares assigned is four. Divide each group’s number of shares by four to find that group’s CPU resource percentage. This CPU resource percentage applies only to those cores available to FSS PRM groups. If PSET PRM groups are configured, the cores assigned to them are no longer available to the FSS PRM groups. In such a case, the CPU resource percentage would be based on a reduced number of cores.

Table 3 Converting shares to percentages

PRM group

CPU shares

CPU resource %

 

 

 

GroupA

1

1/4 = 25.00%

 

 

 

GroupB

2

2/4 = 50.00%

 

 

 

OTHERS

1

1/4 = 25.00%

 

 

 

Shares allow you to add or remove a PRM group to a configuration, or alter the distribution of resources in an existing configuration, concentrating only on the relative proportion of resources and not the total sum. For example, assume we add another group to our configuration in Table 3 (page 19), giving us the new configuration in Table 4 (page 19). To give the new group 50% of the available CPU resource, we assign it four shares, the total number of shares in the old configuration, thereby doubling the total number of shares in the new configuration.

Table 4 Altered configuration

PRM group

CPU shares

CPU resource percentage determined by PRM

 

 

 

GroupA

1

12.50%

 

 

 

GroupB

2

25.00%

 

 

 

GroupC

4

50.00%

 

 

 

OTHERS

1

12.50%

 

 

 

How PRM controls resources 19