Shared processors

Shared processors are physical processors whose processing capacity is shared among multiple logical partitions. The ability to divide physical processors and share them among multiple logical partitions is known as Micro-Partitioning™.

All physical processors that are not dedicated to specific logical partitions are grouped together in the shared processor pool. You can assign a specific amount of the processing capacity in the shared processor pool to each logical partition using the shared processor pool.

The shared processor pool allows you to assign partial processors to a logical partition. A minimum of 0.10 processing units can be configured for any partition using shared processors. Processing units are a unit of measure for shared processing power across one or more virtual processors. One shared processing unit on one virtual processor accomplishes approximately the same work as one dedicated processor.

Some server models allow only a portion of the shared processor pool for use by logical partitions, so you are not always able to assign the full capacity of the shared processor pool to logical partitions. The System Planning Tool (SPT) shows how much of the shared processor pool is available for partitioning use on each server model, so use the SPT to validate your partition plan. For more information about the SPT, see System Planning Tool.

Different partitioning tools assign shared processors to logical partitions in different ways. For more information about shared processor assignment by a specific partitioning tool, see Partitioning tools.

Partitions in the shared processor pool can have a sharing mode of capped or uncapped. An uncapped logical partition is a logical partition that can use more processor power than its assigned processing capacity. The amount of processing capacity that an uncapped logical partition can use is limited only by the number of virtual processors assigned to the logical partition and the amount of unused processing capacity that is available in the shared processor pool. In contrast, a capped logical partition is a logical partition that cannot use more processor power than its assigned processing capacity.

For example, logical partitions 2 and 3 are uncapped logical partitions, and logical partition 4 is a capped logical partition. Logical partitions 2 and 3 are each assigned 3.00 processing units and four virtual processors. Logical partition 2 currently uses only 1.00 of its 3.00 processing units, but logical partition 3 currently has a workload demand that requires 4.00 processing units. Because logical partition 3 is uncapped and has four virtual processors, the server firmware automatically allows logical partition 3 to use 1.00 processing units from logical partition 2. This increases the processing power for logical partition 3 to 4.00 processing units. Soon afterwards, logical partition 2 increases its workload demand to 3.00 processing units. The server firmware therefore automatically returns 1.00 processing units to logical partition 2 so that logical partition 2 can use its full, assigned processing capacity once more. Logical partition 4 is assigned 2.00 processing units and three virtual processors, but currently has a workload demand that requires 3.00 processing units. Because logical partition 4 is capped, logical partition 4 cannot use any unused processing units from logical partitions 2 or 3. However, if the workload demand of logical partition 4 decreases below 2.00 processing units, logical partitions 2 and 3 could use any unused processing units from logical partition 4.

By default, logical partitions that use the shared processor pool are capped logical partitions. You can set a logical partition to be an uncapped logical partition if you want the logical partition to use more processing power than its assigned amount.

Although an uncapped logical partition can use more processor power than its assigned processing capacity, the uncapped logical partition can never use more processing units than its assigned number of virtual processors.

If multiple uncapped logical partitions need additional processor capacity at the same time, the server can distribute the unused processing capacity to all uncapped logical partitions. This distribution process is determined by the uncapped weight of each of the logical partitions.

Uncapped weight is a number in the range of 0 through 255 that you set for each uncapped partition in the shared processor pool. On the HMC, you can choose from any of the 256 possible uncapped weight values. The Integrated Virtualization Manager and the Virtual Partition Manager limit you to only one of several different uncapped weight values. By setting the uncapped weight (255 being the highest weight), any available unused capacity is distributed to contending logical partitions in proportion to the established value of the uncapped weight. The default uncapped weight value is 128.

For example, logical partition 2 has an uncapped weight of 100, and logical partition 3 has an uncapped weight of 200. If logical partitions 2 and 3 both require additional processing capacity, logical partition 3 would receive two additional processing units for every additional processing unit that logical partition 2 receives.

The following Flash demos require the Flash plug-in .

The following Flash demo explains how unused resources in a capped logical partition are used by uncapped logical partitions in the shared processor pool.

Alternatively, you can use the Logical Partitions - Capped Processors of the capped processor demo.

The following Flash demo explains how uncapped logical partitions can exceed their current processing capacity when the shared processor pool has unused processing power.

Alternatively, you can use the Logical Partitions - Uncapped Processors of the uncapped processor demo.