Operating Systems

POWER7 is unusually well-suited for demanding enterprise systems such as SAP. It presents opportunities to exploit the strengths of its chip architecture in terms of four-way Simultaneous Multi-Threading (SMT), partitioning, multi-level cache, and more to achieve high levels of throughput and performance through its inherent parallelism.

Four-way SMT (within a single core) shouldn’t be confused with four-way Symmetrical Multi-Processing (SMP) using four independent cores. SMT doesn’t provide the same capacity, but it does allow for multiple threads within each core to use the compute capacity within each core simultaneously. With SMT, each of the multiple threads is competing for the use of a single core’s resources. This increases thread throughput. Still, by comparison, when executing alone on separate SMP cores, these same threads’ instruction streams each get the full use of the core.

In addition, POWER7 provides what IBM describes as unique virtualization, advanced memory management such as Active Memory Expansion, and breakthrough workload support.

Breakthrough workload support refers to the ability to support a workload without dedicating to it all the resources that workload might need, say, during rare peak times. SAP systems specifically are normally sized for the capacity required to meet those critical peak workloads, which often represents only a small part of the system’s productive day, month, or year. IT can do better.

As explained in an IBM Redbook on POWER and SAP, by consolidating a large number of dedicated servers, each running at an average low utilization with only periodic peaks into a shared infrastructure, less total capacity is required. This also reduces system management and maintenance overhead since there are fewer overall servers in general. And with fewer servers running at a higher utilization, you also reduce the amount of power and space consumed. You do this through virtualization. By virtualizing server resources, you reduce the complexity of the infrastructure and lower the Total Cost of Ownership (TCO) of the IT landscape through consolidation to achieve a more efficient utilization of hardware resources.

SAP on POWER7

The classic architecture of an SAP system isn’t naturally efficient in terms of consuming hardware resources. SAP uses a client/server design that consists of a database and a number of application servers. Of these application servers, the Central Instance (CI) contains resources that are unique to the SAP system, such as the object locking mechanism (enqueue server) and the system communications mechanism (message server). You can construct the system in either two-tier (all components on a single server) or three-tier (application servers distributed over multiple servers). In a three-tier design, the presentation layer (front-end users) counts as a separate tier. Either two-tier or three-tier, SAP will grab all IT resources it feels it needs, even if that need happens only in rare peak instances.

The first step in managing SAP’s resource consumption is to exploit the parallelism of POWER7 for SAP. For this there’s little for the IT staff to do directly, notes Alfred Freudenberger, an IBM Power Systems Solutions specialist. The SAP app server is highly parallelized out of the box. “Parallelism is built into the SAP app server engine,” he notes. This doesn’t leave much opportunity for additional tweaking or tuning by IT.

However, one way where IT staff can make a difference, Freudenberger says, is in tweaking memory. This is done primarily through POWER7’s Active Memory Expansion capability.

Active Memory Expansion

Active Memory Expansion addresses how to make the best use of the entire RAM that’s physically available in the system to execute as many programs concurrently as possible, in the limited space available. Active Memory Expansion, a POWER7 feature, offers a way to make better use of RAM by compressing portions of infrequently called programs into a smaller space within RAM. Such selective compression expands the amount of RAM available to the same or other programs.

With POWER7, RAM is allocated as contiguous memory pages. For an LPAR with Active Memory Expansion enabled, a certain proportion of the RAM is designated for auxiliary storage to hold compressed data. When the operating system determines that a page of uncompressed memory isn’t currently being referenced, and there’s other demand for memory, the contents of the page can be compressed and migrated to the auxiliary storage dedicated for Active Memory Expansion.

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